SERIES IMPEDANCE AND SHUNT A D M I T T A N C E MATRICES OF

A N UNDERGROUND C A B L E SYSTEM

by

Navaratnam Srivallipuranandan B.E.(Hons.), University of Madras, India, 1983

A THESIS SUBMITTED IN P A R T I A L F U L F I L L M E N T OF T H E R E Q U I R E M E N T S F O R T H E D E G R E E O F

M A S T E R OF APPLIED S C I E N C E

in

T H E F A C U L T Y O F G R A D U A T E STUDIES (Department of Electrical Engineering)

We accept this thesis as conforming to the required standard

T H E UNIVERSITY O F BRITISH COLUMBIA, 1986

C Navaratnam Srivallipuranandan, 1986 November 1986

In presenting t h i s thesis i n p a r t i a l f u l f i l m e n t of the requirements for an advanced degree at the University of B r i t i s h Columbia, I agree that the Library s h a l l make i t f r e e l y available for reference and study. I further agree that permission for extensive copying of t h i s thesis for scholarly purposes may be granted by the head of my department or by h i s or her representatives. I t i s understood that copying or publication of t h i s thesis for f i n a n c i a l gain s h a l l not be allowed without my written permission.

Department of

The University of B r i t i s h Columbia 1956 Main Mall Vancouver, Canada V6T 1Y3

Date

6 n /8'i}

SERIES I M P E D A N C E A N D S H U N T A D M I T T A N C E M A T R I C E S

O F A N U N D E R G R O U N D C A B L E

ABSTRACT

This thesis describes numerical methods for the: evaluation of the series

impedance matrix and shunt admittance matrix of underground cable

systems. In the series impedance matrix, the terms most difficult to

compute are the internal impedances of tubular conductors and the

earth return impedance. The various form u hit- for the interim!'

impedance of tubular conductors and for th.: earth return impedance

are, therefore, investigated in detail. Also, a more accurate way of

evaluating the elements of the admittance matrix with frequency

dependence of the complex permittivity is proposed.

Various formulae have been developed for the earth return

impedance of buried cables. Using the Polhiczek's formulae as the

standard for comparison, the formula of Ametani and approximations

proposed by other authors are studied. Mutual impedance between an

underground cable and an overhead conductor is studied as well. The

internal impedance of a laminated tubular conductor is different from

that of a homogeneous tubular conductor. Equations have been

derived to evaluate the internal impedances of such laminated tubular

conductors.

( i i )

Table of Contents

Abstract — - - - l i Table of Contents - i i j List of Table - •• - V List of Figures - -:— VI List of Symbols - V i i i

Acknowledgement , - •- i x

1. I N T R O D U C T I O N .—.: . 1

2. SERIES I M P E D A N C E A N D S H U N T A D M I T T A N C E M A T R I C E S 2.1 Basic Assumptions 6 2.2 Series Impedance matrix [Z] for N Cables in Parallel 7

2.2.1 Submatrix [Z„] 9 2.2.2 Skin Effect 13 2.2.3 Internal Impedance of Solid and Tubular Conductors 14 2.2.4 Submatrix [Z,-yJ - •• 15

2.2.4.1 Proximity Effect 16 2.2.4.2 Proximity Effect of a Single-Phase Circuit of Two Identical Conductors 17 2.2.4.3 Shielding Effect of the Sheath 17 2.2.4.4 Elements of Submatrix [Z„] 19

2.3 Shunt Admittance Matrix [K]; for N Cables in Parallel 22 2.3.1 Leakage Conductance and Capacitive Suceptance 22 2.3.2 Frequency Dependence of the Complex Permittivity 23 2.3.3 Submatrix 27

2.4 Conclusion 29

3. C O M P A R I S O N O F I N T E R N A L I M P E D A N C E F O R M U L A E 3.1 Exact Formulae for Tubular Conductors -•- 30 3.2 Internal Impedance of a Solid Conductor ..— 31 3.3 Internal impedance of a Tubular Conductor ; 36 3.4 Conclusion — -•- 47

( i i i )

4. E A R T H R E T U R N I M P E D A N C E 4.1 Earth Return Impedance of Insulated Conductor 50 4.2 Earth Return Impedance in a Homogeneous Infinite Earth 50 4.3 Earth Return Impedance in a Homogeneous Semi-Infinite Earth 52 4.4 Formulae used by Ametani, Wedepohl and Semlyen 57 4.5 Effect of Displacement Current Term and Numerical Results 61 4.6 Cable Buried at Depth Greater than Depth of Penetration 64 4.7 Mutual Impedance between a Cable Buried in the Earth and an Over­

head Line or vice versa 67 4.8 Conclusion - 69

5. L A M I N A T E D T U B U L A R C O N D U C T O R S 5.1 Internal Impedance of a Laminated Tubular Conductor 70

5.1:1 Internal Impedance with External Return 70 5.1.2 Internal Impedance with Internal Return 72

5.2 Application to Gas-Insulated Substations 73 5.2.1 Case i: Core and Sheath not Coated 74 5.2.2 Case ii: Only Sheath Coated 74 5.2.3 Case iii: Only Core Coated 76 5.2.4 Case iv: Both Core and Sheath Coated 77 5.2.5 Stainless Steel Coating 79 5.2.6 Supermalloy Coating 82 5.2.7 Comparison between Stainless Steel and Supermalloy Coatings

82 5.3 Conclusion 85

6. T E S T C A S E S 6.1 Single-Core Cable 86 6.2 Three-Phase Cable 91 6.3 Shunt Admittance Matrix 93 7. C O N C L U S I O N 94 A P P E N D I X A 96 A P P E N D I X B : 98 A P P E N D I X C 103 A P P E N D I X D 106 R E F E R E N C E S : 114

(Iv);

List of Tables

3.1 Internal Impedance of a Solid Conductor 33 3.2 Internal Impedance Za of a Tubular Conductor 38 3.3 Mutual Impedance (Za(,) of a Tubular Conductor with Current Return­

ing Inside 43 3.4 Internal Impedance Zy of a Tubular Conductor with Current Returning

Outside -. , 44 4.1 Solution of PoIIaczek's Equation by Numerical Integration and Using

Infinite Series 58 4.2 Earth Return Self Impedance with and without Displacement Current

Term .' 1 61 4.3 Earth Return Self Impedance as a Function of Frequency 64 '• 5.1 Resistivity and Relative Permeability of Coating Materials 79 5.2 Skin Depth of Stainless Steel 79 5.3 Skin Depth of Supermalloy 82 6.1 Impedances of Single Core Underground Cable 87 6.2 Mutual Impedance between Two Cables with Burial Depth of 0.75m

and Separation of 0.30m 91

(V)

List of Figures

1.1 Potential Difference V, between Core and Sheath and F 2 between Sheath and Earth 3

2.1 Basic Single Core Cable Construction 7 2.2 Loop Currents in a Single Core Cable 9 2.3 Potential Difference between Two Concentric Conductors 10 2.4 Three Conductor Representation of a Single Core Cable : 10 2.5 Sheath with Loop Currents Ix and I2 ... •- 15 2.6 Two Cable System , -. 16 2.~! Circuit Arrangement of Primary, Secondary and Shielding Conductors,

with Shielding Conductor Grounded at Both Ends".. 18 2.8 Transmission System Consisting of a Single Conductor and a Cable

; 21 2.9 Cross-Section of a Coaxial Cable 23 2.10 (a),(b) - Measurements of e'(<o) and ("(<*) of an OiMmpregnated Test

Cable at 20°cC 24 2.11 Values of e'(o>) and «"((•)) Obtained from the Empirical Formula

26 2.12 Polarization-Time Curve of a Dielectric Material 27 3.0 Loop Currents in a Tubular Conductor 30 3.1 (a),(b) - Impedance of a Solid Conductor as a Function of Frequency

3.2 (a),(b) - Errors in Wedepohl's and Semlyen's Formulae for a Solid Conductor 35

3.3 Cross-Section of a Tubular Conductor 30 3.4 (a),(b) - Impedance Za of a Tubular Conductor (with Internal Return):

as a Function of Freqency 39 3.5 Errors in Wedepohl's, Schelkunoff's and Bianchi's Formulae for Za

40 3.6 Errors in Wedepohl's and Schelkunoff's Formulae for Zab 42 3.7 (a),(b) - Impedance Zb of Tubular Conductor (with External Return)

as a Function of Frequency ..' 45 3.8 Errors in Wedepohl's, Schelkunoff's and Bianchi's Formulae for Zb

:.. „ , 46 4.1 Electric Field Strength at Point P 51 4.2 Error in Replacing a Conductor of Finite Radius by a Filament Con­

ductor —-- 53 4.3 Solution of Real and Imaginary Part of Equation (4.9), for a

Freqency of 1MHz. > - - «... 56 4.4 Relative Error in the Evaluation of Carson's Formulae with an

Asymptotic Expansion 59

(vi)

4.5 Error in the Earth Return Self-Impedance if the Displacement Current is Ignored 62

4.6 (a),(b) - Earth Return Self Impedance as a Function of Freqency , 65

4.7 Errors in Earth Return Self Impedance 60 4.8 Differences in Resistance Values of Semi-Infinite and Infinite Earth

Return Formulae : 68 5.1 (a),(b) - Numbering of Conductor Layers to Find the Internal

Impedances of a Laminated Tubular Conductor 71 5.2 Representation of the nt th Layer 71 5.3 Core and Sheath not coated 74 5.4 Inner Surface of the Sheath only, Coated 75 5.5 Core Alone Coated 76 5.6 Core as well as Inner Surface of the Sheath Coated 77 5.7 Dimensions of the Bus Duct in a Gas-Insulated Substation <• 78 5.8 (a),(b) - Variation of Resistance; and Inductance with Frequency for

the Four Cases; Stainless Steel Coating, Thickness, 0.1mm 80 5.9 (a),(b) - Variation of Resistance and Inductance with Freqency for the

Four Cases; Stainless Steel Coating, Thickness 0.5mm 81 5.10 (a),(b) - Variation of Resistance and Inductance with Freqency for

the Four Cases; Supermalloy Coating, Thickness 0.01mm 83 5.11 (a),(b) - Variation of Resistance and Inductance with Frequency for

the Four Cases; Supermalloy Coating, Thickness 0.05mm 84 6.1 Errors in Ametani's and Wedepohl's Approximations in Zcc 88 6.2 Errors in Ametani's and Wedepohl's Approximations in Zgc 89 6.3 Errors in Ametani's and Wedepohl's Approximations in Zef 90 6.4 Errors in Ametani's and Wedepohl's Approximations in the Mutual

Impedance between Two Cables 92 A.l Three-Phase Cable Set-up for the Study 96 A. 2 Basic Construction of Each Single Core Cable 96 B. l The Relative Directions of the Field Components in a Coaxial

Transmission Line 98 B. 2 Loop Currents in a Tubular Conductor 101 C. 1 Representation of a Buried Conductor in an Infinite Earth 104 D. l Current Carrying Filament in the Air 106 D.2 Current Carrying Filament Buried in the Earth I l l

( V i i )

L I S T O F SYMBOLS

E — electric field strength, H = magnetic field strength / = frequency, IT = 3.1415926 (o = 27rX /, angular frequency

— 1, complex operator fi0 = 4JTX 10 - 7, absolute permeability of free space /xr, — relative permeability of the medium i t*i ~ / io x / i r i> total permeability of the medium « 7 = Euler's constant p, = resistivity of a particular medium t €,((o) = €, — complex dielectric constant or permittivity of a particular

medium » <j> = flux density / = current J — current density c,exp = exponential In = natural logarithm

( joy/ty 1 m, — |:- — o> fi^t- | , known as intrinsic propagation constant of a particu-

lar medium t. If the displacement currents are ignored, then the ( joi / i , I

value of m is equal to I I . Displacement currents are ignored unless li Pi J

otherwise specified /„ = modified Bessel function of the 1st kind and of the nth order Kn — modified Bessel function of the 2nd kind and of the nth order K — characteristic impedance F = propagation constant T = relaxation time of a dielectric R = resistance per unit length L = inductance per unit length G = conductance per unit length C — capacitance per unit length Y = shunt admittance matrix Z = series impedance matrix

(viii)

Acknowledgements

I would like to acknowledge my appreciation to Dr. H. W. Dommel for his

encouragement and supervision throughout the course of this research.

I would also like to thank Mrs. Guangqi Li, Mr. Luis Marti and Mr. C. E.

Sudhakar for their many valuable suggestions.

Thanks are due to Mrs. Nancy Simpson for typesetting the manuscript.

The financial support of Bonneville Power Administration, Portland, Oregon,

U.S.A., and from my brother "Anna" is gratefully acknowledged.

(ix)

-1-

1 . INTRODUCTION Underground cables are used extensively for the transmission and distribution of electric

power. Although expensive when compared with overhead transmission, laying cables under­ground is often the only choice in urban areas. As the urban areas expand, the cable circuits tend to increase in length. At present, cable circuits are being employed which have lengths of the order of 100km. With an increase in system lengths and higher system voltages, the induction effects on nearby communication circuits are becoming more important. Also, for the power system itself, the steady-state and transient behaviour of underground cables must be known. For interference studies as well as for power system studies, methods for finding cable parameters over a range of frequencies are, therefore, needed.

The transmission characteristics of an underground cable circuit or submarine cable cir­cuit are determined by their propagation constant T, and characteristic impedance /<", which may be calculated for an angular frequency w from the following equations

F = V(K + j<aL)(G + juC) (1.1a)

K = V{R + joiL)/(G + j<aC) (1.1b)

Where R,L,G and C are the four fundamental line parameters, i.e., resistance, inductance, conductance and capacitance per unit length. These cable parameters are therefore the basic data for all interference and power system studies.

Cables are principally classified based on

i) their location, i.e., aerial, submarine and underground

ii) their protective finish, i.e., metallic (lead, aluminium) or non-metallic (braid)

iii) the type of insulation, i.e., oil-impregnated paper, cross linked polyethylene (XLPE) etc.

iv) the number of conductors, i.e., single conductor, two conductors, three conductors and so on.

In this thesis, a single conductor aluminium cable with a concentric lead sheath and with insulation of either oil impregnated paper or XLPE is studied in detail (refer to Appendix A).

The series impedance and shunt admittance matrices of a cable system made up of N

cables, can be written as

- 2 -

Z = (Z 2 1 | ( zd • • • 1*8*1

a n d

Y = (nil p y

" l*/w]

1*2*1

PAWI

(1.2a)

(1.2b)

S u b m a t r i x [Z„\ a l o n g the d i a g o n a l o f m a t r i x [Z\ is t h e se l f i m p e d a n c e o f c a b l e i w h i c h c a n

be w r i t t e n as

Z„ = Zw Zc.s. zsic{ zs.s.

(1.3)

w h e r e

Zc.c = se l f i m p e d a n c e o f the c o r e o f c a b l e i"

Zc.s. = Zs;c.= m u t u a l i m p e d a n c e b e t w e e n t h e c o r e a n d the s h e a t h o f c a b l e »

.7j s = se l f i m p e d a n c e o f the s h e a t h o f c a b l e i

T h e o f f - d i a g o n a l s u b m a t r i x 2,; is t h e m u t u a l i m p e d a n c e b e t w e e n t h e c a b l e i a n d c a b l e j w h i c h

c a n be w r i t t e n as

(1.4)

w h e r e

Zee = m u t u a l i m p e d a n c e b e t w e e n t h e c o r e o f t h e c a b l e i a n d c o r e o f t h e c a b l e j

Zci - m u t u a l i m p e d a n c e b e t w e e n t h e c o r e o f t h e c a b l e i a n d t h e s h e a t h o f t h e c a b l e j

z..t. = m u t u a l i m p e d a n c e b e t w e e n t h e s h e a t h o f c a b l e i a n d t h e c o r e o f t h e c a b l e j

ZSi = m u t u a l i m p e d a n c e b e t w e e n t h e s h e a t h o f t h e c a b l e t a n d t h e s h e a t h o f the c a b l e j

E v a l u a t i o n o f a l l t h e s e e l e m e n t s ; o f s u b m a t r i c e s [Z„\ a n d \Ztj\ i s , i n g e n e r a l , n o t e a s y . T h e

b e s t a p p r o a c h s e e m s t o be t h e o n e p r o p o s e d b y W e d e p o h l [22] b a s e d o n t h e e a r l i e r w o r k d o n e

b y S c h e l k u n o f f [6]. B o t h a u t h o r s f i n d t h e s e i m p e d a n c e s f r o m t h e l o n g i t u d i n a l v o l t a g e d r o p s in

t h e c o r e a n d s h e a t h , [Z = V/J). T h e s e l o n g i t u d i n a l v o l t a g e d r o p s c a n be o b t a i n e d f r o m t h e

p o t e n t i a l d i f f e r e n c e V , - b e t w e e n t h e c o r e a n d t h e s h e a t h a n d t h e p o t e n t i a l d i f f e r e n c e

- 3 -

between the sheath and the earth, as shown in Figure 1.1.

The potential differences V1 and V 2 can be expressed as a function of the loop currents /j and J 2 with the help of Schelkunoff's theorems. (Appendix B)

For finding the elements of the shunt admittance matrix, it is a usual practice (22,23,27] to assume the permittivity of the insulation to be a real constant. In reality, the value of the permittivity is a frequency-dependent complex value, the real part of which accounts for the susceptance term and the imaginary part accounts for the conductance term. Therefore, it is necessary to find a general expression for the permittivity as a function of frequency.

Chapter 2 discusses, in detail, the following topics:

i) Formation of series impedance and shunt admittance matrices,

ii) Wedepohl's approach for finding the elements of the series impedance matrix,

iii) Frequency-dependence of the permittivity constant

iv) Proximity effect and shielding effect in the evaluation of the mutual impedance subma-

trix Z,r

As explained in Appendix B, the potential differences V, and V2 can be evaluated in terms of loop currents /, and I2 using SchelkunofTs theorem. For example, the potential difference Vl

can be written as

- 4 -

V, = (Zctt + ZtHl + Z , A , ) / , - Z s i k m 7 2 (1.5)

where

Z c r f = internal impedance of the core with current return outside,

Z,ns — impedance of the insulation between the core and sheath

Ztkl = internal impedance of the sheath with current return inside

ZsKm — mutual impedance between the loops 1 and 2.

The formulae developed by Schelkunbff to evaluate these internal impedances, Zcrt,Zsh,

and mutual impedance, ZtKm, which take the skin effect into account, are exact and given in

terms of modified Bessel functions. These exact expressions are not suitable for hand calcula­

tion purposes. There have been several attempts to obtain approximations to these classical

formulae in order to make them suitable for hand calculations, (22, 23, 24].

Some of these approximate formulae are compared wjjth the exact formulae of (6] in

Chapter 3, in terms of accuracy and computer time. The errors which are caused by neglecting

the displacement current are discussed in Chapter 3 as well.

Generally, the earth acts as a return path for part of the current in the underground or

submarine cable system. The cable parameters are very much influenced by the earth return

impedance. These impedances are obtained from the axial electric field strength in the earth

due to the return current in the ground.

In Chapter 4 the following topics are discussed:

(i) Earth return impedances of cables buried in an infinite earth, where the depth of penetra­

tion of the return current in the ground is smaller than the depth of burial, or in iother

words, where the distribution of return current in the ground is circularly symmetrical.

(ii) Earth return impedances of cables buried in a semi-infinite earth, where the depth of

penetration of the return current in the ground is larger than the depth of burial.

(iii) Error introduced in the answers if the displacement current is neglected in the computa­

tion.

(iv) Approximations proposed by Wedepohl and Semlyen and a comparison of their equations

with the classical formula of Pollaczek [lj in terms of accuracy and computer time.

"I (v) Ametani's (27] cable constant routine in the E M T P program.

(vi) Mutual impedances between a buried conductor and an overhead conductor.

In Chapter 5, we turn our attention from cables made up of homogeneous conductors to

cables whose core and sheath are made up of laminated conductors of different materials. A

practical application of this type of conductor is proposed by Harrington [32]. He suggested

- 5 -

that transient sheath voltages in gas-insulated substations can be reduced by coating the con­ductor and sheath surfaces with high-permeability and high-resistivity materials. Formulae for the internal impedances of such laminated conductors are derived and used to show the damp­ing effect as a function of frequency.

Chapter 6 concludes this thesis by comparing the values of the cable parameters obtained for a particular three-phase cable system shown in Appendix A, by using the exact formulae of Schelkunoff [6] and Pollaczek [l], as well as Ametani's approach [27] and Wedepohl's approxi­mation [22].

- 6 -

2. SERIES IMPEDANCE AND SHUNT A D M I T T A N C E MATRICES

2.1 Basic Assumptions

The transmission characteristics of a conducting system such as an underground cable cir­

cuit or a submarine cable circuit are determined by its propagation constant T and characteris­

tic impedance K, which can be calculated for the angular frequency a) from the formula

T = V(R + {juiL)(G + jcoC)

K =; V(/? + jo>L)/(G + jmC) (2.1)

where R,L,G and C are the four fundamental line parameters - resistance, inductance, conduc­

tance and capacitance, all per unit length. Determination of these parameters in a cable is not

easy, but involves rather difficult analysis. Pioneering work in the calculation of underground

cable parameters has been done by Wedepohl and Wilcox (22], based on the earlier work of

Schelkunoff (6).

The first step in defining the electrical parameters of an underground or submarine cable

system is to set up the equations which describe the electric and magnetic fields. A complete

set of such equations would constitute a perfect mathematical model. If these equations could

be solved without any approximations then the response of the model would be indistinguish­

able from that of the real system, which it represents. In practice, however, this ideal situation

cannot be realized. For example, it is not possible to perfectly represent the electrical proper­

ties such as resistivity, permeability and permittivity of the earth which form the return path

for the currents flowing in the cable. Rigorous representation of such factors would lead to a

set of very complex equations which may be very difficult to solve. In practice, therefore, some

simplifying assumptions are made. The assumptions made in this thesis are:

1. The cables are of circularly symmetric type. The longitudinal axes of cables which form

the transmission system are mutually parallel and also parallel to the surface of the

earth. It is implied in this assumption, that the cable has longtitudinal homogeneity. In

other words, the electrical constants do not vary along the longtitudinal axes.

2. The change in electric field strength along the bngitodinal axes of the cables are negligi­

ble compared to the change in radial electric field strength. This assumption permits the

solution of the field equations in two dimensions only.

3 The electric field strength at any point in the earth due to the carrents flowing in a cable

is not significantly different from the field that would result if the net current were con­

centrated in an insulated filament placed at the centre of the cable and the volume of the

cable were replaced by the soil.

- 7 -

4. Displacement currents in the air, conductor and earth can be ignored.

Assumptions 3 and 4 are justified up to high frequencies (1MHz], as will be demonstrated later in Chapter 3 and Chapter 4.

2.2 Series Impedance Matrix [Z\ for N Cables in Parallel

Let us assume that the transmission system consists of N cables. Each cable has a cross section of the type shown in Figure 2.1, representative of a typical high voltage (H.V.) under­ground cable.

Figure 2.1 Basic Single Core Cable Construction

The core consists of a tubular conductor C with the duct being filled with oil. In the case of solid conductors, the inner radius r 0 would be zero. The insulation between the core and the sheath is usually oil-impregnated paper, surrounded by a metallic tubular sheath 5, and insula­tion around the sheath.

In such systems, there are n = 2N metallic conductors. The soil in which the cables arc ; buried constitutes the ( n - f l)th conductor which is chosen as the reference for the conductor voltages. Such a transmission system can be described by the two matrix equations

4^= -ZI (2.2a) dx

(2.2b)

- 8 -

d]_ dx

= -IV

where'V and / are n-dimensional vectors of voltages and currents, respectively, at a distance z along the longitudinal axis of the cable system. A l l voltages and currents are phasor values at a particular angular frequency a>. The series impedance matrix Z is given by

Z =

\Zn\ l*«l 1*2=1

(*IN1 \ZW\

\ZNN]

(2.3)

Each submatrix, \ZU\ assembled along the leading diagonal is a square matrix of dimension 2 representing the self impedances of cable i , by itself,

*c,c, *£.s; (2.4)

where

Zc.c. = self impedance of the core of cable 1

Z,.s. = Zt.e. = mutual impedance between core and sheath of cable »

Zs.St = self impedance of the sheath of cable »

The off-diagonal submatrix |Z1;] represents the mutual impedances between cable »" and cable j. This submatrix is also a square matrix of dimensions 2,

l*,l Zr - r . Z' c - ? • Z' r -c. Z * .s.

(2.5)

inhere

Z£.c. = mutual impedance between core of cable t and core of cable j

ZCiij — mutual impedance between core of cable 1 and sheath of cable j

Z,.t. = mutual impedance between sheath of cable 1 and core of cable j

Zt.$i = mutual impedance between sheath of cable 1 and sheath of cable j

Similarly, the shunt admittance matrix Y"can be defined as:

- 9 -

Y = (2.6)

•where the submatrices jV„j and |Y"y) can be defined in a similar way, as described in section 2.3.

2.2.1 S u b m a t r i x |Z„J

The elements of the submatrix \Z„] can be determined by considering a single cable whose

longitudinal cross section is as shown in Figure 2.2. The longitudinal voltage drops in such a

cable are best described by two loop equations, with loop 1 formed by the core and sheath (as

return) and loop 2 formed by the sheath and earth (as return).

insulation 2 sheath insulation 1 core

Figure 2.2 Loop Currents in a Single Core Cable.

It has been shown by Carson [4] that the change in the potential difference between j and

(/ + 1) of a concentric cylindrical system as shown in Figure 2.3 is given by

——• + E} — Ej + x —ju>n4>, (27)

where

Ej = longitudinal electric field strength of the outer surface of the conductor j

E ; * j = longitudinal electric field strength of the inner surface of conductor (j'+l)

- 10 -

Axis Conductor j

Conductor (j+l)

Figure 2.S Potential Difference between Two Concentric Conductors.

V. - potential difference between the j and the (j+l) conductor

<t>, = magnetic flux through the area described by the contour ABCD.

Since part of the current in the cable can return through the earth, the cable must be represented by 3 conductors (core, sheath, earth), as shown in Figure 2.4.

Axis

Sheaih

Insulation 2 Earth

Figure 2.4 Three Conductor Representation of a Single Core Cable.

- 11 -

The values of longitudinal electric field strengths Ecre,Esk,,Eihe (i.e., on the external sur­face of the core, internal surface of the sheaih and the external surface of the sheath respec­tively) can be expressed as

Ecre = Z[rt /„ (2.8a)

Eikt = -*,»./, + ZskmI2 i(2.8b)

Eikc = Zikc I2 - Ztkm /, (2.8c)

The electric field strength along the surface of the earth can be written as

Ec = -ZeiI2 (2.9)

where

Zcrc = internal impedance per unit length of the core's external surface with current return­ing through a conductor outside the core.

Zskt = internal impedance per unit length of the sheath's internal surface with current returning through a conductor inside the tubular sheath.

Zskm = mutual impedance per unit length of the sheath which gives the voltage drop on the external surface of the sheath, when current passes through the internal surface or vice versa.

Zike — internal impedance per unit length of the sheath's external surface with current returning through a conductor outside the tubular sheath.

Zes = self impedance of the earth's return path.

Equation (2.7) can be then be written for the contours ABGD and EFGH a i follows:

dx

d\r2

~dx

= Eik, - Ec,c - joL„h (2.10a)

= Et - Eikt - jo>L„/ 2 (2.10b)

In equation (2.10a), the total magnetic flux through the area described by the contour ABCD is

Lcs /j where

L» = itl*(r^) (211)

and r 0 and r, being the outer and inner radii of the insulation. The term L„ can be defined similarly. The parameters jtaLc, and j i * L l t are the impedances Znl and Zn2 of the respective insulations.

- 12 -

Substituting the values for the electric field strengths from equation (2.8) and (2.9) into equation (2.10), we have

dz dV~

dx ~ Zikm Ztht + Zm2 + Zti

(2-12)

The matrix equation (2.12) relating the potential differences between the concentric cylindrical conductors and the loop currents can also be obtained from Figure 2.2. directly, i.e..

d\\

dx zx Zm '/.' d\'2 Zm Z2

(2.13)

dx

where the self impedance of loop 1 consists of 3 parts

Z\ = Zcre + Z i n l + Zitix,

and similarly for loop 2

Z2 ~ Zsh. 4- Z,n2 + Zti,

while the mutual impedance between loop 1 and loop 2 is

Zm = ZShm

Equations (2.12) or (2.13) are not yet in the usual form in which the voltages and currents of the core and the sheath are related to each other. They can be brought ijjto such a form by. considering the appropriate terminal conditions namely

v2 = Vtk I2 = Iik + /„ (2-14)

where

Vcr = voltage from the core to the local ground,

Vsll = voltage from the sheath to the local ground,

IC1 = total current flowing in the core,

Isk = total current flowing in the sheath.

Substituting the values for voltages V,,V2 and currents 7,,/2 from equation (2.14) into equation (2.12), and adding rows 1 and 2, we obtain

- 13 -

dV„ ' ZCre + Z,nl + Zsh, + Ztkc + Icr

dx Z,n2 Zes — 1Zlkm Ze$ ~~ ztkm

dx Zskt ^««2 ^ « — ' ^«i fn -

(2-15)

The impedance matrix given in equation (2.15) is the self impedance submatrix [Z„\ for cable t.

It can be seen that the elements of the impedance submatrix \Z„] are obtained from the internal impedances of tubular conductors • and from the earth return impedance. These impedances are frequency dependent because of the skin effect, which is discussed in the next section.

2.2.2 Skin Effect

In the derivation of formulae for resistance and inductance of conductors, it is often assumed that the current density is constant over the cross section of the conductor. This assumption is justified only if

(i) the resistivity is uniform over the cable cross section, and if

(ii) the conductor radius is small compared to the depth of penetration

However, as the size or permeability of the conductor increases or as the frequency increases (resistivity still being uniform), the current density varies with the distance from the axis of the conductor, current density being maximum at the surface of the conductor and minimum at the centre.

The reason for skin effect is as follows:

In a long conductor of uniform resistivity the direction of current is everywhere parallel to the axis, and the voltage drop per unit length is the same for all the parallel filaments into which the conductor may be imagined to be subdivided, since these filaments are electrically in parallel. The voltage drop in each filament consists of a resistive com­ponent proportional to and in phase with the current density in the filament, and an inductive component, equal to joi times the magnetic flux linking the filament. There is more flux linking the central filament of a round conductor than linking the filaments at the surface, because the latter are surrounded only by the external flux, whereas the former is surrounded also by all the internal flux. The greater the flux linkage and the inductive drop, the smaller must be the current density and the resistive drop in order for the total drop per unit length to be the same. Hence, the current density is least at the centre of the conductor and greatest at the surface [7].

- 14 -

The ac resistance, which is defined as the power lost as heat, divided by the square of the current, is increased by the skin effect, because the increase in loss caused by the increase in current in the outer parts of the conductor is greater than the decrease in loss caused by the decrease in current in the inner parts. The inductance, defined as flux linkage divided by current, is decreased by skin effect because of the decrease in internal flux.

2.2.3 Internal Impedance of Solid and Tubular Conductors

As mentioned in the previous section, the voltage drop per unit length is the same in all the parallel filaments into which the conductor can be subdivided because all filaments are electrically in parallel. The ratio between this voltage drop and the sum of al! filament currents is the internal impedance. For a solid conductor of radius a and resistivity p. the internal impedance is given by [7]

pm I0{ma)

2ira/,(m(i)

where

/„./, = modified Bessel functions of the first kind and of zero and first order, respectively.

m = the intrinsic propagation constant of the conductor of equation (2.16).

The derivation of the internal impedance formula for tubular conductors is more complex due to the boundary conditions. For example, if we consider the sheath in Figure 2.2. the loop

current /,. passes through the inner surface of the sheath and returns internally and the loop

current I2 passes through the outer surface of the sheath and returns externally. This is illus­trated in Figure 2.5.

Therefore, we have to consider the magnetic field strengths on both surfaces (which are then the boundary conditions) while solving the Maxwell's field equations to determine the for­mulae for the internal impedances. A detailed analysis of this problem had been done by Schel-kunoff [6]: his formulae which are relevant to this thesis are summarized in Appendix B.

As shown in Figure 2.5, the return path for the current flowing in a tubular conductor may be provided either inside or outside the tube, or partly inside and partly outside. We designate Z„ as the internal impedance of the inner surface of the tube with internal return, and Zb as the internal impedance of the outer surface of the tube with external return, and Zab

as the mutual impedance between one surface of the conductor to the other. The values of Za,Zb and Zai are given as follows:

Za = ^•[/ 0(ma)K,(mJ) + AT0(ma )/,(*.&)]

- 15 -

Figure 2.5 • Sheath with Loop Currents /, and /j.

Z = P

st 2xabD

(2.17:.,b,c)

w h e r e

D = /j(m6)A',. (ma) - / , ( m a ) AT,(m6),

p = r e s i s t i v i t y o f t h e c o n d u c t o r ,

m = i n t r i n s i c p r o p a g a t i o n c o n s t a n t o f t h e c o n d u c t o r o f e q u a t i o n (2.17),

/<>,/, «= m o d i f i e d B e s s e l f u n c t i o n s o f t h e f i r s t k i n d a n d o f z e r o a n d f i r s t o r d e r , r e s p e c t i v e l y .

/ f 0 ,W, = t h e m o d i f i e d B e s s e l f u n c t i o n s o f t h e s e c o n d k i n d a n d o f r e r o a n d first o rd f - r .

r e s p e c t i v e l y .

U s i n g t h e s e f o r m v . ' a e t h e e l e m e n t s o f t h e s u b m a t r i x [Z.,\ c a n b e f o u n d .

2.2.4 S u b m a t r i x | Z J

T h e o f f - d i a g o n a l s u b m a t r i x [ZtJ\ w h i c h r e p r e s e n t s t h e m u t u a l i m p e d a n c e s b e t w e e n c a b l e i

a n d c a b l e j c a n be b e s t e x p l a i n e d i f w e c o n s i d e r a t r a n s m i s s i o n s y s t e m c o n s i s t i n g o f o n l y t w o

c a b l e s « a n d j as s h o w n i n F i g u r e 2.6. B e f o r e w e a n a l y z e t h e e l e m e n t s o f the s u b m a t r i x , we

- 16 -

Earth

Cable j

Figure 2.6 Tvuo Cable System

will briefly discuss the influence of proximity effects and shielding effects on these elements.

2.2.4.1 Proximity Effect

Skin effect is caused by the non-uniformity of current density in a conductor. This current, density is a function of distance from the axis, but not of direction from the axis. However, in parallel conductor transmission, in addition to the self-magnetic field (field gen­erated by the current flowing through the conductor), there will be magnetic fields generalcd by currents iD adjacent conductors. These fields interact and result in distortion in the ovr;:.l! symmetric field distribution. The effects of the distortion of symmetry are known -j\ |>r«.̂ ii:ii: \ effects, which in most cases affect the distributed parameters of the transmission system {3.3].

A.H.M. Arnold [13], has given a comprehensive treatment on proximity effect resistance ratios for single-phase and three-phase circuits. He has given equations and tubulated func­tions of i (defined below) for determining the proximity effect resistance ratios R'zr' in a single-phase circuit of two identical tubular conductors, with solid conductors being a special case. The ratio R'/R' is defined as the ratio of the effective ac resistance with proximity effect taken into account to the effective oc resistance wbcu the conductors are far apart, such that the proximity efTect is negligible. Further, factors to be applied to the ratio /?'//?'while consid­ering a three-phase circuit with symmetrical triangular spacing or with flat spacing arc also given in the same reference.

- 1 7 -

2.2.4.2 Proximity Effect of a Single-Phase Circuit of Two Identical Conductors

The ratio R'/R', defined earlier, for a tubular conductor with the solid conductor being a

special case, depends upon three variables, i.e., t/d,d/a and x defined as

t/d = ratio of thickness t of the tube to its outside diameter d.(t/d = 0.5 for solid conductor).

d/s = ratio of outside diameter d of a conductor to distance s between the axes of the conduc­

tors.

x = 2nV2ft{d-t)/p

x can be further simplified to [13],

x —

1.52-\/f/Rdc where

= the dc resistance of the conductor in Dim.

The proximity effect resistance ratio is then given by

/?'//?'=- (2.18)

where A. B and C are functions of x and t/d which can be determined from tables given in [13]. Similarly, proximity effect inductance ratios of a single-phase cable can be obtained as well. Also, both proximity resistance and inductance ratios for a 3-phase system can be obtained from the single-phase proximity resistance and inductance ratios.

For the example chosen in this thesis d/s is less than 0.35. For such a value, the proxim­ity effect can be ignored up to frequencies of 1MHz, [13, 33]. Hence, proximity effects are ignored here.

2.2.4.3 Shielding Effect of the Sheath Another factor of importance in determining the mutual impedance, is the shielding effect

of the cable sheath, which is normally grounded at both ends. Consider a primary circuit 0. a parallel exposed secondary circuit x and a shielding conductor * whose ends arc grounded as

d\-'° shown in Figure 2.7- Let be the induced voltage in the exposed circuit duc to the mag-

dx

netic coupling without any shielding conductor and let — — be the induced voltage in the dx

dV? i shielding circuit. The current in the ground shielding conductor is then — , where Z.,

dx Zss

is the self impedance of the shielding conductor with earth return, Now, voltage induced in the

exposed circuit by the current in the shielding conductor is — , where Z.z is the mutual

Figure 2.7 • Circuit Arrangement of Primary, Secondary

and Shielding Conductors, with Shielding Conductor Grounded at Both Ends

impedance between the shielding and the exposed circuit. Therefore, the net voltage induced in the exposed circuit is

d\r

x dV? dVf Z;J

dx dx dx Z S 5

(2.19)

dVf d\r? If the voltages — — and — — are expressed in terms of the current in the primary circuit as

dx dx

dV? d\r° = I0Z0s

a n cl ~.— = IoZoi> then equation (2.19) simplifies to dx dx

£ 1 dx

1 - Os z»z Zn ZQJ

ZQZIQ (2.20)

and the shielding factor of the grounded shielding conductor is then given by

Z,i Z0i

n = l z„z (2.21)

ss Ox

If the shielding and the secondary conductor are exposed to the same field, which is the case for a shielding wire very close to a telephone line, and for the cable sheath around the core con­

ductor, then dX rO

dx dx ' Therefore Z0l = Z0s, which makes the shielding factor to be equal

to

(2.22)

- 19 -

2.2.4.4 E l e m e n t s o f S u b m a t r i x ( Z t ; ) .

K e e p i n g in m i n d t h e s h i e l d i n g e f f e c t d e s c r i b e d a b o v e , w e w i l l n o w d e r i v e t h e e l e m e n t s o f

t h e s u b m a t r i x \Z,}]. A g a i n , l o o p c u r r r e n t s a r e u s e d , as h a s b e e n d o n e b e f o r e f o r d e t e r m i n i n g

t h e e l e m e n t s d f t h e s u b m a t r i x j Z „ j . C o n s i d e r i n g t h e c a b l e s y s t e m s h o w n i n F i g u r e 2 .3 , we c a n

d e f i n e t h e f o l l o w i n g l o o p c u r r e n t s f o r t h e i t h c a b l e

i) l o o p c u r r e n t / ' , , w h o s e p a t h c o n s i s t s o f t h e c o r e ' s e x t e r n a l s u r f a c e a n d s h e a t h ' s i n t e r n a l

s u r f a c e

i i) l o o p c u r r e n t 7 3 , w h o s e p a t h c o n s i s t s o f t h e s h e a t h ' s e x t e r n a l s u r f a c e a n d e a r t h .

S i m i l a r l y , t h e l o o p c u r r e n t s l[ a n d I'2 c a n be d e f i n e d f o r c a b l e j .

If w e c o n s i d e r t h e l o o p c u r r e n t s F2 a n d I{, t h e r e wi l l be n o i n d u c e d v o l t a g e in l o o p 2 o f

c a b l e i d u e t o t h e l o o p c u r r e n t I\ as t h e n e t f i e l d p r o d u c e d b y t h e l o o p c u r r e n t 1\ is z e r o o u t ­

s ide t h e c a b l e j [6], U s i n g the law o f r e c i p r o c i t y o f m u t u a l i m p e d a n c e s , i t c a n be d e d u c e d t h a t

t h e r e wi l l be no i n d u c e d v o l t a g e in t h e l o o p 1 o f c a b l e j d u e t o t h e l o o p c u r r e n t V2. H e n c e ,

r e l a t i n g the l o o p c u r r e n t s w i t h t h e p o t e n t i a l d i f f e r e n c e s b e t w e e n t h e c o n d u c t o r s we o b t a i n :

'dV\/dx z\ Z'm 0 0 r\

diydx zx

m Z'2 0 Zsi ft

d\'\/dx 0 0 z\ Am n

d\"2/dj 0 Z S I Zin Zk

(2.23)

M o s t i m p e d a n c e t e r m s in e q u a t i o n (2 .23) , h a v e a l r e a d y b e e n d e f i n e d e x c e p t the t e r m

Zj. S y, w h i c h is t h e m u t u a l i m p e d a n c e b e t w e e n t h e e a r t h r e t u r n l o o p s 2 o f c a b l e s »' a n d j . If t h e

c a b l e s are b u r i e d in a n h o m o g e n e o u s i n f i n i t e e a r t h , w h e r e t h e p e n e t r a t i o n d e p t h o f t h e r e t u r n

c u r r e n t in t h e e a r t h is less t h a n t h e d e p t h o f b u r i a l , t h e n t h e v a l u e o f Z S i S > is g i v e n b y [9]:

pm"lK0(ms) ' -y _ \_ ' (2 24)

w h e r e

a = d i s t a n c e b e t w e e n t h e c e n t r e s o f t h e c a b l e s ,

r,,Tj = e x t e r n a l r a d i i o f the c a M c s i a n d j , a n d

m = i n t r i n s i c p r o p a g a t i o n c o n s t a n t o f t h e e a r t h .

F o r a h o m o g e n e o u s s e m i - i n f i n i t e e a r t h , w h e r e t h e p e n e t r a t i o n d e p t h i n t h e e a r t h is m o r e

t h a n t h e d e p t h o f b u r i a l , the m u t u a l i m p e d a n c e Zt.Sf is g i v e n b y (22]:

- 20 -

_ jap. K0(m/?) - /C0(mZ) + J

where

d,,d} = depth of burial of cables » and j ,

m = intrinsic propagation constant of the earth,

z = V « ' + (d, + d,y

s = horizontal separation between cables »' and j.

If we measure the voltages with respect to ground, then we can write

v2 = V\h

n = vu

and

V = /'

I\ =

/{ =

n =

I'

ih + Hr

(2.25)

(2.26)

Substituting the values given by equation (2.26), into equation (2.23), and adding rows 1,2 and

rows 3,4, we obtain the series impedance matrix for two cable system, as

(2.27)

From equation 2.27, the impedance submatrix [Z,}] defined earlier in equation (2.5) is

given by

dV\,/dx z\ + 2z; + r2 Z'm + Z'2 Zss Hr

dVJdx z'm + z\ z\ Zss Ilk

dV{,ldx Zss Zss s,s}

Z{ .+ 2ZL + z>2 Zln + Z'2 Hr

dV{hldx Z s i Zss Z'2 + zi z2 7«*.

- 21 -

| 2,l =

It is i n t e r e s t i n g t o n o t e , t h a t t h e m u t u a l i m p e d a n c e b e t w e e n t h e c o r e o f c a b l e i a n d t h e

c o r e o f c a b l e j a n d t h e m u t u a l i m p e d a n c e b o t w e e n t h e c o r e o f c a b l e i a n d s h e a t h o f c a b l e j a r e

t h e s a m e . T h i s r a i s e s t h e q u e s t i o n w h e t h e r t h e s h i e l d i n g e f fec t is p r o p e r l y r e p r e s e n t e d in t h e

e q u a t i o n s . It is i n d e e d i m p l i c i t l y t a k e n c a r e o f i n t h e f o r m u l a t i o n w i t b l o o p c u r r e n t s . T h i s c a n

be i l l u s t r a t e d w i t h t h e h e l p o f a c o n d u c t o r w p l a c e d i n c lose p r o x i m i t y t o a c a b l e b u r i e d in t h e

e a r t h , as s h o w n in F i g u r e 2.8

Earth

(2 .28)

Conductor W

Figure 2.8 - Transmission System Consisting of a Single Conductor and a Cable.

F o v t h e s y s t e m s h o w n in F i g u r e 2 .8 , t h e v o l t a g e s a n d c u r r e n t s o i l the c a b l e c a n be w r i t t e n

as:

dx

dVsh

dx

— %cc Ic ZCSIS + ZCU,IVI

~ Zci Ic + Z,t /, + ZlV) /„

(2 .29a )

( 2 . 2 9 b )

w h e r e

Zet,Ztt

Zca..Zia.

— t h e s e l f i m p e d a n c e o f t h e c o r e a n d s h e a t h o f the c a b l e , r e s p e c t i v e l y ,

— t h e m u t u a l i m p e d a n c e b e t w e e n t h e c o r e a n d the s h e a t h , a n d .

= t h e m u t u a l i m p e d a n c e s b e t w e e n t h e c o r e a n d c o n d u c t o r w a n d b e t w e e n t h e s h e a t h

a n d c o n d u c t o r w, r e s p e c t i v e l y .

- 22 -

Suppose that the cable sheath is not grounded at the ends, but used as the return pa;b for the current flowing in the core. Then there is no magnetic field outside the sheath, and no voltage will therefore be induced in conductor u>. Since this induced voltage is Z^L. + Zs. /,..

with Isk — ~ / £ T . it follows that Z^ — Zsw must be true. On the other hand, if the sheath is grounded at the ends, then there will be a circulating current through the sheath and earth, and Vsk becomes zero. Hence, the value of 7$ can be found from equation (2.29b) as:

^ss

ZSUI

(2.30)

Substituting the value of 7S into equation (2.29a) we obtain

dV,

dx Z-^ z" z..

z„ - zSi

L +

1 -

Zc$' Zs.

~z~

Za ' Zsw

Zls ' Zcv, IwZCu (2.31)

The term 1 -7 7 Za Zcu,

is the shielding factor of the sheath for the field produced by conduc­

tor w. With Zcw = Zsw, it can be simplified to 1 -Zss

which is the same as equation (2.22).

Hence, the shielding effect is implicitly taken care of in the equations.

2.3 Shunt A d m i t t a n c e M a t r i x Y for N Cables in Paral le l

In a manner similar to the series impedance matrix Z, the shunt admittance matrix Y can be expressed in terms of two submatrices [Y„] and [Y,J. Since the soil acts as an electrostatic shield between the cables, the off-diagonal submatrix [>',,] will be a null matrix. Hence, we only have to derive the submatrix [Yj,|. Before we obtain the elements for the submatrix [}'„], the admittance of insulation will be discussed first.

2.3.1 Leakage Conductance and Capaci t ive Susceptance

Figure 2.9 shows a cross-section of a coaxial cable, with insulation between core and sheath, and between sheath and earth. Let us assume that the insulation has a relative permit­tivity of

The admittance Y per unit length of the insulation is defined as

j<i)27reo€* y = G + jB = —

In r2/r1

- 23 -

Figure 2.9 - Croaa-Section of a Coaxial Cable.

77̂ 7 V ~ J

jti)27rf„( (2.32)

In TJT , In rjr,

The first term in the left hand side of equation (2.32), is the leakage conductance of the

insulation. It is the result of the combined effects of leakage current through the insulation

and of the dielectric loss [7]. The second term is known as the capacitive susceptance of the

insulation between the two conductors (core and sheath, or sheath and earth).

2.3.2 Frequency Dependency of the Complex Permittivity

Generally, the dielectric constant i is assumed to be a real constant with the imaginary

part of t neglected due to its relatively small value, (of the order of 10" 4 compared to the real

value [7,13,22.27]). However, the complex dielectric constant i is not a constant as its name

implies. It depends on a number of factors such as the frequency of the applied field, the tem­

perature and the molecular structure of the dielectric substance [15].

Let us consider two commonly used, insulating materials for power cables, namely cross-

linked polyethylene (XLPE) and oil-impregnated paper. The values of i and i for XLPE are

approximately constant for at least up to frequencies of \00MHz [16]. Typically, they have

values of

i = 2.33

t*=4.66 1(r 4 (2.33)

- 24 -

Unfortunate!}', little is mentioned in the literature about the frequency dependency of the permittivity in the case of oil-impregnated paper (18]. Recently Johanscn and Breien [17] pub­lished the measured values of i and t for the oil-impregnated paper for a frequency range of 1Hz to 10Q.4/7/;. The value of i was found to vary by 20%, whereas, the value of ' varies by 200% for the same frequency range, at a temperature of 20°c.

Figure 2.10(a),(b) Measurements of t ( t o )

and t*(co)

j * on an Oil-Impregnated Test Cable at 2(f c.

Figure 2.10(a) and (b) show the experimental data obtained for I and t for the frequency range 10* to 108Hz only. Based on this experimental data, the authors [17] developed an empirical formula for t'(oi).

- " + (, + y J x . o - T " 1 2 3 1 1

Figures 2.11 (a) and (b) show the plot for i and ('obtained from the empirical formula, which closely match the experimental data of Figure 2.10(a) and (b).

- 26 -

Frequency [Hz]

Figure 2.11 Values of t\ui) a n d t"(o>)

Obtained from the Empirical Formula.

- 26 -

A general formula for tbe complex permittivity of any material as a function of frequency

is given by Bartnikas [15]. According to Bartnikas, when a dielectric is subjected to an ac field,

at low electric field gradients, its electrical response will depend upon a number of parameters

such as the frequency of the applied field, the temperature and the molecular structure of the

•dielectric substance. Under some conditions, no measurable phase difference between the

dielectric displacement; D, and voltage gradient E will occur, and consequently the ratio DIE

will be defined by a constant equal to the real value of the permittivity, c'. When a dc field E

is suddenly applied across a dielectric, tbe dielectric will almost instantaneously, or in a very

short time, attain a finite polarization value. This polarization value will be almost instantane­

ous, since it will be determined by the electronic and atomic polarizability effects. The limiting

value of the real dielectric constant <'for this polarization is defined as <«, so that the resulting

dielectric displacement is Dm or imE. The slower processes, due to the dipolc oriontatiou or

ionic migration, will give rise to a polarization which will attain its saturation value consider­

ably more gradually because of such effects as the inertia of the permanent dipoles. The static

dielectric displacement vector, Dit in this case is equal to isE, where t, is the static value of

the real dielectric constant, e.

Figure 2.12 - Polarization-Time Curve of a Dielectric Afaterial

In the idealized polarization time curve depicted in Figure 2.12, Pf is the achieved satura­

tion value of tbe polarization resulting from permanent dipoles or from any other displacement

of free charge carriers. Depending upon the temperature and the chemical and physical struc­

ture of the material, the saturation value, Pt may be achieved in a time that may vary any­

where from a few seconds to several days. If we denote the time-dependent portion of P% as

P(t), the equation of the curve in Figure 2.12 can be represented by a form characteristic of the

- 27 -

charging of a capacitor

P(t) = Ps [l - exp(-f/r)] (2.35)

where T is the time constant of the charging process. The time constant, T is a measure of the

time lag and is referred to as the relaxation time of the polarization process.

Now, the real and imaginary parts of the permittivity of an insulating material can be

expressed as a function of frequency in terms of tm.e, and T as

t = Rc(e') = £«, +

I = Im(£') =

1 + O ) 2 ^

(ts-e„)oiT

1 + o) 2 ^

(2.36a)

(2.36b)

In summary, frequency dependence of the permittivity t is complicated, although, for

some insulating materials, such as (XLPE), i is practically constant. The changes are very sig­

nificant, i.e., of the order of 102, for oil-impregnated paper. Typical values for the real and

imaginary parts of t for X L P E are given in equation (2.33). The real and imaginary values of

t for oil-impregnated paper can be obtained from equation (2.34), based on the reference [17].

A general formula for ''-he complex permittivity of any material is given by equation (2.37a).

2.3.3 Submatrix \YU\

We shall now determine the self admittances of the cable system shown in Figure 2.2.

The loop equations for the current changes along loops 1 and 2 will be:

di,

dx

dx 0 dl2 0 n v2

(2.37a)

?here

Yi = Gx + jBA — admittance of insulation between core and sheath,

Y 2 = G2 + jB2 — admittance of insulation between sheath and earth,

Vj = voltage between the core and sheath,

V2 — volt age between the sheath and earth.

Substituting the values for currents IltI2 and voltages VltV2 from equation (2.14) into

(2.36) and subtracting row 2 from row 1, we obtain:

- 28 -

dlc, dx

dx

Yx -Yr -Y, Yx + Y2 . v » * .

(2.37b)

Hence, the submatrix \Y„] is given by:

Yt ~Yx -Yx Yx + Y2

(2.38)

Recently, Dommel and Sawada [19,20] suggested that the admittance matrix should

include the effect of the grounding resistance as well if the insulation between the sheath and

earth is electrically poor, as in oil or gas pipelines. In such cases, the leakage current flows

through the series connection of the insulation resistance and the finite grounding resistance.

For conduction effects in pipelines they, therefore, use

1 ' insulation R, earth

(2.39)

where the grounding resistance /?e i r th i 3 given by

R earth Pearth

4;r 2J_ + fa V(2ff) g + (t/2)2 + 1/2 D lnV(2H)2 + (1/2)2 - 1/2

(2.40)

with

Pearth = earth resistivity,

H = depth at which the cable is buried,

/ — length of the cable.

Strictly speaking, G2 in equation (2.39) is no longer an evenly distributed parameter

because /? e i r t tin equation (2.40) is a function of length. In [20], it is shown that the change in

the value of C 2.with the length is practically negligible, and treating G2 as an evenly distri­

buted parameter is therefore a reasonable assumption.

2.4 Conclusion

First, the series impedance and shunt admittance matrices were defined. These matrices

are made up of self and mutual impedance (or admittance) submatrices. Elements of the self

impedance submatrices were obtained from the internal impedance formulae for tubular con­

ductor and from the earth return self impedance formula. The elements of (he mutual

impedance submatrices were obtained from the earth return mutual impedance formulae. The

shielding effect of the sheath and the proximity effects between the conductors were studied

next to assess their influence on the elements of the mutual impedance submatrix. Finally, the

- 29 -

self admittance submatrix and mutual admittance submatrix were defined. Since the earth

acts as an electrostatic shield, the elements of the mutual admittance submatrix are zero. The

permittivity c* of the insulation which is needed to evaluate the elements of the self admittance

submatrix, is frequency dependent and complex. An empirical formula for finding the real and

imaginary parts of the permittivity t * as a function of frequency was shown, which can then be

used to find the elements of the self admittance submatrix.

- 30 -

3. C O M P A R I S O N O F I N T E R N A L I M P E D A N C E F O R M U L A E

lu the previous-chapter the scries impedance matrix was assembled from the internal impedances of tubular conductors and from the earth return self and mutual impedances of tubular conductors. The formulae for the internal impedances and earth return impedances are given in terms of modified BesscI functions, which can be expressed as an infinite scries for :

small arguments and as an asymptotic series for large arguments. Before computers became available, exact calculations were almost impossible and approximate formulae were therefore developed. Such approximations had been proposed by several authors (G,22,2'5',"2 l]. In this chapter, approximate formulae for the internal impedances are compared with i lie exact for­mulae in terms of cpu time and in terms of accuracy. The earth return formulae are discussed in the next chanter.

3.1 Exact Formulae for Tubular Conductors

Axis

I 17/

Figure 3.0 - Ix>op Currents in a Tubular Conductor

The internal impedances of a tubular conductor as given in equation (2.17) are as follows:

Z. = f/otmoJK-.M) + KjmaVAmb)] (3.1a)

Zt = -^L. \ I o { r n b ) K l ( m a ) + K^mbVJma)] (3.1b)

Z.» = (3-lc) 2ncbD

where D - /j(ml»)K',(ina) - Il(ma)Kl{mb)

- 31 -

The argumeDts for the modified Bessel functions 7 0 , / i and K0,Ki of the first kind and

second kind are complex because the intrinsic propagation m of the conductor is ^/w/y/p) .

For more exact- calculations, m should be {j<a/i/p— <>>2/i<) , where the first term under the

square root accounts for the conduction current and the second term accounts for the displace­

ment current. The displacement current can be ignored in good conductors as it is negligible

compared to the conduction current up to frequencies of 10MHz. For example, copper conduc­

tors with p = 1.7 10~8/?m resistivity would have a displacement current at 10MHz which is 11 orders of magnitude smaller than the conduction current, and even smaller than that below 10MHz. However, in the earth, the displacement current has an influence on the impedance as the earth's conductivity is of the cider IO - 1 0 smaller than the conductivity of a good conductor such as copper. Displacement currents are therefore taken into account in the earth return impedances discussed in the next chapter.

Subroutine TUBE originally developed by H. W. and I. I. Dommel [30] (and later modified by L. Marti [29]) and Amctani's Cable Constants program implemented in BPA's EMTP, assume that the displacement currents can be neglected. This fact enables us to express the modified Bessel functions of the 1st and 2nd kind in terms of Kelvin functions. For example, the complex functions K0{mr) and I0(mr) can be expressed as real and imaginary parts as fol­lows:

A'„(mr)= K0(VJ\m]r) = Kj\m\r)+ jA' t l(|m|r) (3.2a)

7 0(mr)= /o(V7T^lT)= B„(\m\r)+ jB e i(|m|r) (3.2b)

where B„ and £?,, are Kelvin functions of the first kind, and K~¥ and Kcl are those of the second kind. To evaluate the Kelvin functions, infinite series and asymptotic series can be used for small real arguments and large real arguments, respectively. Subroutine TUBE and Ametani's routine use such series with a sufficient number of terms to guarantee high accuracy,

3.2 Internal Impedance of a Solid Conductor

The exact, formula for the internal impedance of a solid conductor of radius r follows from Zh of equation (3.1) by setting o = 0 and 6 = r:

^1^1 3

2 j i r / , (mr)

where

r = radius of the conductor

- 3 2 -

m = intrinsic propagation constant of the conductor

/ 0 , / 1 = modif ied Bessel functions of the first kind and of zero and first order, respectively.

Wedepohl [22] suggested an approximation to this exact formula, given by

z _ pmcoth(fcmr) + p(l-l/2k)

This approximation was developed by first considering the equation

= pmcoth(fnr) ( 3 5 )

27rr

This equation is known to exhibit similar properties as the exact equation given by Equation

(3.3). For example, at high frequencies, the impedance term Zx tends to be pm/2xr which is a

well known skin effect formula [22], while at lower frequencies it represents pure resistance

although not, in fact, equal to the required value p/xr2. Equation (3.5) can be improved to take

account of the dc resistance more precisely by writing

Z > = - ^ - c o t h ( W ) + ^ - f * ) (3.6) 2nr nr

where k is an arbitrary constant. The second term on the right hand side of this equation

corrects the impedance at direct current. The value of A; chosen to give the correct resistive

component is 0.777.

There is another interesting formula derived by Semlyen in the discussion of reference 24,

where the internal impedance of a solid conductor is given as

Zx = y/R? + Za (3.7)

where Rc is the dc resistance given by p/xr2 and, Za is the impedance at very high frequencies

given by pm/2-r. Table 3.1 shows the values of resistance Rx = Re{Zx} and inductance

Lx = — 7m{Z,} obtained from subroutine T U B E and from Wedepohl's and Semlyen's approxi-(i)

mation formulae.

Figure 3.1(a) and (b) show the resistance and inductance as a function of frequency. The

errors in the values of resistance and inductance in Wedepohl's formula and Semlyen's formula

are plotted in Figure 3.2(a) and (b).

From the table and figures we can see that Wedepohl's formula has an error of 1-3% in

the frequency range 100Hz to 1kHz for the resistive part, and 4% error up to frequency of 1kHz in the inductive part. Semlyen's formula has an error of 4-7% for the frequency range

60Hz to 20kHz, in the resistive part, and an error of 4-11% for the frequency range 20Hi to 300Hz in the inductive part.

- 33 -

T a b l e 3.1

Internal Impedance of a Solid Conductor

(p = 17 10~'Om and r = 0 . 0 2 3 4 m )

FREQUENCY (Hz)

TUBE WEDEPOHL SEMLYEN

RESISTANC ;E (Q/km)

.01 . 1 1 10 100

1,000 10,000 100,000

1 ,000,000 10,000,000

0.0098825 0.0098825 0.0098858 0.0102067 0.0203380 0.0582719 0.1786977 0.5596756 1.7644840 5.5744390

0.0098775 0.0098776 0.0098809 0.0102034 0.0211463

i 0.0592378 0.1797193 0.5607150 1.7655290 5.5754860

0.0098825 ; 0.0098825 0.0098874 0.0103294 0.0190556 0.0561595 0. 1763397 0.5572406 1.7620250 5.5719720

INDUCTANC 3E (jiH/km)

.01 . 1 1 10 100

1,000 10,000 100,000

1,000,000 10,000,000

50.000000 49.999920 49.991580 49.181730 27.567070 8.853760 2.803902 0.886793 0.280432 0.088680

51.800000 51.799920 51.792250

! i 51.042710 i 28.354910 ; 8.868063 2.804328 0.886806 0.280433 0.088680

50.000000 49.999750 49.974780

; 47.836480 25.930690 8.798590 2.802123 0.886736 0.280430 0.088680

The cpu times were found to be:

TUBE

Wedepohl

Semlyen

0.042 ms

0.038 ms

0.022 ms

- 34 -

figure 3.1(a) and (b) Impedance of a

Solid Conductor ae a Function of Frequency.

- 35 -

5-

2 ' U i w -1-1 c ro *—«

CO

0) cc -5-

-7-4 11

12

WEDEPOHL SEMLYEN

-f i x----.:

-

i / / / \ /

10~2 10_1 1 10' 10 2 10 5 10 4 10 s 10 6 10'

8

^ 4

o L— uj o o c

— 4 o

T3

-8

WEDEPOHL SEMLYEN

1/ 10~* 10" 1 1 10' 10 2 10 3 10 4 10 5 10 6 10 7

Frequency [Hz]

Figure 3.2(a) and (b) Errors in Wedepohl's and

Stmlycn'» Formulae for a Solid Conductor.

- 36 -

To verify that the displacement current is indeed negligible, a modified subroutine

TUBEC was written which takes displacement currents into account. Within the accuracy

given in Table 3.1. TUBE and TUBEC produced identical mults.

3.3 Internal Impedance of a Tubular Conductor;

Figure S.S • Cross Section of a Tubular Conductor

There are three impedances associated with a tubular conductor of the type shown in Fig­

ure 3.3:

1) Internal impedance Za, which gives the voltage drop on the inner surface, when a unit

current returns through a conductor inside the tube.

2) Internal impedance Zt, which gives the voltage drop on the outer surface when a unit

current returns through a conductor outside the tube.

3) Mutual impedance Zai of the tubular conductor which gives the voltage drop on the outer

surface when a unit current returns through a conductor inside the tube or vice versa.

The formulae for Z^,Zb and Z,t, originally developed by Schelkunoff, are given in equation (3.11.

These formulae are given in terms of modified Bessel functions, and are obviously not suitable

for hand calculations. Schelkunoff therefore approximated these exact formulae by replacing

the modified Bessel functions J0,IX,K0 and K", by their asymptotic expressions and performing

the necessary division as far as the second term.

Schelkunoff's approximations are as follows [6]:

Zt = -^cotb(m(6-o)) - -^—{^ + ( 3 8 a )

Z i = | ^ c o t h ( m ( 6 - a ) ) + ^ ( 3 / a + l/6} (3.8b)

- 37 -

Zah = ^|LcoBeft(m(6--c)) (3. 8 c)

Wedepohl and Wilcox, give a similar approximation [22] but with a different approarh. The magnetic intensity H and the electric current density / in a tubular conductor can be related by the equations:

f + f-:' <»;»») = m 2 / / (3.9b)

If the tube is thin compared with its mean radius, equation (3.9a) can be written as:

dH 2H ,

Equation (3.9b) and (3.10) lead to a second-order differential equation in H. Now, solving for H from this second-order differential equation and following the same procedure as given in Appendix A for the exact formulae, we obtain the following equations:

Z t = ^ c o i H m { b . a ) ) - - ^ ( 3 , l a )

Z t = ^ c o t H m ( b - a ) ) + ^ ^ { 3.1, b )

Z < » = -7~T C o s e A ( m(6 - a ) ) (3.11c) 7t(a + 0)

There is another approximation derived by Bianchi [23]. If the difference between the radii b and a is very much less than either of the radius o and 6, or in other words, if (6 — a)«a,b then the impedances can be expressed as

Z ° = Z> = / ™ coth(m(6-a)) (3.12a)

Z°> = - 7^«>seA(m(6-a)) • (3.12b)

The impedance Zti obtained by Bianchi in equation (3.12b) is the same as that obtained by

Wedepohl in equation (3.11c).

Using the exact formula (subroutine TUBE) Wedepohl's approximate formula, Schelkunoff's approximate formula and Bianchi's approximate formula, the values of resistance and inductance terms of the impedances ZtZt and Z 4 t were obtained for a typical tubular con­ductor. Table 3.2 shows the resistance and inductance values as a function of frequency for the internal impedance Za. Figures 3.4(a) and (b) show the resistance and inductance for the fre­quency range 0.01 Hz to 10MHz. To highlight the differences in the results, the errors in the

- 38 -

Table S.2

Internal Impedance Zt of i Tubular Conductor with

Current Returning Inside (p = 2 . 1 \0~7Dm,a = 0 . 0 3 8 5 m , 6 = 0 . 0 4 1 3 m )

FREQUENCY TUBE WEDEPOHL SCHELKUNOFF BIANCHI (Hz)

RES s I STANCE ($2/k .m)

.01 0. 299163 0.299163 0.288640 0.299163 . 1 0. 299163 0.299163 0.288640 f 0.299163 1 0. 299163 0.299163 0.288640: 0.299163

10 0. 299163 0.299163 0.2886401 0.299163 100 0. 299169 0.299169 0.288646 0.299169

1000 0. 299761 0.299762 0.289224 0.299741 10000 0. 354424 0.354480 0.343956 0.352539

100000 1. 18036 1.18068 1.17015 1 .14975 1000000 3. 75275 3.75312 3.74259 3.63192

10000000 11 .8915 11.8919 11.8813 11.4852

INI 5UCTANCE ( MH/ 'km)

.01 4. 84605 4.84848 4.84848 4.67836 . 1 4. 84605 4.84848 4.84848 4.67836 1 4. 84605 4.84848 4.84848 4.67836

10 4. 84605 4.84848 : 4.84848 4.67836 100 4. 84603 4.84846 4.84846 4.67834

1000 4. 84339 4.84581 4.84581 4.67578 10000 4. 60048 4.60256 1 4.60256 4.44106

100000 1 . 89284 1.89296 1.89296 1.82654 1000000 0. 59905 0.59906 0.59906 0.57804

10000000 0. 18944 0.18944 0.18944 0.18279

approximate formulae are plotted in Figure 3.5. From these figures and the table it can be seen that

1. Wedepohrs formula has almost no error up to a frequency of 1MHz for both the resis­tance and inductance.

2. Schelkunoff's approximation has an error of 2-4% up to a frequency of 10kHz in the resis­tive part. The inductance value obtained by SchelkunofTs approximation is the same as that obtained by Wedepohl's approximation.

- 39 -

10-

v o c _p

or

TUBE WEpEPOHL SCHELKUNOFF BIANCHI

O . H

10" 10-

10" 10' 102 103 10' 105 10' 10

E \ x j t aj i -o c _o "o T> C

TUBE WEDEPOHL SCHELKUNOFF BIANCHI

0.1+-10" 10" 10' 10J 103

Frequency [Hz] 10* 10' 10'

Figure S.4(a),(b) • Impedance Z, of a Tubular Conductor (with Internal Return) as a Function of Frequency.

10

- 40 -

^ 2

o " 0 tt> o c (0

c o " t o tu _-> cc z

WEDEPOHL

SCHELKUNOFF

B I A N C H I

\ . - ' "

\

\ / ' A

10~2 10"' 1 101 102 10J 104 10s 106 107

e£ 2

o k_ k_

U l 0 tv> o c <0

u "D

c - 2

WEDEPOHL

SCHELKUNOFF

B I A N C H I

I . M i l l , , IHj I 1,1,11V ! I l l l l l l , I . I I I I t T , I I I M i l l , I I l l l l l l , I I I I . I l l , I l l l l l l f

10"2 10"1 1 10' 102 105 10" 105 10s 107

Frequency [Hz]

Figure S.5 -Errors in Wedepohl'*, Sehelkunoff'e and Bianchi'a Formulae

forZt.

- 41 -

3. Bianchi's approximation formulae is good for frequencies less that 10 kHz in the case of

resistance, where the error is almost zero, but beyond that frequency the error increases.

In the inductance the error is around 3% for the whole frequency range.

The cpu time for the routines were found to be

TUBE 0.059 ms

Wedepohl 0.037 ms

Schelkunoff 0.036 ms

Bianchi 0.033 ms

Similar comparisons were made for the mutual impedance Zab and the impedance Zt.

Table 3.3 gives the values of resistance and inductance of the impedance term Z..^ obtained

from routine TUBE, from Wedepohl's approximation formula and from Schelkunoff's approxi­

mation formula at different frequencies. Since the approximations proposed by Wedepohl and

Bianchi are identical, only Wedepohl's approximation was considered.

The errors in the approximate formulae are shown in Figure 3.6. It can be seen from Fig­

ure 3.6 that the errors in both Wedepohl's and Schelkunoff's approximations are less than 0.5^ up to a frequency of 1MHz. Wedepohl's approximation is closer to the exact formula.

The cpu times were found to be:

TUBE 0.056 ms

Wedepohl/Bianchi 0.038 ms

Schelkunoff 0.037 ms

In a similar manner, the resistance and inductance components of Zt were calculated.

Table 3.4 shows the values of these parameters using the exact formula and various approxi­

mations at different frequencies.

Figures 3.7(a) and (b) show the resistance and inductance as a function of frequency for

the range 0.01 Hz to 10MHz. The errors in the approximations are plotted in Figure 3.8. We

note the following for the impedance Z 4

- 42 -

0.4

0.1-

ui CP o c CD

- - o . H T J C

— 0.3 f 111 (Mill I I I l l l l l ,

W E D E P O H L

S C H E L K U N O F F

\

TT!!I, 1 1 l l l T H t ' T 1 MTIir, I I 1 l l l l l l ' 1

\2 4/r>3 i i inn: - ' i i mill i linn

10~2 10" 1 10' 102 10 s 10" 10* 106 107

Frequency [Hz]

Figure S.6

Errors in Wedepohl's and Sehetkunoff'e Formulae for Ztl

- 43 -

Table S.S Mutual Impedance (Z e 6, of a Tubular Conductor

(p = 2.1 10" 7/?m,o = 0.0385m ,6 = 0.0413m)

FREQUENCY (Hz)

TUBE WEDEPOHL SCHELKUNOFF

RESISTANC :E (8/km)

.01 . 1 1

10 100

1 ,000 10,000 100,000

1,000,000

0.29916343 0.29916343 0.29916343 0.29916338 0.29915838 0.29865873 0.25299441 -0.06964902 0.00001930

0.29916343 0.29916343 0.29916343 0.29916338 0.29915838 0.29865846 0.25297170

-0.06962398 0.00001929

0.29934775 0.29934776 0.29934776 0.29934771 0.29934270 0.29884247 0.25312757 -0.06966688 0.00001930

INDUCTANC 3E (uH/km)

.01 . 1 1 10 100

1,000 10,000 100,000

1 ,000,000

-2.3386052 -2.3386052 -2.3386052 -2.3386052 -2.3385803 -2.3361094 -2.1095191 -0.0097380 0.0000082

-2.3391813 -2.3391813 -2.3391813 -2.3391810 -2.3391563 -2.3366832 -2.1099033 -0.0097146 0.0000082

-2.3406226 -2.3406226 -2.3406226 -2.3406223 -2.3405975 -2.3381230 -2.1112033 -0.0097206 0.0000082

T h e e r r o r s i n W e d e p o h l ' s a p p r o x i m a t i o n a re p r a c t i c a l l y n e g l i g i b l e f o r b o t h r e s i s t a n c e a n d

i n d u c t a n c e u p t o a f r e q u e n c y o f 10MHz. T h e e r r o r s i n S c h e l k u n o f f ' s a p p r o x i m a t i o n a r e a r o u n d 3.5% u p t o a f r e q u e n c y o f 10kHz a n d d e c r e a s e s f o r h i g h e r f r e q u e n c i e s . S c h e l k u n o f f ' s a p p r o x i m a t i o n g i v e s t h e s a m e v a l u e s

as W e d e p o h l ' s a p p r o x i m a t i o n f o r t h e i n d u c t a n c e t e r m .

T h e e r r o r s i n B i a n c h i ' s a p p r o x i m a t i o n a r e n e g l i g i b l e f o r f r e q u e n c i e s u p t o 1kHz bu t

i n c r e a s e t h e r e a f t e r f o r t h e r e s i s t a n c e t e r m . T h e e r r o r i n t h e i n d u c t a n c e is 3.8% f o r t h e

w h o l e f r e q u e n c y r a n g e .

- 44 -

Table S-4

Internal Impedance Zk of a Tubular Conductor

tvith Current Returning Ouleide

(p = 2.1 10~ 7/?m,a = 0.0385m ,6 = 0.0413m)

FREQUENCY TUBE WEDEPOHL SCHELKUNOFF BIANCHI (Hr)

RE< 51 STANCE ( j B / J ;m)

.01 0. 299163 0.299163 0. 309686 0. 299163 .1 0. 299163 0.299163 0. 309686 0. 299163 1 0. 299163 j 0.299163 0. 309686 0. 299163

10 0. 299163 0.299163 0. 309686 0. 299163 100 0. 299169 0.299169 0. 309691 0. 299169

1000 0. 299720 0.299721 0. 310243 0. 299741 10000 0. 350679 0.350729 0. 361252 0. 352539

100000 1. 120643 1.120912 ; i . 131444 ; T .

149755 1000000 3. 518632 3.518953 3. 529473 • 3. 631926

10000000 11 .10564 11 .10605 11 .11652 1 1 .48527

INI HJCTANCE (uH/ 'km)

.01 51759: 4.51977 i *• 51977 I 4. 67836 . 1 : 4 . 51759 j 4 . 5 1 9 7 7 i 4. 51977 i 4. 67836 1 4 . 51759 1 4 . 5 1 9 7 7 ! 4. 51977 ! 4. 67836

10 4 . 51759 4 . 5 1 9 7 7 4 . 51977 4. 67836 100 4 . 51756 4 . 5 1 9 7 5 4. 51975 4. 67834

1000 4 . 51510 4.51728 4. 51728 4. 67578 10000 4 . 28865 4 . 2 9 0 5 2 4 . 29052 4. 441 06

100000 1 . 76453 1 .76463 1. 76463 1 . 82655 1000000 0. 55844 0.55844 0. 55844 0. 57804

10000000 0. 17660 0.17660 0. 17660 0. 18280

- 45 -

°icr2 io"' 1 io' 101 103 io 4 io 5 10s io 7

Frequency [Hz]

Figure S.7(a),(b) - Impedance Zb of a Tubular

Conductor (with External Return) as a Function of Frequency.

- 46 -

2-

O k-UJ 0 <o o c to

to _•>

s /

V /\

-s

/

V /\

WEDEPOHL

SCHELKUNOFF

B I A N C H I

o fc— fc—

UJ 0

a> u c -to u 3

TJ c -2

10~ 2 1 0 " ' 1 10 ' 1 0 2 1 0 S 1 0 " 10* 10* 1 0 7

WEDEPOHL

SCHELKUNOFF

B I A N C H I

— A ] II. i i i . i II,i| i i i . nn, • J • i in., .i i . i . i i , i . , i mi, III nm, .MI mil i i 11 in 1 0 ~ 2 1 0 " 1 1 1 0 ' 1 0 2 1 0 S 1 0 4 1 0 S 10* 1 0 7

Frequency [Hz]

Figure S.8 - Errors in Wedepohl's Schelkunoff's

and Bianchi's Formulae for Zt

- 47 -

The cpu times were found to be:

TUBE 0.058 ms

Wedepohl 0.038 ms

Schelkunoff 0.037 ms

Bianchi 0.033 ms

3.4 Conclusion

The exact or classical formulae for finding the internal impedances for a tubular conduc­tor were given by Schelkunoff. Since these formulae were not suitable for hand calculation pur­poses, approximations for these classical formulae were developed by many authors, including Schelkunoff himself. In this chapter, the accuracy and the cpu time taken by these approxima­tions were compared with the classical formulae for the tubular conductor of a typical cable.

The displacement current term is neglected in subroutine TUBE and in Ametani's cable constant routine, which use the exact formulae. Though not discussed in detail, a modified subroutine TUBEC was developed which takes the displacement current into account. In all cases, the results from TUBEC and TUBE were identical within the accuracy shown in the tables.

Subroutines were then written for the approximate formulae of Wedepohl, Schelkunoff and Bianchi, and the values obtained from these approximations were compared with the values obtained from the classical formula. Wedepohl's approximation formulae were indeed very good if the conductor thickness is small compared to its mean radius. The approximation proposed by Schelkunoff is similar to that proposed by Wedepohl except for the 2nd term. Schelkunoff approximated the modified Bessel functions in the exact formulae by the asymp­totic series and retained only two terms, which produces reasonably accurate results as long as the argument is larger than 8 [34]. In the example, the argument terms | ma | and | mb | did not reach the value 8 up to frequencies of 2kHz. Hence, approximations based on the asymp­totic series would obviously produce errors at low frequencies.

Bianchi's approximation is only good at low frequencies less than 1kHz, for the resistance and has an acceptable error of 3-4% in the inductance up to frequencies of 10MHz. It should also be noted that all the above approximations are valid only if the thickness of the conductor is smaller than the mean radius of the tubular conductor.

- 4 8 -

The routines for Wedepohl's approximation - WEDAP, Semlyen's approximation -SEMAP, Schelkunoff's approximation - SCHAP, Bianchi's approximation - BNCAP and TUBEC were written by the author. If these routines were written by a more experienced programmer, they might consume less cpu time than shown earlier. Even then, the cpu time for the exact formula (TUBE) would not be much more than that of the approximations. Therefore, the exact formula with subroutine TUBE is recommended for computer solution. For hand calcula­tion or for calculations with electronic calculators, Wedepohl's formulae are recommended.

- 49 -

4. EARTH RETURN IMPEDANCE The self and mutual impedance of conductors with earth return are of importance in stu­

dies of inductive interference in communication circuits from nearby overhead lines or under­ground cables. Also they are important in the calculation of voltages in power lines or com­munication circuits due to lightning surges or other transients [14]. Generally, the earth acts as a potential return path for currents in the underground, aerial or submarine cables. The values of cable constants therefore depend on the earth return impedances. In practical situa­tions, the earth's electrical characteristics such as resistivity, permeability and permittivity are not constant. However, simulation results came reasonably close to fied test results if a homo­geneous earth is assumed. The equations for self and mutual impedances are therefore developed with that assumption.

The impedances are obtained from the axial electric field strength in the earth due to the return current in the ground, which in turn, can be obtained from Maxwell's equations. If a cable is assumed buried in an infinite earth, (where the depth of penetration of the return current in the ground is smaller than the depth of burial, or, in other words, the distribution of return current is circularly symmetrical) the electric field strength can be easily derived, because only the earth medium must be considered in Maxwell's equations (Appendix C). On the other hand, if the earth is treated as semi-infinite, (where the depth of penetration of the return current is larger than the depth of burial so that the depth of penetration of the return current is not circularly symmetrical) the problem of finding the axial electric field strength in the earth is quite complex, because both air and earth media must be considered in Maxwell's equations (Appendix D). The solutions for the electric field strengths, for both infinite and semi-infinite earth are derived, assuming first that the conductor is a filament with negligible radius. In the case where the return current in the ground is circularly symmetric, exact equa­tions are still easy to derive for cables of finite radius. It is quite difficult, however, to extend the equations for the filament conductor to a conductor of finite radius in the case where the earth return current distribution is not circularly symmetric. In this chapter, we discuss the conditions under which these equations derived for the filament conductor can be extended to a conductor of finite radius. Furthermore, the effect of neglecting the displacement current term is discussed as well.

Ametani's Cable Constant routine in EMTP uses different formulae for these impedances. His approach, as well as the approximations proposed by Wedepohl and Semylen are discussed and compared with the exact equations. Another impedance of interest is the mutual impedance between a buried conductor and an overhead conductor. This topic is also covered here.

- 50 -

4.1 Earth Return Impedance of Insulated Conductor

The simplest underground cable consists of a conductor laid at depth d below the surface of the ground with insulation around it which forms a concentric dielectric cylinder of external radius a. The earth then forms the return path. The ground return self impedance Zti is defined as the ratio of the axial electric field strength at the external surface of the insulation to the current flowing in the cable. The earth return mutual impedance between the loops of two buried, insulated conductors is defined as the ratio of the axial electric field strength at the external surface of the insulation to the current flowing in the other conductor and vice versa.

As a preliminary step, the self and mutual impedances will be first found on the assump­tion that the cables are buried in an earth which is homogeneous and infinite in extent. Clearly, this situation does not arise in practical applications, although it is a reasonable approximation if the cable is buried at great depth, or if it is at modest depth but the frequen­cies are so high that the return current will flow very close to the cable. Furthermore, this treatment will be found useful in justifying the simplifying assumption 2 in section 2.1 of Chapter 2, and will also be helpful in interpreting the results for the more general case of earth return impedance in a homogeneous semi-infinite earth.

4.2 E a r t h Return Impedance in a Homogeneous Infinite E a r t h

The calculation of earth return impedance in an infinite earth is relatively easy since there is no surface discontinuity as in the semi-infinite case (earth and air).

With the assumptions mentioned]in Chapter 2 (except for assumption 3), it is shown in Appendix C that the electric field strength at a radial distance r from a cable of insulation outer radius a carrying current / which returns through the earth can be written as:

E = . _ pml Ko(mr) ( 4 ,) ITXQ. A'j(mo)

where

p = resistivity of the earth

m = intrinsic propagation constant of the earth.

The earth return self impedance per unit length of the cable is obtained from equation (4.1) by substituting r = a together with the general relation E = —ZI. It also follows from Zb

of equation (3.1b), if we regard the earth as a tubular conductor with inside radius a whose outside radius b goes to infinity.

- 51 -

The mutual impedance between the cable and a filamentary insulated conductor at a radial separation R is obtained by substituting r = R. The mutual impedance between two cables with finite radii over their insulation is different from the case of a cable of finite radius and a filament conductor. It can, however, be deduced from the mutual impedance between the cable and a filamentary type conductor by invoking the law of reciprocity of mutual impedance [9].

We have already shown in equation (4.1) that the mutual impedance between a cable of

radius o and a filament conductor separated radially by a distance R is given by:

Z --pm K0(mR)

2na A'j(ma) (4.2)

Hence, from the law of reciprocity of mutual impedance, the electric field strength on the surface of the cable due to a current / in the filament can be written as:

E = -pmIK0(mR)

2za K t ( m a ) (1.3)

Now consider a cable of radius 6 carrying a current / as shown in Figure 4.1. The electric

field strength at a point P at a radial distance R is given by:

Earth

Figure 4-1 - Electric Field Strength al point P

pm I'K0(mR)

27rbKl(mb)

The same field strength will be experienced at point P, due to a filament conductor placed along the axis of the cable and carrying a current f, but now the equation will be:

pmfK^mR)

2nb(\/mb)

(as 6 - 0 , Ki(mb)-\/mb)

- 52 -

From equations (4.4) and (4.5) we obtain the value of f to be equal to f/mb Kx(mb).

Therefore, the electric field strength in the soil outside a cable of radius b carrying current / is indistinguishable from that of a current filament placed along the axis of the cable and carry-:

ing a current f/mbK^mb).

Hence, the electric field strength at the surface of the cable of radius o, which is at a radial distance R from a cable of radius b is found from equation (4.3) by substituting for I the value of f. Therefore, we obtain the field to be:

_ Pm*fK0(mR) .

27imaK1(ma)mbK1(mb) ' ' '

Hence, the mutual impedance between two cables of radii o and 6 respectively, buried in a homogeneous infinite earth is given by:

pm2K0(mR) - • (''•") 27tma Ki(ma)mb K}(mb)

Now we can deduce an interesting result. The series expansion of A'j(r) shows that as z-0, K1(x)~l/x. Therefore, for small values of \ma\ and |mi|, or in the limiting case of o,6-0 (filament conductors), the self and mutual impedances obtained from equation (4.2) and (4.7) will be given by:

Zs = -^Koima) ... (4.8a)

Zm = ̂ ~K0(mR) (4.8b)

It is interesting to know the error if the cable of finite radius is replaced by a filament conductor placed along its axis. For p = 10O— m (low earth resistivity), a = b — 7.5cm (large radii), and a separation of 30 cm, values which perhaps represent a worst case, the errors in the resistance and inductance from equation (4.8b), as compared with equation (4.7), is plotted in Figure 4.2.

From Figure 4.2 we see that the approximate formulae have an error of less than 29o, up to \ma\ =? |m6| = 0.1. This happens at a frequency of IMHz. For much lower values, the error is practically negligible. This result is important as it will be used in justifying the exten­sion of formulae for filament conductors to cables of finite radii.

4 . 3 E a r t h R e t u r n Impedance in a Homogeneous S e m i - I n f i n i t e E a r t h

The self and mutual impedances of cables buried in semi-infinite homogenous earth are deduced from the electric field strength in the ground due to a buried filament conductor.

- 53 -

w H

0) o c 1 0 1

—I-t o t o c u az

-3 10

25

15

cu o c CO o

T 3 C

-15

/ ' Y

/

10"s 10" 10~3

I I I! I I ! I 1 T T1!| •-' !

10"2 10"'

\

'1 - T TTTT i

10'

-25 -f 1 1 I 1 Mll| I 1 I I I M i l 1 U I I I I I H l | 1 1 I 1 I Ml| 1 1 l l l l l l ,

10" 10" 10" 10" 10" 10- 10' ma

Figure 4-2 - Error in Replacing a Conductor of Finite Radius

by a Filament Conductor

- 54 -

The electric field strength in the ground due to a buried insulated filament carrying a

current which returns through the soil was first deduced by Pollaciek [l). In fact, he derived

formulae for four cases:

(1) The electric field strength in the air due to a current-carrying conductor in the air.

(2) The electric field strength in the earth due to a current-carrying conductor in the air.

(3) The electric field strength in the air due to a current-carrying conductor in the earth.

(4) The electric field strength in the earth due to a current-carrying conductor in the earth.

The mathematical derivation in all four cases become complicated by the plane of discon­

tinuity at the earth's surface. Pollaczek [l] does not discuss the derivations and only mentions

that they have been obtained through the reciprocity of Green's functions.

Recently Mullineux [10,11,12] obtained expressions for the fields produced in air and earth

due to an overhead conductor by using double integral transformation. This technique is

equally applicable to buried cable systems [22], and is used in Appendix D to obtain the four

types of fields.

Comparisons between filamentary type conductors and conductors of finite radii for the

infinite earth give every reason to expect that these formulae in Appendix D for filament con­

ductors will be accurate enough for cables of finite radii provided that the condition | ma | <0.1 is satisfied. Hence, for the case of a buried cable at depth h, the electric field strength in the

soil resulting from a net current I flowing in the cable is given by equation (D.32(c)) in Appen­

dix D as:

E__ — | e x p [ - ( o g + m2)\h-y\] - e X P [ - ( ^ + m2)\h + y\\

2(p- + m~)

+ r exp(-|ftlfyl VV+ttr) exp(jd>x )d<t> (4.9)

where . ' ; " .

x = horizontal distance between the filament and the point at which the field is being deter­

mined,

y = depth at which the field is being determined,

p = resistivity of the earth, and

m = intrinsic propagation constant of the earth.

The first integral term is identified as Jfc:0(mr?) - rTo(mZ)j [1,22]; where R = Vi" + (h-yf

and Z = V i s + (/i + y)2. The second integral part can be numerically evaluated [28] or

- 55 -

expanded into an infinite series [22]. The series expansion is in the form of modified Bessel

functions. Therefore, equation (4.9) can be written in the series expansion form as follows:

= -^~\KdmR) - K0(mZ) -f —K^mZ) + ̂ "*^Ki\mZ) In \ Z mZ

- } (* •+ ml)P-mt ~ ̂ F(mZ,\x |,/)) (4.10)

where

F(mZ,\x\,l)= f f c V l ^ T 2 ^==r\e-mZtdt (4.11) 1/2 I Vl-t2 J

Now the earth return self and mutual impedance terms may be extracted from equation (4.10) or from numerically integrating equation (4.9). The self impedance term is obtained by choos­ing the coordinates z and y to correspond to the location of the external surface of the cable and the mutual impedance by simply inserting the coordinates of the second cable axis .

The numerical integration of equation (4.9) is quite difficult as the solution is highly oscil­latory. For example. Figure 4.3 shows the solution for the resistance and inductance of the earth return self impedance at a frequency of 1MHz, for the cable discussed in Appendix A. For such cases, special numerical integration techniques have to be applied [28]. The numerical integration routine available in the UBC system library DCADRE [26], which uses a cautious adaptive Romberg extrapolation technique, has been used in obtaining the solution for the equation (4.9). The solution converges fast for the case when the two cables are buried at d i f ­ferent depths below the earth, but in the case when the cables are at the same depth or in the case of finding the earth return self impedance, the convergence is rather slow. To explain this phenomena, consider the first integral part on the right hand side of equation ( 4 . 9 ) , i.e..

e x p [-(*2 + m 2 ) |A- j , | ] - exp[-(cA2 + m2)\h + y\]

?(c*2 + mz) expO^x )</<•> ( 4 . 1 2 )

If h = y then the first exponential term within the closed brackets { } will become 1 and the integration now becomes

j l - e xp[-(cr+m 2 ) | / i+y | j (<p2+m2)\h+y\ Uxp{j<t>x)d<t>

- 56 -

10

6-T

o> o c JO

V) o>

- 2

-6

-10 10'

50

£

o o c o

30-

10

o ~ 1 0

D

- 3 0

- 5 0 10'

A

V

1 "' '^f 1 T I 1 1 I 1 1 1 1 | 10 2 10 3

I I I i l l

\ \

\ /

7— ••T-",,T "I m I I I 1 I I I I i

10* 1CV

Interval

104

i i i i i i 104

Figure 4-8 - Solution of Real and Imaginary Part of Equation (4-9),

for a Frequency of 1MHz .

- 57 -

Even though the exponential term within the closed brackets { } approaches zero very fast, we are left with

which causes the slow convergence.

On the other hand, if h^y then both the first and second exponential terms within the brackets { } of equation (4.12) will approach zero and hence the convergence is faster.

Due to the slower convergence, the cpu time taken to compute the self impedance or the mutual impedance in case of two cables buried at the same depth is relatively high. For the self impedance, the computer cost varied between $.50 and $1.00, for one particular frequency. However, the results obtained by applying the DCADRE integration routine to equation (4.9) and the results obtained from equation (4.10) are almost identical, as shown in Table 4.1 for the mutual impedance between two cables with the following data:

depth of cable 1, y = 0.75m

depth of cable 2, h = 0.76m

radial distance between the two cables = 0.5m

earth resistivity peyAh = 100/7— m.

Hence, from Table 4.1 we can see that the equation (4.10), which is the series expansion for the classical equation (4.9) is accurate enough for practical purposes. Therefore, equation (4.10) is taken as the standard equation for finding the self and mutual impedances of buried cables in a semi-infinite earth, and the results obtained by the other formulae proposed by Semlyen [24], Ametani [27], and Wedepohl [22] are compared with respect to it.

4.4 Formulae Used by Ametani, Wedepohl and Semlyen.

Ametani's approach is implemented in BPA's Cable Constant routine of EMTP, and is based on Carson's formulae for overhead conductors. The self and mutual impedances of over­head conductors with earth return effects can be derived from equation ((D.32(b)) Appendix C) together with the relation E — — ZI.

In the case of overhead conductor at a height h from the ground, the electric field strength in air at a point (whose height is y and which is at horizontal distance x from the con­ductor) due to current / flowing in the conductor is given by

- 58 -

Table 4.1 Solution of Pollaczek'o Equation by Numerical

Integration and Using In finite Series

FREQUENCY (Hz)

NUMERICAL INTEGRATION

EQUATION 4.10

RESISTANCE (8/kr n)

.01 . 1 1 10 100

1 ,000 10,000 100,000

1,000,000 10,000,000

0.00000986985 0.00009870394 0.00098721243 0.00987746990 0.09894307000 0.99462669000 -10.0999510000 105.026550000 1119.21050000 10416.7130000

0.00000986986 0.00009870399 0.00098721145 0.00987752230 0.09894471000 0.99467682000 10.1014090000 105.062940000 1119.73660000 10410.0480000

REACTANCE (fi/km

.01 . 1 1 10 100

1 ,000 10,000 100,000

1,000,000 10,000,000

0.00014814024 0.00133672137 0.01192043120 0.10477298900 0.90245276590 7.57239400000 61.8788630000 461.037952000 3018.40132100 13049.3294800

0.00014814024 0.00133672130 0.01192028400 0.10472980000

I 0.90245110000 ! 7.57234170000 . 61.8625200000 460.988500000 3017.08610000 13031.2620000

= — 2x

ln(Z/R) + J (4.15)

fhere Z - V i 2 + (A+y ) z ,

- 59 -

m = intrinsic propagation constant of the earth.

The integral part of equation (4.15) can be further simplified to

o U | + \ U 2 +• m2 (4.16)

Equation (4.16) is widely known as Carson's formula. Strictly speaking, this formula is only valid for the case of overhead conductors. Ametani used this correction term in finding the earth return self and mutual impedances of buried conductors, instead of the second integral term used in equation (4.9).

Carson's formula given by equation (4.16) can be numerically integrated [28] or can be expanded into an infinite series [5], in terms of r = | mZ \. Ametani chooses the latter approach. His procedure uses 2 different series, one when r S 5 and the other one when r>5. Recently, Shirmohamadi of Ontario Hydro [28] and L. Marti at UBC discovered that the error between the numerical evaluation and the asymptotic expansion is as high as 5-8% as shown in Figure 4.4, for the values of r between 5-10, depending on the value of 8.

) 0 0 0 4 0 0 0

r«CQUCNCT ( H i )

Figure 4-4 - Relative Error in the Evaluation of Carson's Formula

with an Asymptotic Expansion.

- 60 -

Shirmohamdi avoids this error by using Gauss-Legendre quadrature technique for the direct evaluation of equation (4.13) in this region of r, while L. Marti [24] avoids these errors by extending both the asymptotic and infinite series, and by using a switchover criterion which depends on the geometry of the line (ie. on the value of 0).

The use of Carson's formula for underground systems will be reasonably accurate at low frequencies because the value of m 2 with the exponent term exp(—| h+y\ Vc4 2+m 2) in equation (4.9) is very small compared to the value of 4>z at low frequencies, and, therefore, can be ignored. At high frequencies, however, that term becomes quite significant and, therefore, can­not be ignored. For this reason, the resistance and inductance obtained by Ametani's method has an error in the order of 10% or more for frequencies above lKHz when compared with Pollaczek's equation, i.e., equation (4.9).

Wedepohl and Wilcox [22], who proposed the infinite series expansion form of equation (4.9) gave an approximation to the infinite series expansion (equation (4.10)) which is valid only if the condition |mZ| <0.25 is satisfied. Their closed-form approximation for the self and mutual earth return impedances are given by:

Z, = pm

2n

pm

2ir

.lnhHEl + 0 5 _ ± m h

2 3

•In— L + 0.5 ml 2 3

(4.17a)

(1.17b)

where

7 = Euler's constant,

h = depth of burial of the conductor,

I = sum of the depths of burial of the conductors,

R = V i 2 + (h-yf and

m = intrinsic propagation constant of the earth.

Semlyen and Wedepohl [24] developed another interesting formula for the self impedance of a cable of radius r in terms of complex depth which is nothing but 1/m, defined here as p.

Accordingly, the self impedance term is given by

Zs = " ^ M r + V'r) (4.18)

- 61 -

4.5 Effect of Displacement Current and Numerical Results

So far. the effect of displacement currents has been ignored. As shown in Chapter 2 .

there is no noticeable error in the internal impedances of tubular conductor if displacement currents are ignored. Hut unlike in good conductors, the displacement currents in the earth arc noticeable, at least at high frequencies. The displacement current term can be easily incor­porated in equation (4-10) by snbsti :ing Vm"-t> J/i( for in. Table 4.2 shows the values of resistance and inclu;'. :nce with and without the displacement currents.

Tabic 4.2 Earth Return Self Impedance with and without

Displacement Current Term

FREQUENCY (Hz)

WITH DISPLACEMENT

CURRENT

WITHOUT DISPLACEMENT

CURRENT

RESISTANCE (8/ki

.01 . 1

1 10

1 00 1 000

10 000 100 000

1 oob ooo 10 000 000

0 . 0 0 0 0 0 9 8 6 9 8 5 0 . 0 0 0 0 9 8 7 0 3 9 3 0 . 0 0 0 9 8 7 2 0 9 8 6 0 . 0 0 9 8 7 7 4 7 6 7 1 0 . 0 9 8 9 4 3 7 1 2 3 0 0 . 9 9 4 6 8 7 0 0 0 0 4 1 0 . 1 0 5 6 6 3 3 5 0 4 1 0 5 . 5 7 2 6 4 6 5 6 8 1 1 7 2 . 7 4 9 2 5 5 8 8 1 5 1 7 0 . 9 6 0 5 5 3 9

0 . 0 0 0 0 0 9 8 6 9 8 6 0 . 0 0 0 0 9 8 7 0 3 9 3 0 . 0 0 0 9 8 7 2 0 9 8 2 0 . 0 0 9 8 7 7 4 7 3 4 0 0 . 0 9 8 9 4 3 3 9 5 9 5 0 . 9 9 4 6 5 5 3 7 1 8 7 1 0 . 1 0 2 4 6 0 2 9 2 6 1 0 5 . 2 3 9 9 0 9 5 6 4 1 1 3 6 . 3 5 1 8 0 5 4 6 1 1 5 9 3 . 2 5 6 9 4 5 0

INDUCTANCE ("H/ nm)

o

—

oo

­o

oo

­OOOOO­

OOOOOO—

•

o

OOOOOOO — —

— 2 . 8 2 4 7 8 6 9 2 8 4 4

2 . 5 9 4 5 1 9 8 3 7 3 7 2 . 3 6 4 2 3 4 1 5 0 9 9 2 . 1 3 3 8 8 9 7 4 4 4 6 1 . 9 0 3 3 5 9 8 2 7 5 6 1 . 6 7 2 2 4 5 2 5 3 7 4 1 . 4 3 9 3 0 2 4 4 3 3 3 1 . 2 0 0 8 0 3 8 4 1 19 0 . 9 4 7 1 2 6 1 4 3 5 7 0 . 6 4 9 8 0 3 4 1 2 1 4

2 . 8 2 4 7 8 6 8 9 7 1 2 2 . 5 9 4 5 1 9 7 9 2 9 7 2 . 3 6 4 2 3 4 1 1 6 6 1 2 . 1 3 3 8 8 9 7 1 3 3 2 1 . 9 0 3 3 5 9 7 9 9 1 6 1 . 6 7 2 2 4 5 2 0 0 2 0 1 . 4 3 9 3 0 1 5 6 5 1 4 1 . 2 0 0 7 8 3 2 9 1 3 9 0 . 9 4 6 9 4 2 4 2 9 6 3 0 . 6 6 8 2 2 9 4 1 7 1 8

- 62 -

The errors in the answers obtained by neglecting the displacment current arc shown in

Figure 4.5. The error in the resistance is less than 3% up to a frequency of 1MHz and

increases to 209o in the frequency range 1MHz- lOMHr.

o i_ k. UJ

CD o c to

CO CD

CC

- 2 5

- 5 -

-15

i i i IIIMI— n i ] — i i i i n n , — i i i M i n i — i i i inn, 1 i i mii| nil 1 i i mill i i i mil! 10~ 2 1 0 " 1 10' 10 2 10 J 10 4 10 5 10 6 io 7

Frequency [Hz]

Figure 4-5 - Error in the Earth Return Self Impedance

if the Displacement Current is Ignored

- 63 -

Hence, we can neglect the effect of displacement current terms up to a frequency of 1MHz which is well within the limits of practical interest.

The earth return self impedance is compared next for the following approaches, with the displacement current term neglected:

1. Pollaczck's original formula,

2. Wedepohl's approximations, -

3. Ametani's approach,

4. Semlyen's approximation.

The value of resistance and inductance at different frequencies are tabulated in Table 4.3. Figures 4.6(a) and 4.6(b) illustrate the variation of resistance and inductance in the frequency range 0.001 Hz to 10MHz. The errors are plotted in Figure 4.7 for the same frequency range. Wedepohl's approximation gives an error of less than 1% up to a frequency of 100kHz, for the resistive part, thereafter it increases steadily. It is around 2 5 % at a frequency 1MHz. The error in the inductive part is almost zero up to a frequency of 1MHz. The reason for the noticeable error in the resistance, beyond a frequency of 100kHz, is that the condition | mZ | <0.25 is violated. Semlyen's approximation is good at low frequencies for the resistive part but at high frequencies the error is higher. It has an error of around 4% in the case of inductive part over the whole frequency range. As mentioned earlier, the error i i i Ametani's

procedure is not significant at low frequencies, but increases from 2 % to 20% iu (.-be frequency range 10kHz to 1MHz as shown in Figure 4.7.

Three routines were developed for the calculation of earth return self and mutual impedances with Pollaczek's original formula. The part which is .difficult to evaluate is the integral term F(mZ,\x\,l) (equation (4.11)) in equation (4.10).. This part can either be expanded into a series with a suitable number of terms and each term can then be integrated, or a suitable library subroutine for numerical integration can be used. Routine SEARTH developed by the author uses the first approach by considering 15 terms. Routine LEARTH developed by Luis Marti uses the UBC library subroutine DCADRE for the evaluation of the integral. One more routine, namely CEARTH was developed by the author, which can take the displacement current into account. This routine also uses the UBC library subroutine DCA­DRE to evaluate the function discussed earlier. Routines SEARTH and LEARTH give identical answers but differ in the cpu time. The former one takes 3.4 ms while LEARTH takes 2.9 ms for the evaluation of resistance and inductance at a particular frequency. Routine CEARTH takes 23.0 ms for the same evaluation. The routine for Ametani's approach takes 5.30 ms, while the routines for Wedepohl's and Semlyen's approximations take 0.30 ms and 0.24 ms of cpu time, respectively.

Table 4.S

Earth Return Self Impedance

as a Function Frequency

F R E Q U E N C Y P O L L A C Z E K W E D E P O H L A M E T A N I S E M L Y E N (Hz)

R E ; > I S T A N C E (£2/1 cm)

. 0 1 0 . 0 0 9 8 8 2 5 0 . 0 0 9 8 8 2 5 0 . 0 0 9 8 7 7 5 0 . 0 0 9 8 8 2 5

. 0 1 0 . 0 0 0 0 0 9 9 0 . 0 0 0 0 0 9 9 0 . 0 0 0 0 0 9 9 0 . 0 0 0 0 0 9 9 . 1 0 . 0 0 0 0 9 8 7 0 . 0 0 0 0 9 8 7 0 . 0 0 0 0 9 8 7 0 . 0 0 0 0 9 8 7 1 0 . 0 0 0 9 8 7 2 0 . 0 0 0 9 8 7 2 0 . 0 0 0 9 8 6 7 0 . 0 0 0 9 8 7 0

1 0 0 . 0 0 9 8 7 7 5 0 . 0 0 9 8 7 7 5 0 . 0 0 9 8 6 1 8 0 . 0 0 9 8 6 9 2 1 0 0 0 . 0 9 8 9 4 3 4 0 . 0 9 8 9 4 5 7 0 . 0 9 8 4 5 1 9 0 . 0 9 8 6 8 4 0

1 0 0 0 0 . 9 9 4 6 5 5 4 0 . 9 9 4 8 5 6 1 0 . 9 7 9 5 2 6 3 0 . 9 8 6 5 7 8 4 1 0 0 0 0 1 0 . 1 0 2 4 6 0 1 0 . 1 1 9 2 8 8 9 . 6 5 6 4 6 8 0 9 . 8 5 7 5 3 1 3

1 0 0 0 0 0 1 0 5 . 2 3 9 9 1 1 0 6 . 5 9 1 7 2 9 3 . 4 9 8 2 7 0 9 8 . 3 1 5 0 5 3 1 0 0 0 0 0 0 1 1 3 6 . 3 5 1 8 1 2 3 6 . 6 4 3 8 9 1 3 . 0 3 2 1 4 9 7 4 . 9 9 1 2 3

1 0 0 0 0 0 0 0 1 1 5 9 3 . 2 5 7 1 7 7 6 5 . 2 8 8 1 0 8 2 6 . 0 5 8 9 4 9 8 . 8 3 9 4

I N I X J C T A N C E (mHy 'km)

. 0 1 2 . 8 2 4 7 8 6 9 2 . 8 2 4 7 8 3 8 2 . 8 2 4 7 9 1 7 2 . 7 0 1 6 0 4 8 . 1 2 . 5 9 4 5 1 9 8 2 . 5 9 4 5 1 6 7 2 . 5 9 4 5 4 1 8 2 . 4 7 1 3 4 6 7 1 2 . 3 6 4 2 3 4 1 2 . 3 6 4 2 3 1 0 2 . 3 6 4 3 1 0 5 2 . 2 4 1 0 8 9 5

1 0 2 . 1 3 3 8 8 9 7 2 . 1 3 3 8 8 6 6 , 2 . 1 3 4 1 3 7 8 2 . 0 1 0 8 3 5 2 1 0 0 1 . 9 0 3 3 5 9 8 1 . 9 0 3 3 5 6 4 1 . 9 0 4 1 5 0 1 1 . 7 8 0 5 8 9 8

1 0 0 0 1 . 6 7 2 2 4 5 2 1 . 6 7 2 2 3 8 6 1 . 6 7 4 7 4 1 4 1 . 5 5 0 3 7 2 9 1 0 0 0 0 1 . 4 3 9 3 0 1 6 1 . 4 3 9 2 6 2 9 1 . 4 4 7 1 0 6 4 1 . 3 2 0 2 4 5 9

1 0 0 0 0 0 1 . 2 0 0 7 8 3 3 1 . 2 0 0 4 1 1 8 1 . 2 2 4 5 1 5 7 1 . 0 9 0 4 0 3 2 1 0 0 0 0 0 0 0 . 9 4 6 9 4 2 4 0 . 9 4 2 9 8 1 3 1 . 0 1 2 6 2 2 8 0 . 8 6 1 4 5 9 7

1 0 0 0 0 0 0 0 0 . 6 6 8 2 2 9 4 0 . 6 2 6 7 9 7 5 0 . 7 9 6 2 6 8 9 0 . 6 3 5 3 5 6 5

4.6 Cables Burled at Depth Greater than Depth of Penetration

If the depth of burial is greater than the earth return current's depth of penetration, or other words, if the distribution of return current is circularly symmetrical, then the cable c be considered to be buried in an infinite earth. In practice, this can arise in two situations:

1. Cables arc buried at large depths below the ground,

\

- 05 -

POLLACZEK

WEDEPOHL

AMETANI SEMLYEN

0_,j •• , , , , „ „ ....t r-̂ >..nr 1 0" J 10" 1 10' io2 10

Frequency [Hz] Figure 4.6(a),(b)

Earth Return Self Impedance

as a Function of Frequency

10' 105 10s 107

- 66 -

ui

25

15-

5-

o -C <o - 5 . CO in CD

or -15

WEDEPOHL

AMETANI SEMLYEN /

/

— 25 "1—1 i 1111111—1 1 11111 n 1 1 11 IIIII 1 1 1111111—1 1 1 . .1.11 1 1 . null—1 1 1 m i i | 1 1 1 i i u i | 1 1 1 m i r

10" 2 10"' 1 10' 10 2 10 J 10 4 10 5 10 6 10 7

25

15-

5-

LU CD o c - 5 ra o

T3 C

— -15

WEDEPOHL

AMETANI

SEMLYEN

>

— 2 5 ~f 1 1 T mrn r-i TTTTTTI 1 i 11 ini[ 1 i i itui] r - r r i - T T T T i r—r-rrrmj r i it ini] r i i rim\ r - r r m r t } 10~ 2 10"' 1. 10' 10 2 10 J 10 4 10 S 10 6 10 7

Frequency [Hz]

Figure 4- 7 Errors in Earth Return Self Impedance

- 6 7 -

2. C a b l e s are b u r i e d a t n o r m a l d e p t h s ( l - 2 m ) b u t a re u s e d a t h i g h f r e q u e n c i e s ( 1 0 0 k H z a n d

a b o v e ) .

In s u c h s i t u a t i o n s , t h e d e p t h o f p e n e t r a t i o n i n t h e e a r t h is g i v e n b y :

d = 5 0 3 . 3 \ / p e « u / / (4 .19)

( w h e r e d is in m , / is i n fi—m a n d / is i n H z )

a n d b e c o m e s s m a l l e r t h a n the d e p t h o f b u r i a l .

T h e s e c o n d p o s s i b i l i t y d o e s n o t a r ise n o r m a l l y i n p o w e r s y s t e m s t u d i e s . F o r a t y p i c a l

u n d e r g r o u n d t r a n s m i s s i o n s y s t e m , w i t h a n e a r t h r e s i s t i v i t y o f 10 J ? — m , a n d a b u r i a l d e p t h o f

l m , t h e f r e q u e n c y a t w h i c h the p e n e t r a t i o n o f t h e r e t u r n c u r r e n t in t h e e a r t h b e c o m e s less

t h a n l m is 3 M H z o r h i g h e r . In p o w e r s y s t e m s , o n e r a r e l y e n c o u n t e r s s u c h f r e q u e n c i e s . If s u c h

c a s e s d o a r i s e , h o w e v e r , the in f in i te e a r t h r e t u r n i m p e d a n c e f o r m u l a e g i v e n b y e q u a t i o n (4.2)

a n d (4.7) c o u l d be u s e d t o find the e a r t h r e t u r n i m p e d a n c e s .

It is i n t e r e s t i n g t o k n o w w h e t h e r e q u a t i o n (4 .10) f o r t h e s e m i - i n f i n i t e c a s e is s t i l l v a l i d if

the b u r i a l d e p t h is l a r g e . T h i s c a n be c h e c k e d as f o l l o w s :

If the e a r t h r e s i s t a n c e is a s s u m e d t o be 10 17— m, t h e n t h e d e p t h o f p e n e t r a t i o n g i v e n b y

e q u a t i o n (4.19) wi l l be less t h a n 5.5 m , f o r f r e q u e n c i e s 0.1 M H z a n d h i g h e r . H e n c e , i f a

c a b l e is b u r i e d at a d e p t h o f 5 . 5 m , t h e n t h e v a l u e s o b t a i n e d f o r e a r t h r e t u r n se l f a n d

m u t u a l i m p e d a n c e s , u s i n g t h e e q u a t i o n s (4 .2) o r (4.7), s h o u l d be t h e s a m e as t h o s e

o b t a i n e d u s i n g e q u a t i o n (4.10) f o r f r e q u e n c i e s a b o v e 0 . 1 M H z . F i g u r e 4.8 s h o w s t h e d i f f e r ­

e n c e in the v a l u e o f r e s i s t a n c e o b t a i n e d b y P o l l a c z e k ' s f o r m u l a a n d e q u a t i o n (4.2) f o r t h e

e a r t h r e t u r n se l f i m p e d a n c e in t h e f r e q u e n c y r a n g e 1 0 k H z t o 1 M H z . T h e d i f f e r e n c e

d e c r e a s e s f r o m 1 9 % t o less t h a n 1 % w h i l e t h e d e p t h o f p e n e t r a t i o n d e c r e a s e s f r o m 1 5 . 9 m

t o 1 .59 in . H e n c e , it a p p e a r s t h a t P o l l a c z e k ' s f o r m u l a is c o r r e c t e v e n a t l a r g e d e p t h s o f

b u r i a l e v e n t h o u g h it is b e t t e r t o use t h e e q u a t i o n s (4.2) a n d (4.7) f o r t h e c a s e o f i n f i n i t e

e a r t h [9,22].

S u b r o u t i n e T U B E has a n o p t i o n f o r finding t h e in f in i te e a r t h r e t u r n se l f i m p e d a n c e , b u t it

c a n n o t be u s e d for f i n d i n g the m u t u a l i m p e d a n c e . T h e r o u t i n e T U B E C d e v e l o p e d b y the

a u t h o r has o p t i o n s f o r finding b o t h se l f a n d m u t u a l i m p e d a n c e s , a n d c a n t a k e d i s p l a c e m e n t

c u r r e n t s i n t o a c c o u n t as we l l .

4.7 M u t u a l I m p e d a n c e B e t w e e n a C a b l e B u r i e d i n t h e E a r t h a n d a n O v e r h e a d L i n e

o r V i c e V e r s a

A n o t h e r i m p e d a n c e o f i n t e r e s t t o p o w e r e n g i n e e r s as we l l as t o c o m m u n i c a t i o n e n g i n e e r s

is t h e m u t u a l i m p e d a n c e b e t w e e n a n u n d e r g r o u n d c a b l e a n d a n o v e r h e a d l ine o r v i c e v e r s a .

T h e e l e c t r i c field s t r e n g t h s in a i r d u e t o a c u r r e n t c a r r y i n g c o n d u c t o r b u r i e d in the e a r t h

- 68 -

c u u c CD

CU u c CO

to CO cc

2 0

1 2 -

"* 4 -

-4

- 1 2

- 2 0 +

104 10s

Frquency [Hz]

- i — i — • I I I

106

Figure 4.8

Differences in Resistance Values of Semi-Infinite

and In finite Earth Return Formulae

or the field Zs_+ in earth due to a current-carrying conductor in the air is given by Equations (D.32(b)) and (D.32(d)), respectively, in Appendix D. In both cases the mutual impedance is given by:

0 e x p j - / i | c*| -dV<t>2+ m 2j exp(j>| x | )d<f>

| c6| + V(t>2+m2

•where

A = height of the conductor in air,

d = depth of burial of the buried conductor,

| i | = the horizontal distance between the conductors,

m = intrinsic propagation constant of the earth.

This integral can be evaluated in terms of infinite series in somewhat the same way as was done for E + + in [4,27] and for E__ in [22].

- 69 -

4.8 Conclusion

To summarize, the self and mutual impedances of conductors with earth return were derived for two situations, namely for ' •

1. Cables buried in infinite earth, and for

2. Cables buried in semi-infinite earth.

The impedances were obtained from the axial electric field strengths in the earth due to return currents in the ground. These electric field strengths were derived from Maxwell's equations, for filamentary type conductors of negligible radius. Since we were interested in cables of finite radius, the solutions for filamentary type conductors were extended to cables of finite radius.

The solutions for the earth return impedance with semi-infinite earth is in infinite integral form. Wedepohl [22] transformed this infinite integral equation into an equation con­sisting of Bessei functions. It was found that the values obtained from the numerical integra­tion of the infinite integral and from Wedepohl's transformation were very close. Ametani's approach for finding earth return impedances which is implemented in Cables Constants rou­tine in the BPA's EMTP and other approximations suitable for hand calculations were com­pared for typical cable data. Ametani's approach gave erroneous results at high frequencies due to an erroneous assumption. Wedepohl's approximation was found to give reasonably accurate answers and is well suited for hand calculations.

At the end of the chapter, the evaluation of mutual impedance between a buried conduc­tor and overhead conductor, and vice versa, is briefly discussed.

- 70 -

5. Laminated Tubular Conductors

In Chapter 3, formulae for internal impedances of homogeneous tubular conductors were derived. These formulae are used in this chapter to obtain the impedances of cables whose core and sheath are made up of laminated conductors of different materials. A practical appli­cation of this type of conductor was recently proposed by Harrington [32]. He suggested that the transient sheath voltage rise in a gas-insulated substation can be reduced by coating the conductor and sheath surfaces with high-permeability materials, thereby increasing the impedance of the surfaces for surge propagation, which in turn will damp out high frequency transients.

5.1 Internal Impedances of a Laminated Tu b u l a r Conductor

The internal impedances needed for laminated conductors are the same as those needed for homogeneous conductors, namely:

1. The internal impedance z0(J of the laminated tubular conductor which gives the voltage drop on the inner surface when unit current returns through a conductor inside the tube.

2. The internal impedance zbb of the laminated tubular conductor which gives the voltage drop on the outer surface when unit current returns through a conductor outside the tube.

5 . 1 . 1 Internal Impedance with External Return

Let us first number the layers consecutively with the inner most layer being number 1 as shown in Figure 5.1. For the analysis, we start with the mth outer most layer shown in Figure 5.2.

Let

= internal impedance of the mth layer with current returning inside,

= internal impedance of the mth layer with current returning outside,

Z^ = mutual impedance between the two surfaces,

zb

m

h — internal impedance of all m layers when the current return is external

For the very first layer, we note that Zb[^ = z$\ If we use concentric loop currents as before in Chapter 3, then the loop current 7m_, of the first m —1 layers combined, returns on the inner surface of the mth layer, while loop current Im flows on the outer surface. Using Schelkunoff's theorem 2 from Appendix B, the electric field strength along the inner surface of the mth layer becomes

- 71 -

Axis

Layer (m-l) Layer m

Figure 5.1 Numbering of Conductor Layers to Find the Internal Impedances of a Laminated Tubular Conductor

r m

Figure 5.2 Representation of the mth Layer

dV dx

— (ZTb lm %aa 'm-l) (5.1)

But the inner surface of the mth layer is the outer surface of the first m - l layers combined for which the electric field strength is given by

. ~ zbi 'm-l dx

Therefore we can find a relationship between I„ and /„_, from equations (5.1) and (5.2),

lm + Zli '

(5.2)

(5.3)

- 72 -

Now let us consider the electric Held strength on the outer surface of the mth layer. On one hand it is — z^Im, and on the other hand it is —(Zb

m

tIm — Z^/ m _i) using Schclkunoff's theorem 2. Thus we have the following identity:

m m m ^ m ~ * Zbb = %bb ~ %ab ~~j . ' ' (5.4)

m

Substituting for 7m_i//m from equation (5.3), we obtain

• m _ 7m J^i! '/<:«;» Zbb ~ *bb m , m - x ( o - 5 )

•^aa + f»6

which gives the internal impedance of all m layers of the laminated tubular conductor, with current return on the outside. Starting with the first layer where z$ = Zby, we add the remaining layers one by one until we obtain the impedance of the complete laminated conduc­tor made up of m layers.

(Zlb?

Z'at + Ab Ab = ZU - „ , , • = 2, • • • m (5.6)

5 .1 .2 I n t e r n a l I m p e d a n c e w i t h I n t e r n a l R e t u r n

Similarly we can find the internal impedance of a laminated tubular conductor with current returning inside. Let Z™, Zb

m

h and Z™b related to the same internal impedances defined in the previous section. Let z*a be the internal impedance of all m layers when the current return is internal. Also, we note that for the very last layer, i.e. layer m in Figure 5.1, ZTa = zTn- Using Schelkunoff's theorem 2, we find the electric field strength along the outer surface of the 1st layer as

= ~(Zb\h ~ Z,\lo) (5-7)

But the outer surface of the 1st layer is the inner surface of the rn —1 outer layers combined, for which the1 electric field strength can be written as

= (5-8)

Therefore, we can find a relationship between I0 and Ix from equations (5.7) and (5.8),

/, za\ h Zt\ + z-

(5.9)

Now, consider the electric field strength on the inner surface of the first layer. On one hand it is —{ — za\lo)t a n d o n t n e other hand it is — (Z^/i — Z}J0) using Schellkunoffs theorem 2.

Therefore we have the following identity,

- 73 -

= ZL ~ Z^-j- (5.10) 'o

Substituting for / j / / 0 from equation (5.9) we obtain

i _ . _ (̂ )2

zaa ~ Zaa _] 2 ,5.11) ^bb

+ zat

which gives the internal impedance of all m layers of the laminated tubular conductor, with current return on the inside. Starting with the last layer, i.e. layer m, where z™ = Z™, v.v add the remaining layers one by one until the impedance of the complete conductor made up of m

layers is obtained,

IZ' )2

= -Zla ~ _, '* • » = m - l , m - 2 > - l . (5.12) ^lb + zaa

5.2 A p p l i c a t i o n to Gas-Insulated Substations

The equations for the internal impedance of laminated conductors will now be used to obtain the surge propagation characteristics in a gas-insulated substation with conductor coatings.

Gas-insulated substations are subjected to transient sheath voltage rises whenever switch­ings or fault surges occur. These surges propagate along the outer surface of the inner conduc­tor and the inner surface of the sheath, as if the two surfaces were cylindrical wave guides, as well as along the outer surface of the sheath and the ground. The impedances of these surfaces play an important role in attenuating the surges, and thereby the transient sheath voltage rise. Since these surface impedances depend on the resistivity and the magnetic permeability of the material, it has been proposed by Harrington [32], to coat these surfaces with material of high resistivity and high permeability for surge suppression purposes. The coating should be such that its thickness is less than its current penetration depth at power frequency (60Hz or 50Hz), so that the resistance is not changed during steady-state operation. In addition to the base case without coatings, three different coating configurations are examined. The four cases con­sidered are as follows:

Core and sheath not coated.

i. Only the inner surface of the sheath coated.

ii. Only the outer surface of the inner core coated.

v. Both the outer surface of the inner core as well as the inner surface of the sheath coated.

For each of these cases, the formulae for the impedances for surge propagation arc derived.

5.2.1 C A S E i : C ore and Sheath not Coated

Axis

Core

S h c a l h v/ssssss//////////////////////J^ J

Earth \ % , J

Figure 5.3 - Core and Sheath not Coated

This is the simplest of all the cases where the impedance for surge propagation in loop 1

is given by

Z = Z:rc + Z,ns + Ziht (5.13)

where Zcre ( core - with external return) can be obtaind from equation (3.3) if the core conduc­tor is solid, or from equation (3.1b) if it is tubular. Z M S can be obtained from equation (2.11), and Zih, (sheath - with internal return) from equation (3;.la).

5.2.2 C A S E i i : Only Sheath Coated

In this case the surface for the surge propagation consists of the outer surface of the core conductor and the inner surface, of the laminated conductor made up of coating paint layer and sheath. Hence, the impedance for surge propagation between core and sheath is given by

Z = Zcre + Z,ns + zll (5.14)

where Zcre and Z,ni are the same as explained for case i. zll is the internal impedance of the laminated conductor with internal return. This is obtained from equation (5.12} where layer 1 is the sheath and layer 2 is the coating material (superscipt "sp" denotes the paint layer on sheath and superscript "sh" denotes the sheath). Hence

- 75 -

Axis

Core

Paint Sheath Earth

Figure 5.4 Inner Surface of the Sheath only Coated.

sp\2 y*V - T V _

Zll + z, sk (5.15)

The total impedance Z can then be written as

Z — Z:rc + Zins + Zip, (Zspm )2

zspc Zik, (5.16)

where Zsp, (sheath coated with paint layer - with internal return) and ZsK, (sheath - with inter­nal return) can be obtained from equation (3.1a). Zipm (mutual between sheath conductor and paint layer) is found from equation (3.1c), and Zspe (sheath coated with paint layer - with exter­nal return) from equation (3.1b). Equation (5.16) can also be derived from the loop equations of the loops 1,2 (figure 5.4),

dx

oT's ~di~

= -(zj, + ZM

= ~(ZnIi:+ Z2I2) (5.17a,b)

where

V, = potential difference between the core and paint

= potential difference between the paint layer and sheath

Zms "** Zsp,

— Z2i Zspm

v2

Zi

zm

Z2 — Zipt + z$k,

Since the paint layer and the sheath are at the same potential we have V2 = 0. Hence from equation (5.17b),

- 7 6 -

I2 = ( - Z m / Z 2 ) / 1

S u b s t i t u t i n g t h e v a l u e o f I2 i n t o e q u a t i o n (5 .16a) g i v e s

dV,

(5 .18)

(5 .19)

Therefore the impedance for surge propagation between core and sheath is given by

z = z, - —

or

7 - 7 _L 7 J - 7 _ ( ^ s y m

w h i c h is i d e n t i c a l w i t h e q u a t i o n 5 .16 .

5.2.3 C A S E i i i : O n l y C o r e C o a t e d

Axis

Sheath Earth |

/ / / / / / / / / / / / / / / / / / / / / / / / / / / / / /

1,

Figure 5.5 Core Alone Coated

T h e s u r f a c e f o r s u r g e p r o p a g a t i o n i n t h i s c a s e c o n s i s t s o f t h e s u r f a c e o f t h e l a m i n a t e d

c o n d u c t o r ( m a d e u p o f c o r e a n d t h e c o a t i n g ) a n d t h e i n n e r s u r f a c e o f t h e s h e a t h . T h e

i m p e d a n c e f o r s u r g e p r o p a g a t i o n b e t w e e n c o r e a n d s h e a t h is t h e r e f o r e g i v e n b y

Z = z$ + Znt + Z a , (5 .20)

w h e r e Z , A , a n d Zint a r e t h e s a m e i m p e d a n c e s e x p l a i n e d e a r l i e r . z$ is t h e i n t e r n a l i m p e d a n c e o f

t h e l a m i n a t e d c o n d u c t o r w i t h e x t e r n a l r e t u r n . T h i s c a n be o b t a i n e d f r o m e q u a t i o n (5.6), w h e r e

l a y e r 1 is t h e c o r e a n d l a y e r 2 is t h e p a i n t o n c o r e ,

Substituting ztf into equation (5.20) gives

(Z )2

Z = Zcre - 7 + Z,ns + Ztk, (5.22)

where Zcre is the same as explained earlier. Z^ (core coated with paint layer - with external return) can be obtained from equation (3.1b). Zcpm (mutual between paint layer and core con­ductor) from equation (3.1c), and ZCfi (core coated with paint layer - with internal return) from equation (3.1 a)

5 . 2 .4 C A S E i v : B o t h C o r e a n d S h e a t h C o a t e d

A x i s

Paint Sheath Earth

Figure 5.6 Core aa well as Inner Surface

of the Sheath Coated

In this case, the outside of the core and the inside of the sheath arc coated. The surfaces for surge propagation consist of.the surface of the laminated conductor 1 (made up of the core and coating paint layer on the core conductor) and the inner surface of the laminated conduc­tor 2 (made up of the sheath and the coating paint layer on the sheath conductor). The impedance for surge propagation between core and sheath is given by

Z = z% + Zml + z?.

Using the values of z$ and z\\ from equations (5.15) and (5.21) Z can be written as

z zcpc + z,RS + Zipt (5.24)

where all the impedance values have been explained earlier.

- 78 -

R e s u l t s

Two coating materials arc considered, i.e., stainless steel and supermalloy. These routing materials would be applied iu the form of paints. The bus duct of the gus-"insuiatccl substation is assumed to have the dimensions given in Figure 5.7.

Axis .

Sheath w/////r7/777777777777777777>

C

\/77//7777777777777777//777777/,

Earth j

-ORCR IRCR -IRSH ORSH

Figure 5,7 Dimensions of the Bus Duct in a Gas-htsulatcd Substation

Inner radius of the core, IRCR = 10.0mm

Outer radius of the core, ORCR = 65.0mm

Inner radius of the sheath, IRSH = 350mm

Outer radius of .the sheath, ORSH = 380mm

The values of relative permeability and resistivity of the coating materials (stainless steel and supermalloy) and of the core and sheath material (aluminium) are given in Table 5.1.

5 . 2 . 5 S t a i n l e s s S t e e l C o a t i n g

The skin depth for a particular material is given by

6 = V2p /M/ i (5.25)

- 7 9 -

Table 6.1 Resistivity and Relative Permeability of Coating Materials

T Y P E MATERIAL RELATIVE

PERMEABILITY RESISTIVITY

Core and Sheath Aluminum 1.0 2.62E-08

Paint

(a)

(b)

Stainless Steel

Supermalloy

1500.0

100 000.00

4.70E-07

0.00E-07

Using the values of relative permeability and resistivity for "stainless steel, the skin depth at various frequencies was calculated and tabulated in Table 5.2.

Table 5.2 Skin Depth of Stainless Steel

FREQUENCY

(Hz)

SKIN DEPTH (mm)

10.0 2.81723

60.0 1.15013

100.0 ; 0.89089

1 000.0 0.28172

10 000.0 0.08909

Since the thickness of the coating should be very much smaller than the skin depth cf the material at normal operating frequency, coating thickness of 0.1mm and 0.5mm were assumed to be practical values. Figures 5.8(a) and (b) show the variation in resistance and inductance for a coating thickness of 0.1mm and Figures 5.9(a) and (b) for a coating thickness of 0.5mm.

— 6 U "

100

10 E

c

in

a) rr 0.1

0.01

0.001

C A S E 1

CASC 2

CASE_3

C A S E 4

1CT 10" 10' 103 103 10' Frequency [Hz]

10s 10' 10'

x

O.B-

0.6

c o o o.*

~o c

0-2.

0-+ 10"

\

\ \

w CAST 1

CASE.. 2

C_ASE_3

CASE ̂

• 10- 10' 10J 105 10' Frequency [Hz]

•10s 10* 10'

Figure 5.8(a),(b) Variation of Resistance and Inductance

•with Frequency for the Four Cases;

Stainless Steel Coaling, Thickness 0.1mm.

- 82 -

5.2.6 Supermalloy Coa t i n g

The high resistivity and high permeability of supermalloy make its skin depth very small even at low frequencies, as shown in Table 5.3.

T a b l e 5.3 Skin Depth of Supermalloy

FREQUENCY (Hz)

SKIN DEPTH (mm)

1.0 1.23281

10.0 0.38985

60.0 0.15915

100.0 0.12328 *

1 000.0 0.03898

10 000.0 0.01233

Since the coating thickness should be smaller than the skin depth at normal operating fre­quency, it would be necessary to keep the coating thickness to less than 0.1mm. Figures 5.10(a) and (b) show the variation of resistance and inductance with frequency for a coating thickness of 0.01 mm and Figures 5.11(a) and (b) for a coating thickness of 0.05mm respec­tively.

5.2.7 Comparison between Stainless Steel and Supermalloy Coatings

For the case of stainless steel, we note from the figures 5.10(a) that there is no noticeable difference in the resistance up to a frequency of 100Hz for all four cases if the coating thickness is 0.1mm. Beyond that it increases sharply for cases 2, 3 and 4 as compared to the base case. When the coating thickness is increased-to 0.5mm, the differences are pronounced at frequen­cies as low as 1Hz, as shown in Figure 5.11(a). This indicates that the coating thickness should not be increased beyond 0.1mm, since it would change the resistance at steady state operating frequency (50Hz or 60Hz) too much, and thereby increase the losses as well as the operating temperature.

Due to the high permeability of stainless steel, the inductance is very high for cases 2, 3 and 4 as compared to case 1, as shown in Figures 5.10(b) and 5.11(b). However, the increase in inductance should not cause any problems in bus ducts which are very short compared to the length of transmission lines.

- 83 -

\

£ ^ \ \ C A s e 2

\

c o y 2 •o c

\ 5 X \

CASE 1

<2 \ i - 3~ V LAIL3.

CASE i

10"2 10" 1 10' 10J 105 10." 105 10' 10 Frequency [Hz]

Figure 5.10 (a) and (b) Variation of Resistance and Inductance

with Frequency for the Four Cases;

Supermalloy Coating, Thickness 0.01mm.

- 84 -

1000CH

1000-

100-

E

u c D

tn

ca

10-

0.1.

0.01.

0.001J

CASE 1

CASE 2

1CT1 10"'

2 0 -

10' 102 103

Frequency [Hz] 10' 105 10s 1 0 '

\

15-

E

<u 10-u c o "o 3 -"D C

X \

\ CASE 1

CASE 2 CASE_3 CASE *

0 + 10" 10" 10' .10* 10J 10"

Frequency [Hz] 10* 10e 10'

Figure 5.1l(a),(b) Variation of Resistance and Inductance

with Frequency for the Four Cases;

Supermalloy Coating, Thickness 0.05mm.

- 85 -

In the case of supermalloy, due to its higher resistivity and very lur^e permeability the coating thickness should not be increased beyond 0.01mm for the same reasons explained ear­lier for stainless steel.

The practicality of stainless steel or supermalloy coatings for surge suppression has been questioned by Boggs and Fujimoto [32]. Such coatings may, be cost effective. Using highly resistive materials such as steel for the entire sheath has been considered as well. This would be feasible with single-point ground, which would prevent currents from circulating through the sheath, thereby avoiding sheath losses. However, single-point grounding has adverse implica­tions for transient ground rise, however. If switching surges are produced, transient overvol-tages would appear at many points within the gas-insulated substation.

5.3 Conclusions

The internal impedances of tubular laminated conductors have been derived. These equa­tions are used to find the internal impedances of bus ducts in gas-insulated substations whose core and/or sheath are coated with high-resistivity paints for the suppression of surges.

- 86 -

6. TEST CASES T h e i n t e r n a l i m p e d a n c e f o r m u l a e f o r t u b u l a r c o n d u c t o r s a n d t h e e a r t h r e t u r n i m p e d a n c e

f o r m u l a e w e r e d i s c u s s e d i n d e t a i l i n C h a p t e r s 3 a n d 4, r e s p e c t i v e l y . T h e s e i m p e d a n c e s m a k e

u p t h e e l e m e n t s o f s u b m a t r i c e s [ Z „ ] a n d [Zi}\. In t h i s c h a p t e r t h e v a l u e s o f t h e s e s u b m a t r i c e s

a r e o b t a i n e d f o r a s p e c i f i c u n d e r g r o u n d c a b l e s y s t e m , u s i n g t h e e x a c t f o r m u l a e as we l l as

a p p r o x i m a t i o n s .

T h e a p p r o x i m a t e f o r m u l a e p r o p o s e d b y W e d e p o h l [22] a g r e e v e r y c l o s e l y w i t h t h e e x a c t

f o r m u l a e a n d t a k e v e r y l i t t le c p u t i m e . T h e y a l s o p r o v i d e s i m p l e e x p r e s s i o n s f o r h a n d c a l c u l a ­

t i o n p r u p o s e s . T h e r e f o r e o n l y t h e a p p r o x i m a t e f o r m u l a e p r o p o s e d b y W e d e p o h l [22] a re c o n ­

s i d e r e d in th is c h a p t e r . T h e r e s u l t s a re a lso c o m p a r e d w i t h v a l u e s o b t a i n e d f r o m t h e C a b l e

C o n s t a n t s r o u t i n e in t h e E M T P , w h i c h w a s d e v e l o p e d b y A m e t a n i [27].

6.1 S i n g l e C o r e C a b l e

T h e i m p e d a n c e s o f a s ing le c o r e c a b l e a re g i v e n b y a 2X2 m a t r i x o f t h e f o r m

m =

w h e r e al l e l e m e n t s were d e f i n e d e a r l i e r in C h a p t e r 3 . W i t h t h e d a t a o f t h e t e s t c a s e d e s c r i b e d

in A p p e n d i x A , t h e v a l u e s o f t h e s e e l e m e n t s w e r e o b t a i n e d f r o m t h e e x a c t f o r m u l a e , f r o m

W e d e p o h l ' s a p p r o x i m a t i o n f o r m u l a e a n d f r o m A m e t a n i ' s C a b l e C o n s t a n t r o u t i n e , as t a b u l a t e d

in T a b l e 6 .1 . F i g u r e s 6.1 d e p i c t s t h e e r r o r s in A m e t a n i ' s a n d W e d e p o h l ' s a p p r o x i m a t e f o r m u ­

lae f o r the i m p e d a n c e Z.c. F i g u r e s 6.2 a n d 6 .3 , r e s p e c t i v e l y , s h o w t h e e r r o r s f o r the i m p e d a n c e s

Zs. a n d Zss T h e e r r o r s in A m e t a n i ' s a n d W e d e p o h l ' s f o r m u l a e a re n o t s i g n i f i c a n t at low f re ­

q u e n c i e s , b u t f o r h i g h e r f r e q u e n c i e s t h e y c a n n o t be n e g l e c t e d . W e a l s o n o t i c e t h a t the e r r o r s in

Zcc, ZCi a n d ZS! a l l h a v e s i m i l a r v a l u e s . T h e y are e s s e n t i a l l y c r e a t e d b y the e r r o r s in the e a r t h

r e t u r n f o r m u l a e u s e d b y A m e t a n i a n d W e d e p o h l , as s h o w n in F i g u r e 4 .7 . A s m e n t i o n e d e a r l i e r ,

W e d e p o h l ' s a p p r o x i m a t e e a r t h r e t u r n f o r m u l a is v a l i d o n l y i f t h e c o n d i t i o n \ mZ \ <0.25 is s a t i s ­

fied. T h i s is o n l y t r u e at low f r e q u e n c i e s . A t h i g h f r e q u e n c i e s , t h e i n t r i n s i c p r o p a g a t i o n c o n ­

s t a n t m , b e c o m e s l a r g e r , a n d th is c a u s e s t h e e r r o r s in the r e s u l t s . T h e e r r o r s a lso i n c r e a s e i f

t h e s e p a r a t i o n b e t w e e n t h e c a b l e s b e c o m e s l a r g e r .

T h e r e a s o n s f o r the e r r o r s in A m e t a n i ' s e a r t h r e t u r n i m p e d a n c e f o r m u l a e at h i g h f r e q u e n ­

c ies h a s a l r e a d y been d i s c u s s e d in C h a p t e r 4 .

z. (6.1)

- 87 -

T a b l e 8.1 Impedances of a Single Core Underground Cable.

Zee (fi/km)

E X ; AME1 ' ' A N I W E D E : P O H L

F R E Q (Hz)

R X R X R X

1 10

100 1000

10000 100000

. 0 1 0 8 7 3

. 0 2 0 0 8 4

. 1 1 9 3 0 3 1 .05509 1 0 . 4 8 0 3 1 0 8 . 2 4 0

. 0 1 6 0 8 2

. 1 4 6 2 9 9 1 . 3 0 4 5 6 1 1 . 4 7 5 9 9 9 . 6 8 4 3 8 3 9 . 8 4 8

. 0 1 0 8 7 3

. 0 2 0 0 6 9

. 1 1 8 8 0 5 1 .03996 1 0 . 0 3 4 3 9 6 . 4 9 7 3

. 0 1 6 0 8 3

. 1 4 6 3 1 5 I . 30504 I I . 4916 1 0 0 . 1 7 5 8 5 4 . 7 6 1

. 0 1 0 8 6 8

.020081

. 120114 1 .05626 1 0 . 4 9 8 3 1 0 9 . 5 9 3

. 0 1 6 0 9 3

. 1 4 6 4 1 6 I . 30506 I I . 4759 9 9 . 6 8 2 2 8 3 9 . 6 1 4

Zcs (£2/ km)

1 10

100 i o o o

10000 100000

. 0 0 0 9 8 7

. 0 0 9 8 7 8

. 0 9 8 9 5 4

. 9 9 5 7 1 7 10 .2001 1 0 6 . 4 3 0

. 0 1 5 0 9 7

.136501 1 .22016 1 0 . 7 4 9 4 9 2 . 8 2 9 5 7 7 5 . 5 2 4

. 0 0 0 9 8 7

. 0 0 9 8 6 2

. 0 9 8 4 6 5

. 9 8 0 5 9 0 9 . 7 5 4 1 4 9 4 . 6 8 7 6

. 0 1 5 0 9 8

. 1 3 6 5 1 6 1 .22066 10 .7651 9 3 . 3 2 0 1 7 3 0 . 4 3 8

. 0 0 0 9 8 7

. 0 0 9 8 7 8

. 0 9 8 9 5 6

. 9 9 5 9 1 9 1 0 . 2 1 7 0 1 0 7 . 7 8 2

. 0 1 5 0 9 7

.136501 1 .22016 1 0 . 7 4 9 4 9 2 . 8 2 7 2 7 7 5 . 2 9 1

i Zss Wl im)

1 10

100 1000

10000 100000

.300151

.309041

. 3 9 8 1 1 2 1 .29438 10 .4531 106 .361

. 0 1 5 0 8 3

. 1 3 6 3 5 4 1 .21869 1 0 . 7 3 4 7 9 2 . 6 9 6 9 7 7 5 . 5 1 8

. 3 0 0 1 5 0

. 3 0 9 0 2 5

. 3 9 7 6 1 2 1 . 2 7 9 2 5 10 .0071 9 4 . 6 1 8 0

. 0 1 5 0 8 3

. 1 3 6 3 6 9 1 .21919 1 0 . 7 5 0 4 9 3 . 1 8 7 5 7 9 0 . 4 3 2

.300151

.309041

. 3 9 8 1 1 5 1 .29458 1 0 . 4 7 0 0 1 0 7 . 7 1 3

. 0 1 5 0 8 3

. 1 3 6 3 5 4 1 . 2 1 8 6 9 1 0 . 7 3 4 7 9 2 . 6 9 4 6 7 7 5 . 2 8 5

- 88 -

50

30-

1 0 -

u c « -10 cn io cu rr

- 3 0

/ 7

AMETANI

WEDEPOHL

— 5 0 \ i i i Mini i i i nun —n T T T T T T T J — i T I i t i i T f nr T T H T T T ; T-TTTTTTT] r i i mit| " i T T m n | " i i r i t i n

10~ 2 1 0 " 1 10' 10 2 10 3 10 4 10 5 10 6 10 7

20

12

4-

01 o c to u ca cu CC

-4

-12

\ AMETANI

WEDEPOHL

-20 1 III Mini i i i mi l l i i i IIIIII i i i m m i i i i i u i i — i i 11mil i i 11iui| i i 11mil i i 11mi 10" 2 10"' 1 10 1 10 2 10 5 10 4 10 s 10 6 10 7

Frquency [Hz]

Figure 6.1 - Errors in Ametani's and Wedepohl's Approximations in Zcc

- 89 -

50

30-

0) u c

tn cu tc

10-

-10-

- 3 0 -

/ /

AMETANI

WEDEPOHL

— 50 1 T ITTIfH ' I '1 M l l T i r I 'T I 1 i m ] ' ~ T ' T T T T m r T"T T^TIITf I ' T l T n i l } ' 1 I 1 11'ltT) 1 "TTTTnTJ I I T'H'ffl

10"2 10"' 1 10' 102 103 104 10s 106 107

20

12

cu o c CO *-> o ra CD CC

- 4 -

-12-

\ AMETANI

WEDEPOHL

— 20 | i i 1 1 — i i i IIIIII I i i M i n i 1 i i m i l l 1 i . I I I I I I — i i i M i n i 1 i I M i n i — i i i u t i i | i l l M i n i

10"2 10"' 1 10' 102 10J 104 105 106 107

Frquency [Hz]

Figure 6.2 - Errors in Ametani 'a and Wedepohl's Approximations in Za

- 90 -

LU

5 0

30-

10

co u « - 1 0 +—<

m cc

- 3 0 -

/ /

AMETANI

WEDEPOHL

—50 | I I I I I I I I I i i i I I I I I I — i i i I I I U I i i 11uii| i i 1 1 m i l I i 11uii|—> i i I I I I I I I I I I I I I I I — i i i i n n 1

TO" 2 1 0 " ' 1 1 0 ' 1 0 2 1 0 3 10" 1 0 5 1 0 6 1 0 7

2 0

CD u c to o co CD

at - 1 2 AMETANI

WEDEPOHL

—20 I I I I n i M | i i i i i i i i ,

1 0 " 2 1 0 " ' 1 rrm] 1 i i i i n i j i i I I I I I I ; I I I I I I M I I I I I I I I I ) I I I I I I I I I I i i m i q

1 0 ' 1 0 2 1 0 3 10" 1 0 S 1 0 6 1 0 7

Frquency [Hz]

Figure 6.3 - Errors in Ametani's and Wedepohl's Approximations in Z,

- 91 -

6.2 Three-Phase Cable

In the case of a three-phase cable system, the series impedance matrix is given by

1*1-\Zn\ |Z 1 2] \ZK)

\2A \z*\ (0.2)

where [Z„] is the self impedance submatrix of cable i as given by equation (0.1). The mutual

impedances between cable i and cable j are represented by submatrix |Z t ;] of the form:

Z - .f . Z r r •

Zj.ry Zs.s. (0.3)

As shown in Chapter 2, all four.elements of submatrix are equal to each other. The values

from the exact formula (4.10) and from Ametani's and Wedepohl's approximate formulae are

tabulated in Table 6.2 for the three-phase cable system described in Appendix A.

T a b l e 6.2 Mutual Impedance between Two Cables with

Burial Depth of 0.7om and Separation of 0.80m.

Z i j (fi/kin)

E X ; ^ C T AME*] rANI WEDI 3POHL

FREQ RES REA RES REA RES REA

1 10

100 1000

10000 100000

.000987

.009877

.098943

.994644 10.1015 105.154

.012562

.111152

.966670 8.21457 67.5095 525.238

.000987

.009862

.098452

.979517 9.65569 93.4318

.012563

. 111167

.967169 8.23027 68.0000 540.150

.000987

.009878

.098946

.994856 10.1193 106.592

.012562

.111151

.966668 8.21452 67.5070 525.000

- 92 -

5 0

3 0

O ) 0

UJ

cu o li -to-cn cn tn

- 3 0 H

- 5 0 —i—n: TTTT. i—r^rrrrrrr-10~2 10"'

5 0

3 0 -

O 1 0 -

UJ cu o c

0> CC

- 1 0

- 3 0

/ /

AMETANI

WEDEPOHL

ni 1 i i I I I I I I 1 11 MM; 1 \ i i mi, 1—i t mil, 1—t 11 nn' 1 10' 10 2 10 3 10 4 10 S 10 6 io 7

\ AMETANI

WEDEPOHL

5 0 -f 1 i i mii| 1 i i inn, 1 i i mii| 1 i i iiiiii 1 i ' mii; 1 i mm, 1 i i i nn; 1 , 1 1 ""1 ' 1 11"" 1 0 - 2 10" 1 10' 10 2 10 S 10 4 10 S 10 6 10 7

Frequency [Hz]

Figure 6.4 - Errors in Ametani's and Wedepohl's Approximations

in the Mutual Impedance between Two Cables.

- 93 -

The errors in Ametani's and Wedepohl's approximate formulae are plotted in Figure 6.4.

The reasons for the errors are essentially the same as those discussed in Section 6.1.

6.3 Shunt Admittance Matr ix

The elements of the shunt admittance matrix obtained from Ametani's Cable Constant

routine in the EMTP shows that the relative permittivity t is assumed to be real and con­

stant. As explained earlier in Chapter 2, the relative permittivity is complex as well as

frequency-dependent, but this data is usually difficult to obtain. A real, constant permittivity

should give reasonable answers in many cases.

- 94 -

7. CONCLUSION

Various formulae proposed in the literature for the series impedance and shunt admit­

tance matrices of underground cable systems have been compared in this thesis. The elements

of the series impedance matrix are evaluated from formulae for the internal impedance of tubu­

lar conductors and from formulae for the earth return impedance. Exact equations for the

internal impedance of tubular conductors were first derived by Schelkunoff [6j. They are given

in terms of modified Bessel functions, and are therefore not suitable for hand calculations.

Since then closed-form approximations suitable for hand calculations have been proposed by

many authors, including Schelkunoff. A comparison of these approximate formulae shows that

the formulae proposed by Wedepohl [22] give answers which are usually accurate enough for

engineering purposes. With computers being almost universally available nowadays, approxi­

mate formulae are no longer that important, however, and programming the exact formulae

may therefore be the best approach.

The displacement current term is usually neglected in the formulae for the internal

impedances of conductors. It is shown that it can indeed be neglected for frequencies up to

10MHz. The shielding effect of grounded sheaths is explained as well, and it is shown that it is

implicitly accounted for in the mutual impedances.

The permittivity of the insulating material is needed for the elements of the shunt admit­

tance matrix. Its value is frequency dependent as well as complex. In some cases, (e.g., cross-

linked polyethylene), the permittivity can be assumed to be constant and real up to very high

frequencies, while in other cases (e.g., oil-impregnated paper) the changes with frequency are

quite significant. Two insulating materials, namely cross-linked polyethylene and oil-

impregnated paper, are discussed in detail because they are the materials most often used in

power cables. A general formula for the complex permittivity of insulation materials is given

by Bartnikas [15], based on the relaxation time of the dielectric material. Ametani's Cable

Constants routine in the E M T P [27] assumes that the permittivity is real and constant which

may not always be accurate enough.

The earth return impedance formula derived by Pollaczek [l] for the case of a semi-

infinite earth is valid only for filamentary type conductors of negligible radius. This formula

can be used for a conductor of finite radius a, if the condition | mo | <0.1 holds. This condition

is satisfied up to a frequency of 1 MHz even for a worst case low earth resistivity of 10 fl— m.

Hence Pollaczek's formula is recommended as the accurate formula. Values obtained from

various approximate formulae and from Ametani's Cable Constants routine in the E M T P were

compared against Pollaczek's formula. The results agree closely at low and medium frequen­

cies but significant differences arise at high frequencies.

- 95 -

Equations for the internal impedances of a laminated tubular conductor have been derived from the equations for homogeneous tubular conductors. They are used to study the increase in the surface impedances of bus ducts in gas-insulated substations if the conductors are coated with high-resistivity magnetic material. This coating technique has been proposed by Harring­ton [32] for reducing the transient sheath voltage rise during switching operations, although others have criticized it as impractical, [discussion 32]

- 96 -

A P P E N D I X A Test Examples for Buried Cables

Earth

d,

G A

x'- x»

Figure A.l - Three-Phaae Cable Setup for the Study

Each cable is of a single core type with dimensions as given below

conducting sheath

central conductor

insulation

Figure A.2 - Basic Construction of each tingle core cable

di,d2,dt

X12

X ja

= 0.75m, depth of burial of each cable

= 0.30m, horizontal distance between cables 1 and 2

= 0.30m, horizontal distance between cables 2 and 3

*2

Peon

P- r core? r shea.tb»

/ ' f ex r tb ' / ' r i l r

0.0234m, radius of the core

0.0385m, inner radius of the sheath

0.0413m, outer radius of the sheath

0.0484m, outside radius of the cable

100 fl—m, resistivity of the earth

1.7X10"8J?-m resistivity of the core material

2.1xi0"7/?-m resistivity of the sheath material

= 1.0, relative permeability of the core, sheath, earth, and air respectively.

- 98 -

A P P E N D I X B Internal Impedances of a Tubular Conductor

Based on tbc work of Schelkunoff [6], the derivation of the internal impdance formulae for tubular conductors is summarized here.

B.l Circularly Symmetric Magnetic Fields

In polar coordinates, Maxwell's equations assume the following form:

dllz rd<*>

3//r

dz

dz

dHz

= (\lp + »"toe)E,,

= (l/> + i<ai)Et,

3Ez rd(j>

3E, dz

dEt

dE,

— — iliifiH,

= —ioifill^

ifdlrHJ 3H,\ 1 (9[rE4) ^ dEt\

— 1 — — ' = + i«e)E r. T T f = ~ r ( dr d<p ) r { dr 3<t> )

(B-1)

where H and E are electric and magnetic field strengths, respectively. Here we are interested in the circular magnetic field around conductors, with its lines of force forming a system of coaxial circles. Such circular magnetic fields are associated with currents flowing in isolated wires, as for example in a single vertical antenna, or between the conductors of a coaxial cable, as shown in Figure B.l.

Figure B.l - The relative directions of the field components

in a coaxial transmission line.

- 99 -

From equation (B.l) we see that when the quantities are independent of the angle <f>, one of the independent subsets composed of the 1st and 3rd equation on the left of equation (B.l), together with the 2nd equation on the right, define the circular magnetic field strengths as fol­lows:

dlrllA . - (1/p + i<at)rE, (B.2a) or

a,

dE2 dE

-(l/p + itoe)Er (B.2b)

iattH4. (B.2c) dr dz

It has been shown by Schelkunoff that H^,Er and Ez have components which vary exponentially along the longitudinal axis of the cable, i.e., along the z axis in Figure B.l. If we express the exponential variation of the quantities E,,EZ and as E,eCr*, Eze~Tz and H^,e~r', then the quantities E,,E2 and H# are functions of r only. Substituting these values into equation (B.2) we obtain

Er = f • H4 (B.3a)

dE, iuuHt = —— + TE, (B.3b)

dr

^—A. = (Up + iu>f)rE, (B.3c) dr

where the quantity T is called the longitudinal propagation constant. Now solving for H# from Equation (B.3), we obtain

where m2 = I ' co 2/ze|. This quantity m is called the intrinsic propagation constant of I P

the conductor material. For solid conductors, the term u>2ut which accounts for the displace­ment current is negligibly small compared to the conduction current. Hence we can neglect it up to quite high frequencies. The intrinsic propagation constants of metals are relatively large quantities even at low frequencies as shown in Table B.l for copper.

- 100 -

Table B.l

Propagation Constant of Commerical Copper

p = 1.7 X 10" 8tt-m

iH,) y/a){i/p = | m |

0 0.0

1 21.40

10 87.67 100 214.00

10,000 2140.00

1,000,000 21400.00 100,000,000 214000.00

On the other hand, the longitudinal propagation constant T is relatively very small, even at

high frequencies. For example, if air is the dielectric between the conductors T will be of the

order of (l/3)ia>10~10. Hence, even at high frequencies T 2 is negligibly small by comparison with

m2. Therefore, we can write equation (B.4) as

d2H* i dH, dr' dr sT = mXH*

The solution for Ht of equation (B.5) is in the form of Bessel functions given by:

Alx{mr) + BK^mr)

(B.5)

(B.6)

Since we are interested in longitudinal voltage drops, we must find the longitudinal elec­tric field stength first. This can be obtained from equation (B.3) and (B.6) along with the fol­lowing rules of differentiation for modified Bessel functions of any order n,

dx

_d_ dx

(x*Kn)= -x*Kn.x

(B.7a)

(B.7b)

The solution for the longitudinal electric field strength then becomes

- 101 -

E, = pm\AI0(mr) - BK0{mr)\ (B.8)

In a tubular conductor whose inner and outer radii are a and b, respectively, coaxial return path for the current may be either outside or inside the tube or partly inside and partly outside. We designate Zt as the internal impendance of the tubular conductor with internal return and Zb as the internal impedance with external return. If the return path is partly internal and partly external, we have in effect a two-phase transmission line with a distributed transfer impeduace Zab between the two loops of internal and external return.

In order to determine these impedances, let us assume that a total current (/, + /.) is

flowing in the tubular conductor, with part /„ returning inside and part Ib returning outside.

Figure B.2 - Loop Currents in a Tubular Conductor

Since the total current enclosed by the inner surface of the conductor is — Ia and that enclosed by the outer surface is Ib (70 + Ib — /„), the magnetic field strengths at these two surfaces take the values ( — Ia/2na) and (Ib/2nb) respectively. Hence from equation (B.6) we have

A/,(7»io)+ M,(ma) = -lJ2na (B.9a)

A li(mb) + BKx(mb) = Ib/2r.b (B.9b)

From these two equations the values of A and B can be evaluated as

2-naD 2nbD

= _ /»/»("») ( B 1 0 b )

2naD 2xbD

where

- 102 -

D = r1(mb)k1(ma) - I^majk^mb) (B.II)

Substituting these values in (B.7) and using the identity /„ (z)/C"i(z) + K0(x)Ix(x) = 1/x,, we obtain the longitudinal electric field strength at any point on the conductor. However, we are interested in its' values at the surfaces as they constitute the surfaces of propagation. Hence, equating r successively to a and 6 we obtain

E,(a) = ZJa + ZabIb (B.12a)

E,(b) = ZcbIa + ZbIb (B.12b)

where

2naD

7 - Pm

Z> ~ 2xbD

7 = P

[irimajK^mb) + /v"0(mo)/,(m6)

^/ 0(m6)K'i(r7ja) + AT0(m6)/1(ma) j

2nabD

Schelkunoff stated these results in the following two theorems.

(B.13)

Theorem 1

If the return path is wholly external (Ia = 0) or wholly internal (/;, = 0), the longitudinal electric field strength on that surface of a tubular conductor which is nearest to the return path equals to the corresponding surface impedance per unit length multiplied by the total current flowing in the conductor and the field strength on the other surface equals to the transfer impedance per unit length multiplied by the total current.

Theorem 2

If the return path is partly external and partly internal, the separate components of the field strength due to the two parts of the total current are calculated by the above theorem and added to obtain the total field strength.

- 103 -

A P P E N D I X C Calculation of Earth Return Impedances

in an Infinite Homogeneous Earth

If the return current distribution in the ground is circularly symmetrical, then we refer to

such a case as infinite earth. This happens in practice when the cables are either buried at

large depth or when the frequency is very high. In both cases, the penetration depth d given

by 503 • I t ) m, becomes smaller than the depth of burial. Then only the earth medium

must be considered, which simplifies the solution. If the cables are buried close to the earth's

surface on the other hand, which is usually the case, then the distribution of current in the

ground is no longer symmetrical (at least at low frequencies), and the magnetic field both in air

and earth must then be considered which makes the solution more complicated.

Consider a cable lying along the Z-axis of the cartesian coordinate reference frames as

shown in Figure C . l . Let the positive direction be along the Z-axis, and let the conductor

carry a current I flowing in the positive direction returning through the ground. Let the radius

over the outer insualtion be a. From Ampere's Law (neglecting the displacement current term)

the magnetic field strength H at a radius r & o is given by

2-nrH = I + J 2nrJdr

i.e.

T

H = —+-fjrdr (C.l) 2nr rJ v

where J is the current density in the ground.

Suppose that the earth is subdivided into concentric cylindrical shells of radius r and

thickness dr in which the current density / and magnetic field strength / /are constant. Then

the magnetic flux per unit length of such a shell is given by

d<f> = BdA = ulldr (C.2)

Substituting for H from equation (C.l) yields

dtp = udr -!-+±Jjrdr 2xr r •*

(C.3)

Now let us write Kirchhoff's voltage law around the rectangle ABCD of unit length and width

dr. The net resistive voltage drop is — ',|~^:~J^r a n c * t n e induced voltage is jmd<l> or jtaiilldr.

- 104 -

Figure C.1 - Representation of a Buried Conductor

in an In finite Earth.

Since the sum of these two voltages must be zero, we obtain

dJ — p dr + jtauH dr = 0

dr Substituting for//from equation (C.1), we have

pdJ

dr •dr + jtAfidr — + -fjrdr

2itr = 0

(C.4)

(C.5)

Multiplying this equation by ~- and differentiating with respect to r we have par

d2J + dJ_ _ jvuJ dr2 rdr p

= 0 (C.6)

If we substitute m* for J f a > / i, then equation (C.4) can be written as P

- 105 -

£ «•»««*. (C7)

and equation (C:6) can be written as

d2J . dJ m

lJ = 0 (C.8) dr' r dr

Equation (C.8) is immedately recognizied as a Bessel equation whose solution is of the form

/ - AI0{mr) + BK0{mr) (C.9)

We note that I0(x) approaches infinity as z approaches infinite. However, we cannot permit a

solution of J to increase indefinitely as r approaches infinity and we must conclude that ,4=0

PI-

Hence, equation (C.9) becomes

/ = BK0(mr) (C.10)

Using equation (C.7) we find a solution for the magnetic field strength H as

m2H = -BKAmr)m ( C l l )

Now applying the boundary condition that H = I/2ira in the ground immediately adjacent to

the cable, we obtain the value for the constant B from equation ( C l l ) as

B = ~ 2nal<!(ma) ( C ' 1 2 )

Using the equation E = pJ, the solution for the electric field strength at any point in the

soil is found to be

pml K0{mr) . E~~2*a K^ma) ( C 1 3 )

The earth return self impedance as well as the mutual impedance between two buried cables

can be deduced from this equation (see Chapter 4).

- 106 -

A P P E N D I X D

Calculation of Earth Return Impedances

in a Semi-Infinite Homogeneous Earth

The limited conductivity of the ground path for the return currents as well as conductor skin effects result in the frequency dependence of the line parameters. The parameters of a transmission line over a ground of perfect conductivity are given by textbook formulae, but the earth return effects and skin effects need special treatment. While a complete solution of the actual problem is impossible, on account of the uneven surface under the line and the lack of conductive homogeneity in the earth, a solution of the problem, where the actual earth is replaced by a plane homogeneous semi-infinite solid, gives reasonably accurate answers. The same applied to the underground case, too.

The first step in finding the earth return impedances is to derive the respective longitudi­nal electric field strengths. Let us first consider au overhead line and derive the electric field strengths in air and in earth.

A y p . (cc.h)

X

Earth

Q* (x.y)

Figure D.l - Current-Carrying Filament in the Air

Let medium 1, denoted by subscript 1, correspond to air and medium 2, denoted by subscript 2, correspond to earth. Let point P{a,h) correspond to the current-carrying filament lying

- 107 -

along the Z-axis of the Cartesian coordinate system. Let E+ + (x,y;a,h) be the electric field strength in air at a point Q(x,y) and E- + (z,y';a,h) be the electric field strength in the earth at a point Q'(x,y'). Note that the y axis is positive in the air and that the y' axis is positive in the earth, as shown in Figure D.l.

From Maxwell's theory, the general equation for electromeganetic wave propagation is given by

V 2E - V(V£) = - tovjtf (D.l)

where p,fi and e correspond to the respective medium to which this equation is applied. Using assumption 4 (Chapter 2), we can say that VE — 0 in both air and earth. Hence, equation (D.l) can be written as

V 2E - CO [It (D.2)

Now let us define the fields which we would like to derive as follows

E++ = E+ + z = Electric field strength in the air due to the current-carrying filament in the air

E-+ — E- + z

= Electric field strength in the earth due to the current-carrying filament in the. air

If we assume that a sinusoidal current I of angular frequency' o> is passing through a filament concentrated at the point (a,h) in the x—y plane as illustrated in Figure D.l, then the current density is zero everywhere in the air except at the point (a,h) where it is infinite. Such an idealized situation can be represented by the Dirac delta function 5(x-^a) defined as

in such a way that

/ 5(x-a)dx = 1 ! (D.4) — CB

which implies that if f(x) is continuous at x = 0 and bounded elsewhere [12]

J f(x)5(x-a)dx = f(a) (D.5)

Hence in the air, the current density can be expressed in the form

- 108 -

I6(x-a)S(y-h) (D.6)

Now keeping this result in mind and noting that we are interested only in the electric field 9EZ ' -

strength along the Z-axis and = 0 (using assumption 2 from Chapter 2) equation (D.2)

can be written for the case of air as

d2E+ + d2E.+ 2 3z a By'

m2E++ + plmfIS(x — a)6(y — h) (D.7)

where

Pi

For the earth, equation (D.2) can be written as

a2£_ + a2E_+

dx2

• here

m2 =

= m|E_. (D.8)

I P2 2 2 — CO p i2

The solutions for E++ and E_+ should be obtained in such a way that they satisfy the follow­ing boundary conditions:

1. Continuity of E at the surface.

Lim £++ = Lim E_+ — Lim £ _ + = /?0(say) V - * 0 y — 0 jr'-+o

Vertical component of B is continuous at the surface

dx dx

Horizontal component of H is continuous at the surface

dE++ a£_+ aE_+

(D.10)

Pidy P2&y p-z^y' (D.il)

The solutions for E+ + and E_ + can be found by using integral transform techniques. Taking the Fourier complex transformation of equations (D.7), (D.8), (D.9) and (D.ll) with respect to x, with 6 as the parameter, we obtain the following equations:

- 109 -

d 2 E + +

-62E^ + = m f E + + + /j,m 2/exp(-^a)%-A) (D.12)

<f 2 £.. -62E_ + + j ^ - mf E.+ - (D.13)

£ + + | y-0 — E-+ | Y ' - 0 — £ Q

1 dE+* j i d £ _ +

(D:14)

• 1 OD _ + I

Hi dy fi2 dy'

Now taking the Fourier sine transformation of equation (D.12) with respect to y with 0 as the parameter, we have:

-62E^+ — 02 E+ + + 0EO = m 2 E + + + p^rn? Iexp{-j6a)sin{0h) (D.16)

i.e.,

(02 + C f ) E + + = 0EO - Plm2lexp(-jea)sin(0h) (D.17)

where Cf = 02 + m 2

Similarly, taking the Fourier sine transformation of equation (D.13) with respect to y' with 0' as parameter, we have

- 0 2 E _ + + fl'2E.+ + /J*E 0 = m 2 £ _ + (D.18)

Hence

+ Cf)£_ + = £'E0 (D.19)

where C'| = t?2 + m 2

: .

Now, taking the inverse Fourier sine transformation of equation (D.17) with respect, to 0 we have

= E0exp( —C,y)

-•^ L/exp(-^a)|exp(-C 1 | / i-j/|)-exp(-C,|/ l+y | ) j (D.20)

Similarly, taking the inverse Fourier sine transformation of eqution (D.19) with respect to fl' we have

F_+ = E0exp(-C2y) (D.20)

By taking the derivative of Z? +* with respect to y and the derivative of E_+ with respect to y.

- no -

and substituting in equation (D.l 5), we obtain the value of E0 as follows:

— P\fn f /exp( — j6a)exp{—Cxh) ui

(C, + :—<7a) u2

(D.22)

Substituting E0 in equation (D.20) and taking the inverse complex transformation with respect to 0, we obtain the value of E + + as follows:

.2/ - [expf-C,| h-y | )-exp(-C 2| h +y |)]

2 C ,

exp{-C1\h+y\)

C. + — C2

exp(j6\ i - a | )d0 (D.23)

Similarly, substituting E0 in equation (D.21) and taking the inverse transformation with respect to 6, we obtain the value of E. + as follows

Pimfl "r exp{— C^y — C1h}exp(j6\ x — a\)d6 E-+ = ~ J

2n Ci + — C 2

U2

(D.25)

Now that we have derived the equations for the electric field strengths in the air and in the earth due to an overhead conductor, we will turn our attetion to the case of an underground conductor.

Electric Field Strength in the Air and in the Earth due to Current Carrying Filament Buried in the Earth.

Let

= E--Z~ Electric field strength in the earth due to the current-carrying filament buried in the earth.

E+. = is + _/= Electric field strength in the air due to the current-carrying filament buried in the earth.

Similar to equations (D.7) and (D.8), Maxwell's equations for electromagnetic wave propagation in air and earth respectively, for this case are given by,

d 2 £ + _ d2E+

dx5 By 2 = m , E , . y ^ O , (D.26)

- I l l -

H y ? (x.y)

x _ i

• (x,y') . p.- W)

Earth

Figure D.2 - Current Carrying Filament Buried in the Earth.

32E__ 82E__ , , . + — = m | £ _ _ + p2m2fS(x — a)5(y — h ) ax1 ay (D.27)

Similar to the procedure used in the derivation of fields and we solve for and

such that they satisfy the boundary conditions given by equation (D.9) through (D.l 1). Hence we have

E+. = p2m2I " e x p { — Cly — C2h)

2TT -»- — C 2

?2

•xp{j6\x-d\ )d0 (D.28)

£•_ _ .= -p2m2I

2n

[ e x p ( - C 2 | h'-y'\ )-exp(-C 2| h'+y'\ ]

2C,

exp(-C 2 | / i ' - r V | )

C, + —C2

»2

exp(jO \ x-a'\ )d8 (D.29)

If we assume that the relative permeability of air and earth are the same, i.e., uTl = u,2 then

- 112 -

ril = fi2 and we can show that the equations derived for E+ + ,E_ + ,E^_ and E+_ are the same as those derived by Pollaczek [lj.

Now using the standard results

i \ c exp{—aV^-+m2}exp . . . , • ' _ • . K0(mr)= J y x 9 \ y(jis)d3 (D.30)

2Vr+m2

where r = and where K0 is the modified Bessel function of the 2nd kind and of the zeroth order, we can write E+ + and E__ as follows:

E „ = pm,2/ (

+ / ; = ( j t f | x - o » | )rfg

\A2 + rnf + \ / V +

where /?, ~ V {T - af+(h - y?, Zx = V ( z - a ) 2 + (h+yf

pm2I (

— — j A ' o ( m 2 ^ 2 ) — KG(m2Z2)

r exp{-|j/+/i V^+m22}exP( + / . (j*l z - a |)««

— \A2 + m? + \A2 + ™ 2 (D.31(a,b))

where /?2 .=• V ( z - o ] ' + (A'=7P, Z 2 = V(z-a') 2 + (A'+^j2

Further using assumption 4 in Chapter 2, we can neglect the displacment current up to a fairly high frequency, and also noting the fact that the resistivity of air, i.e., p, is very large, we can conclude that the term mf ~0. This produces the final equations:

7(011 aI ( E++ = —UniZM

+ / ^ - ^ ^ " ^ ^ e x n l ^ l z - a l ^

— \ e\ + y/e2 + m2

E_+ = - joidol - exp{-A I 0\ -y'Ve2 + rn2} J w exp(;6l| z-a] )d8

2n |*| + \/e2+m2

- 113 -

2TT

1 ( -\l<0(m2R2)-K{

,{m2Z2)

e x p l - b ' + A ' l V<?2+m|}

\e\ + V̂ +̂ I exp(j'tl| i - a ' | </0

jco/ip/ ; exp{-y | 6\-h\/e2 + m22}

E + _ = / e x p ^ | x - a | ) d *

2* — |*|. + V^+m|

(D.32(a.b,o,d))

- 114 -

[6

[7

[8

[9

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- 115 -

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- 116 -

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