Understanding Insulation Coordination - University of · PDF fileUnderstanding Insulation...

Alberta Power Industry Consortium & University of Alberta

Professional Development Course

Understanding

Insulation Coordination

Organized

By Alberta Power Industry Consortium & University of Alberta

AESO AltaLink ATCO Enmax Epcor FortisAlberta

Instructed

By

Dr. David Peelo

May 13 & 15, 2014

Calgary & Edmonton, Alberta, Canada

i

Abstract

Insulation coordination is fundamental to the design and operation of power systems. In turn,

overvoltage limitation is an important part of insulation coordination. Means for overvoltage

limitation have evolved from rod gaps through gapped type surge arresters to the metal oxide

surge arresters of today. This course will discuss the basic principles of insulation coordination

and will then cover arrester application and selection. The course is intended for APIC company

staffs who are not specialized in insulation coordination but want to gain an adequate

understanding on the basics of insulation coordination and surge arrester application. It is hoped

that the knowledge will help them to better appreciate the challenges and requirements of

insulation coordination and equipment protection, which, in turn, will facilitate the execution of

engineering projects involving multiple technical subjects including insulation coordination for

T&D equipment.

ii

Confidentiality Requirement

This course material was prepared by the University of Alberta for the ultimate benefit of the

Alberta Power Industry Consortium members (hereinafter called “SPONSORS”). It may contain

confidential research findings, trade secrets, proprietary materials (collectively called

“Proprietary Information”). The term Proprietary Information includes, but is not limited to,

plans, drawings, designs, specifications, new teaching materials, trade secrets, processes,

systems, manufacturing techniques, model and mock-ups, and financial or cost data.

The document is made available to the sponsors only. The Sponsors will use all reasonable

efforts to treat and keep confidential, and cause its officers, members, directors, employees,

agents, contractors and students, if any, (“Representatives”) to treat and keep confidential, and

Proprietary Information in the document and the document itself. This course material shall not

be disclosed to any third party without the consent of the Alberta Power Industry Consortium.

Disclaimer

This document may contain reports, guidelines, practices that are developed by the University of

Alberta and the members of the Alberta Power Industry Consortium (APIC).

Neither the APIC members, the University of Alberta, nor any of other person acting on his/her

behalf makes any warranty or implied, or assumes any legal responsibility for the accuracy of any

information or for the completeness or usefulness of any apparatus, product or process disclosed,

or accept liability for the use, or damages resulting from the use, thereof. Neither do they

represent that their use would not infringe upon privately owned rights.

Furthermore, the APIC companies and the University of Alberta hereby disclaim any and all

warranties, expressed or implied, including the warranties of merchantability and fitness for a

particular purpose, whether arising by law, custom, or conduct, with respect to any of the

information contained in this document. In no event shall the APIC companies and the University

of Alberta be liable for incidental or consequential damages because of use or any information

contained in this document.

Any reference in this document to any specific commercial product, process or service by trade

name, trademark, manufacture, or otherwise does not necessarily constitute or imply its

endorsement or recommendation by the University of Alberta and/or the APIC companies.

iii

About the Alberta Power Industry Consortium:

The Alberta Power Industry Consortium consists of six Alberta utility companies (AESO,

AltaLink, ATCO, Enmax, Epcor and FortisAlberta) and the University of Alberta. Established in

the fall of 2007, its goal is to bring Alberta power companies together, with the University of

Alberta as the coordinating organization, to solve technical problems of common interest, to

produce more power engineering graduates, to support the professional development of current

employees, and to promote technical cooperation and exchange in Alberta’s power utility

community.

iv

About the instructor:

Dr. David Peelo, P.Eng., is an independent consultant. He graduated from University College

Dublin in 1965 and worked first for the ASEA Power Transmission Products Division in

Sweden. He joined BC Hydro in 1973 where he rose to the position of specialist engineer for

switchgear and switching and took early retirement in 2001 to pursue a second career as a

consultant. In 2004 the Eindhoven University of Technology awarded him a PhD for original

research on current interruption using air-break disconnect switches. He has published more than

60 papers on switching and surge arrester application and is actively involved with IEEE, CIGRE

and IEC. He is a past convener of IEC Maintenance Team 32 Inductive Load Switching and IEC

Maintenance Team 42 Capacitive Current Interrupting Capability of Disconnectors and a

member of the Canadian IEC National Committees for switching equipment and for surge

arresters. He is the author of a textbook on current interruption transients calculation and a co-

author of a textbook on switching in power systems both due for publication in 2014.

v

Course Outline

1. Basic principles of insulation coordination

Overvoltages in power systems

The concept of insulation levels

The role of surge arresters

Simplified approach to insulation coordination

2. Power system overvoltages

Origins of overvoltages: lightning, switching and temporary overvoltages

Characteristics of overvoltages in power systems

Traveling waves

3. Metal oxide surge arresters

Evolution of overvoltage protection devices

The design of metal oxide surge arrester

Characteristics and applications of metal oxide surge arrester

4. Surge arrester standards

Evolution of surge arrester standards

IEC versus IEEE standards

Which is the recommended standard to follow?

5. Arrester application in substations

Types and characteristics of incoming surges

Distance effects

Arrester selection

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© DF Peelo & Associates 2014

APIC 2014

Understanding Insulation Coordination

David PeeloDF Peelo & Associates Ltd.


APIC 2014

Basic principles of insulation coordination

• Overvoltages in power systems• Concept of insulation levels• Role of surge arresters• Simplified approach to insulation coordination

2

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APIC 2014

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What is insulation coordination?

IEEE definition: The selection of insulation strength consistent with expected overvoltages to obtain an acceptable risk of failure

IEC definition: The selection of dielectric strength of equipment in relation to voltages which can appear on systems for which the equipment is intended and taking into account the service environment and the characteristics of available protective devices

4

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5

Types of overvoltages and their causes:

6

Insulation types and strengths:

• External insulation

• Internal insulation

• Self-restoring insulation

• Non self-restoring insulation

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APIC 2014

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8

Internal insulation - transformers

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Insulation Coordination Course

9

Insulation strengths:

• Atmospheric air: statistical based on overvoltage type and insulation terminal configuration

• Liquids and gases: deterministic or statistical based on type of equipment


10

Overvoltage limitation:

• Air gaps: dependent on electrode configuration and type of overvoltage

• Silicon carbide surge arresters: voltage limited rating basis

• Metal oxide surge arresters: energy absorption limited rating basis

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11

Basic definitions:

Terms that we will encounter throughout the various part of the course


12

Nominal voltage of a system: A suitable approximate value of voltage used to designate or identify a system: example 230 kV system.

Highest voltage of a system: The highest value of operating voltage which occurs under normal operating conditions at any time and at any point in the system: 245 kV on 230 kV system.

Highest voltage for equipment (Um): The highest rms value of phase-to-phase voltage for which the equipment is designed in respect of its insulation as well as other characteristics which relate to this voltage in the relevant equipment Standards

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13

Isolated neutral system: A system where the neutral point is not intentionally connected to earth (ground)

Solidly earthed (grounded) neutral system: A system where the neutral point or points are directly connected to earth (ground)

Impedance earthed (grounded) system: A system whose neutral points are earthed (grounded) through impedances to limit fault current


14

Earth (ground) fault factor: The ratio of the highest power frequency voltage on an unfaulted phase during a line-to-earth (ground) fault to thephase-to-earth (ground) power frequency voltage without the fault

Overvoltage: Voltage, between one phase and earth (ground) or between two phases, having a crest value exceeding the corresponding crest of the highest voltage of the system. Overvoltages may be classified by shape or duration as either temporary or transient

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Performance criterion: The basis on which the insulation is selected so as to reduce to an economically and operationally acceptable level the probability that the resulting voltage stresses imposed on the equipment will cause damage to the equipment insulation or affect continuity of service. This criterion is usually expressed in terms of an acceptable failure rate of the insulation configuration


16

Examples of performance criteria

Transmission lines:

2 lightning flashovers per 100 km-years exposure

1 switching surge flashover per 100 switching operations

Substations:

Generally, station reliability criteria is 10 times line criteria

Also, transformers and other non-self restoring insulation equipment arrester protected due to failure consequences

Air insulated stations: MTBF of 50 to 200 years

Gas insulated stations: MTBF up to 800 years due to failure consequences

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17

Withstand voltage: The value of the test voltage to be applied under specified conditions during which a specified number of disruptive discharges may be tolerated:

• conventional assumed withstand voltage: number of disruptive discharges tolerated is zero and corresponds to a probability of withstand Pw = 100%

• statistical withstand voltage: number of disruptive discharges tolerated is related to a specified withstand probability and is specified at Pw = 90%


18

Representative overvoltages (Urp): Overvoltages assumed to produce the same dielectric effect on the insulation as overvoltages of a given class occurring in service due to various origins

Coordination withstand voltage (Ucw): For each class of voltage, the value of the withstand voltage of the insulation configuration, in actual service conditions, that meets the performance criterion

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Insulation coordination procedure

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20

Required withstand voltage (Urw): The test voltage that the insulation must withstand in a standard withstand test to ensure that the insulation will meet the performance criterion when subjected to a given class of overvoltages in actual service conditions and for the whole service duration

Standard withstand voltage (Uw): The standard value of the test voltage applied in a standard withstand test. It is the rated value of the insulation and proves that the insulation complies with one or more required withstand voltages

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21

Coordination factor (Kc): The factor by which the value of the representative overvoltage must be multiplied in order to obtain the value of the coordination withstand voltage

Atmospheric correction factor (Ka): The factor to be applied to the coordination withstand voltage to account for the difference between the average atmospheric conditions in service and the standard reference atmospheric conditions. The factor applies only to external insulation


22

Standard reference atmospheric conditions: The standard reference atmospheric conditions are:

• temperature to = 20°C

• pressure bo = 101.3 kPA (1013 mbar)

• absolute humidity hao = 11 g/m³

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Safety factor (Ks): The factor to be applied to the coordination withstand voltage, after application of the atmospheric correction factor (if required), to obtain the required withstand voltage, accounting for all the differences between the conditions in service and those in the standard withstand test

Test conversion factor (Kt): The factor to be applied to the required withstand voltage, in the case where the standard withstand voltage is selected of different shape, so as to obtain the lower limit of the standard withstand test voltage that can be assumed to prove it


Insulation coordination procedure

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Insulation coordination simplified approach

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27Source IEC 62271-1


28Source IEC 62271-1

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References:

1. IEC and IEEE insulation coordination standards noted earlier.

2. IEC 60099-4 Surge arresters – Part 4: Metal-oxide surge arresters without gaps for a.c. systems.

3. IEC 60099-5 Surge arresters – Part 5: Selection and application recommendations.

4. Insulation Coordination for Power Systems (Book), A.R. Hileman, Marcel Dekker, Inc 1999.

5. Insulation Coordination for High-Voltage Electric Power Systems (Book), W. Diesendorf, Butterworth & Co. 1974.

6. Surge arrester manufacturer websites: ABB and Siemens in particular have comprehensive selection and application guides.


Power system overvoltages

• Lightning, switching and temporary overvoltages• Characteristics of overvoltages in power systems• Travelling wave basics• How do surge arresters work?

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Spectrum of power system overvoltages:


32

Overvoltage class: transient

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33

Overvoltage class: low frequency continuous and temporary


34

Temporary overvoltages: causes and characteristics

1. Ferranti effect:

Steady state voltage (V2) at the open receiving end of an uncompensated line (no shunt reactors) is always higher than the voltage (V1) at the sending end; occurs because the (leading) capacitive charging current flows through the series inductance of the line

where L is the line length in km and B the phase constant (7.2 degrees/100 km at 60 Hz)

)BLcos(1

VV

1

2 =

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35

Ferranti effect

V1 and V2 sending and receiving end voltages.

1: no compensation 2: 50% series capacitor compensation 3: 50% series capacitor and 70% shunt reactor compensation.


36

2. Faults:

A line to ground fault represents a typical example of a temporary undamped overvoltage that may be sustained on the unfaulted phases for up to hundreds of milliseconds

The magnitude of the overvoltages on the unfaulted phases depends on the shift of the electrical neutral caused by the fault – earth fault factors

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37

SLG EFFs:

Maximum line to ground voltage at any fault location and under any fault condition for effectively grounded system. Numbers on curves are maximum line to ground voltages on any phase in percent of line-to-line voltage.


38

Earth fault TOVs:

Category EFF (pu) Duration

Grounded• Network, high SC*• Line radial line, low SC

1.2 – 1.41.2 – 1.5

1 s1 s

Resonant grounded• Network or mesh• Long radial line

1.731.2 – 1.5

8 hours – 2 days1 s

Isolated/Ungrounded• Distribution with O/H lines• Industrial with cable

1.73 1 – 2 s(with fault clearing)

SC – short circuit

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3. Load rejection:

Load rejection occurs when a remote circuit breaker on a transmission line carrying a substantial load is opened due to system condition or an error. A voltage rise follows because:

• the reduced current means a lower voltage drop across the internal system impedance

• generators tend to overspeed to produce higher voltages


40

Load rejection overvoltages:

Category Magnitude Duration (s)

Load rejection• in a system• generator-transformer

− steam turbine− hydro

1.05

1.1 – 1.41.15 – 1.5

> 10

11

Note: Load rejection may be combined with the occurrence of a fault producing a combined effect

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4. Resonance:

Resonance in power systems can take two forms:

• linear resonance when inductive and capacitive elements in series form a series resonant circuit; for example, an unsaturated transformer and a shunt capacitor bank

• ferroresonance when saturated iron-cored inductive and capacitive elements in series form a series circuit; for example, the grading capacitors on an open circuit breaker in series with a magnetic PT


42

Overvoltages associated resonance situations can be very significant and rapid intervention is necessary

Resonance Magnitude (pu) Duration (s)

Unsaturated phenomena < 2 ≤ 0.5

Saturated phenomena 2 – 3+ 0.5 – 10

Coupled circuits 3 – 5+ 0.5 – 10

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TOVs due to Magnitude (pu) Duration

Line energization and re-energization ≤ 1.5 < 1 s

Stuck breaker pole ≤ 2 Steady state

Backfeeding ≤ 2 Seconds

5. Other TOV situations:


44

Interesting facts about TOVs:

1. TOVs DO NOT CONTRIBUTE TO INSULATION DESIGN…HOWEVER

2. TOVs DETERMINE THE VOLTAGE RATINGS OF THE METAL OXIDE ARRESTERS TO BE APPLIED ON THE SYSTEM

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45

Slow front switching surges:

• Line switching

• Making and breaking reactive currents

• TRVs across circuit breakers


46

Line switching:

Due to travelling wave effects, line switching can result in overvoltages of significant magnitude. Worst case is re-energization of a line with a full DC trapped charge

The resulting switching surges are slow-front with times to peak in the order of hundreds of microseconds

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47

Switching surge overvoltage distribution with various control measures


Voltage profile on line

48

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49

Typical switching overvoltage magnitudes:

Condition Magnitude (pu)

Energizing discharged line 1.5 – 2.0

Re-energizing with no overvoltage control 3 – 3.4

Re-energizing with pre-insertion resistors 2 – 2.2

Re-energizing with controlled closing 1.2 – 1.7


50

Making and breaking reactive currents:

• Inductive current switching

transformer magnetizing current

shunt reactors at EHV, HV and MV

• Capacitive current switching

shunt capacitor banks

lines and cables

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51


52

Fast front overvoltages:

Fast front overvoltages are those due to lightning and have the dimensioning role at system voltages below EHV levels (at EHV levels switching surges have the dimensioning role)

The overvoltages are characterized by fast rise times of 0.1 to 20 µs and associated in the range 5 to 200 kA.

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Very fast front overvoltages:

Very fast front overvoltages are generally associated with gas insulated switchgear (GIS) applications and in particular with the live operation of disconnect switches

The overvoltage are not dimensioning with respect to the insulation coordination of GIS; however, present standards for disconnect switches now incorporate very severe tests to demonstrate that live operation of the switches does not compromise the GIS insulation structure


54

What is lightning?

• Lightning is the breakdown of atmospheric air; in dry air at sea level breakdown voltage is 30 kV/cm but, at higher altitudes in a region filled with water droplets, it is about one-third this value

• Lightning strikes are not instantaneous starting as ‘local’ events and progressing in steps to the other electrode which could be earth (ground) or another cloud

• Lightning strikes are very statistical in terms of current magnitude, number of strokes per strike and striking point

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55

Global electric circuit


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58

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60

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Travelling wave basics:Fundamental circuit for travelling waves

Vi = incident voltageZi = surge impedance of ViZt = surge impedance of VtVr = reflected voltage = αVi where α = (Zt – Zi)/(Zt + Zi)Vt = transmitted voltage = βVi where β = 2Zt/(Zt + Zi)

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Voltage waves:

CaseSurge

Impedance

VrReflected Voltage

VtTransmitted

Voltage

No change Zi = Zt 0 Vi

Open circuit Zt >> Zi Vi 2 Vi

50% reduction Zt = 0.5 Zi -0.33 Vi 0.67 Vi

Short circuit Zt = 0 -Vi 0

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Surge current (directional): I = V/Z

CaseSurge

ImpedanceIr

Reflected CurrentInet

(Zi side)

No change Zi = Zt 0 Ii

Open circuit Zt >> Zi Ii 0

50% reduction Zt = 0.5 Zi -0.33 Ii 0.67 Ii

Short circuit Zt = 0 -Ii 2 Ii


66

CO operation on 100 km unloaded transmission line

Sending end

Receiving end

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67

We can now consider the principle of overvoltage surge protection using arresters:

V1 V2

V3

Z1 Z2

Z3

I3

V2 = V3


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We can write:

32 VV =

32

32T ZZ

ZZZ where+

=

1T1

T VZZ

Z2

+

=

( )( )

++

+=

3221

32212 ZZ1ZZ

ZZ11ZV2V

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69

If no arrester, Z3 = ∞

If Z2 also ∞ as at open-ended line or transformer

132 V2VV == (voltage doubling)

21

2132 ZZ

ZV2VV+

==


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We can also write:

If ideal arrester (short-circuit), Z3 = 0 and

113 ZV2I =

33323 ZVZVI ==

+

+

+

=32

321

32

21 ZZ

ZZZZZ

ZV2

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We now have the two extreme points on the circuit load line:

( )31121

22 I ZV2

ZZZV −

+

=

V2

I3

2V1*Z2

Z1+Z2

2V1

Z1


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Add the arrester VI-characteristic:

( )ar1121

2ar I ZV2

ZZZV −

+

=

V2

I3

2V1*Z2

Z1+Z2

2V1

Z1

Var

Iar

Arrester VI-characteristic

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73

Load line for transformer case: Z2 = ∞

ar11ar I ZV2V −=

V2

I3 2V1

Z1

Var

Iar

Arrester VI-characteristic

2V1


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Question: how does travelling wave theory treat the effect of a surge arrester in clamping the voltage to the protective level at its location?

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APIC 2014

Metal oxide surge arresters

• Evolution of overvoltage protective devices• Design of metal oxide surge arresters• Characteristics and application of metal oxide surge

arresters

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History of surge arresters:

• 1898 – rod gap design• 1908 – electrolyte arrester with non-linear resistive

element to limit follow current and enable arcinterruption

• 1930 – silicon carbide arresters with silicon carbideresistive elements in series with gaps

• 1957 – silicon carbide arresters with current limitinggaps

• 1976 – metal oxide surge arresters with extremenon-linear characteristics

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Structure of metal oxide surge arresters:

• Consist simply of zinc oxide varistor disks of varying sizes connected in series

• Zinc oxide varistors are ceramic semiconductor devices with verynon-linear voltage-current characteristics and very good energy absorption capability


APIC 2014

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Zinc oxide disks are about 90% zinc oxide (ZnO) by weight plus various other metal oxides to provide specific properties – for example:

bismuth + CaO, CoO, BaO, SrO, MnO:non-linearity of the VI characteristic

K2O: inhibits grain growth

Cr2O3: enhances thermal stability

Ga2O3: increases exponent (α) of VI characteristic

Sb2O3: grain grown enhancer

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Varistor microstructure key elements:

• Zinc oxide grains

• Bismuth-rich intergranular layer

• Spinel grains

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Varistor microstructure:

Source: ABB

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VI characteristic: three conductive regions

0

0.5

1

1.5

2

2.5

1E-05 0.0001 0.001 0.01 0.1 1 10 100 1000 10000 100000

Current (A)

Per u

nit p

eak

rate

d vo

ltage

Region 1 Region 2 Region 3

LIPL

SIPLRated voltage

MCOV

25 C 150 C

Resistive leakage current


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Region 1: pre-breakdown region is the low current region associated with steady state operation; material resistivity temperature dependent with negative temperature coefficient

Region 2: breakdown region is the highly non-linear region associated with TOVs and switching surges; very small temperature dependence and exponent α = 30 to 50

Region 3: high current region is the region associated impulse currents > 1 kA due to lightning;non-linearity much less than in the breakdown region

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Current conduction: equivalent circuit


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Pre-breakdown: in this region current conduction is determined by the high “resistance” associated with grain boundaries with a significant temperature dependence

Breakdown: at a certain applied voltage across grain boundaries, the intergranular layer “resistance” drops allowing a large increase in current; energy is therefore being absorbed in the intergranular layer

High-current: the zinc oxide grain resistance dominates in this region

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Comparison: metal oxide versus gapped silicon carbide arresters – fundamentally different:

• Metal oxide arresters are rated on the basis of ability to absorb energy and maintain thermal stability at rated voltage followed by MCOV

• Gapped silicon carbide arresters are rated on the basis of ability to reseal – interrupt follow current – after discharging a lightning or switching surge

86

Grading resistors and capacitors

Active gaps

SiC resistors

SiC gapped arresterZnO arrester

Courtesy of ABB

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87

88

Rated voltage and energy considerations: metal oxide arresters are energy (temperature) limited

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89

Effects of aging: over lifetime

90

IEC 60099-4 specifies five (5) line classes:

0

1

2

3

4

5

6

7

1 1.5 2 2.5 3 3.5

Ures/Ur

Spec

ific

ener

gy (k

J/kV

Ur) Line class 1

Line class 2Line class 3Line class 4Line class 5

Former IEC rating basis now changed to a thermal energy rating and a repetitive charge transfer rating

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91

Former IEC rating basis now changed to a thermal energy rating and a repetitive charge transfer rating


APIC 2014

Thermal Energy Rating Wth

• Maximum specified energy given in kJ/kV of Ur that may be injected into an arrester within 3 minutes time duration without causing thermal runaway.

• This rating is verified in the revised operating duty test –above injection preceded by conditioning consisting of two high current impulses only.

• The rating is strictly thermal and no longer relates rated energy to protective levels; temperature coefficient for metal oxide material 0.33 ⁰C/J/cm3.

92

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Repetitive Charge Transfer Rating Qrs

• Maximum specified charge transfer capability of an arrester in the form of a single event or group of surges that may transferred through an arrester without causing mechanical failure or unacceptable electrical degradation to the MO resistors.

• The charge is calculated as the absolute value of current integrated over time. This is the charge that is accumulated in a single event or a group of surges lasting for no more than 2 seconds and which may be followed by a subsequent event at a time interval not shorter than 60 seconds.

93

94

TOV versus time curves:

0.7

0.8

0.9

1

1.1

1.2

1.3

0.1 1 10 100 1000 10000

Time (s)

TOV

fact

or k

tov

With prior energyWithout prior energyMCOV

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95

Overvoltages: types and shapes

Source: IEC 60071-1


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Energy absorption:

• TOVs: system is a voltage source

• lightning and switching surges: system is a current source

• For energy absorption studies, the minimum VI characteristics should be used

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97

Overvoltages and associated energies:

Type/Cause Magnitude Waveshape

EnergykJ/kV rated

voltage

Atmospheric Overvoltages:Lightning strikes or induced by lightning (multiple strikes may occur)

> 5 pu Front: 1 – 6 µsTail @ 50%: 50 µs 0.5

Switching Overvoltages:Line autoreclosing, switching capacitor banks, shunt reactors, issue more at EHV

2 – 4 pu Front: 30 – 300 µsTail @ 50%: 100 – 2000 µs 3 – 5

Temporary Overvoltages:SLG faults in ungrounded systems, Ferranti effect, loading shedding++ 1 – 1.5 pu Power frequency

(may be distorted)

Very high(fast remedial

action required)

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Protective and related characteristics: 192 kV arrester

0

100

200

300

400

500

600

700

1E-05 1E-04 0.001 0.01 0.1 1 10 100 1000 10000 1E+05Current (A)

Vol

tage

(kV

peak

)

MCOV 218 kV peakRated voltage 272 kV peak

SIPL 384 kV peak

LIPL 470 kV peak

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Because we are dealing with a VI characteristic, the respective protective levels are defined at standard normal discharge currents. For lightning protection:

System voltageStandard nominal discharge current*

(8/20 µs)

Distribution 5 kASub-transmission up to 72.5 kV 5 or 10 kA

72.5 to 245 kV 10 kA245 kV and up 10 kA or 20 kA≥ 500 kV 20 kA

* values also define so-called Arrester Classification


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For switching surges:

Arrester classification Peak currents*(A)

20 kA 500 and 2000

10 kA 250 and 1000

10 kA distribution class 125 and 500

* in switching impulse residual voltage test, higher numbers used to define the protective level

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101

VI characteristic: switching and lightning currents

0.60.70.80.9

11.11.21.31.41.5

1 10 100 1000 10000 100000

Current (A)

Volta

ge (p

u of

10

kA v

alue

)

4/10micros8/20micros36/90micros

102

Extract from ABB surge arrester catalog:

Guaranteed protective characteristics

Recommended for system

voltageRated

voltage

Max. cont. operating voltage (COV)

MCOV as per ANSI

testsTOV

capability for

Maximum residual voltage with current wave

Switching surges 8/20 µs

kVrms kVrms kVrms kVrms

1 skVrms

10 skVrms

1 kAkVcrest

2 kAkVcrest

3 kAkVcrest

5 kAkVcrest

10 kAkVcrest

20 kAkVcrest

40 kAkVcrest

EXLIM P-A and P-B245 180 144 144 209 198 351 362 371 392 414 452 497

192 154 154 223 211 374 386 396 418 442 482 530

198 156 160 230 218 386 398 408 431 456 497 547

210 156 170 244 231 409 423 433 457 483 528 580

219 156 177 254 241 426 441 452 477 504 550 605

228 156 180 264 251 444 459 470 496 525 573 630

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Energy absorption:

• TOVs: system is a voltage source

• lightning and switching surges: system is a current source

• For energy absorption studies, the minimum VI characteristics should be used


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Selection procedure:• Electrical characteristics

Rated voltage and MCOVNominal discharge currentThermal energy rating Wth

Repetitive charge transfer rating Qrs

Lightning and switching surge protection levels• Mechanical characteristics

Strike and creepage distancesShort-circuit withstandSeismic and tensile loads

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Selecting the housing: briefly the following applies

• Arresters are self-protecting and the housings do not require the same withstand capability as other station equipment

• Creepage distances should be the same as for all station equipment

• Pollution or exposure to salt contamination can require longer creepage distances but consideration should also be given to using higher rated voltage arresters or more appropriate arrester designs


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• Short-circuit testing is required on all arrester types over a range of current; for example, an arrester with a ratedshort-circuit current rating of 50 kA would also be tested at 25 kA, 12 kA and 600 kA

• Selection of the short-circuit current rating should be based on the maximum expected short-circuit current magnitude at the arrester location

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Insulation coordination – iterative process


108

Station layout considerations:

• Location of the arrester relative to the protected equipment is important and will be discussed later

• Location of the arrester relative to other energized equipment on the same phase also requires attention

• Question: how does the protected equipment ‘know’ that the arrester is ahead of it?

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Testing:

• Type testing is covered in great detail in IEC 60099-4 as is basic routine testing

• As noted earlier routine testing is at least as important as type testing; in making comparisons between arresters from different manufacturers, compare the tests performed on each block before considering award


Surge arrester standards

• Evolution of surge arrester standards• IEC versus IEEE standards• Which is the recommended standard to use

110

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111

IEEE and IEC differ in approaches:

• IEEE: rated voltage is the duty cycle ratingwhich is a holdover from gappedsilicon carbide arresters and has norelationship to absorbed energy

• IEC: rated voltage is a TOV to be withstoodfollowing absorption of defined energyunder specific circumstances


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IEEE rating basis: from IEEE C62.11-2005

3.29 duty cycle voltage rating: The designated maximum permissible voltage between its terminals at which an arrester is designed to perform its duty cycle.

8.14 Duty-cycle test

The purpose of the duty-cycle test is verify that the arrester can withstand multiple lightning type impulses without causing thermal instability or dielectric failure.

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Former IEC rating basis now changed to a thermal energy absorption rating


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Importance of routine testing:

• Absolutely no redundancy in metal oxide surge arresters: one bad block and the arrester will fail –guaranteed!

• Routine testing has therefore equal or arguably greater importance than type testing

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APIC 2014

Arrester application in stations

• Type and characteristic of incoming surges• Distance effects• Arrester selection

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Stations: insulation coordination

• Transmission lines are normally shielded except where the soil resistivity is such that it is not possible to achieve low tower footing impedances at a justifiable cost

• Lines, however, are typically shielded about 1 km or more out from the station in order to limit the occurrence, magnitude and steepness of incoming surges

• Two types of lightning related failures are of interest: shielding failures where a lightning strike occurs directly to the line conductor and back flashover (also known as a backflash) where the tower or ground wire is struck and a flashover then occurs to a phase conductor.

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Lightning strike to a phase conductor

Only close-in strikes are of interest because of the damping effect of corona on the traveling wave.

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Travelling wave damping due to corona

Conductors Zo(ohms) Kc (km-kV/us)

Single 450 700

2 Conductor Bundle 350 1000

3 or 4 Conductor Bundle 320 1700

6 or 8 Conductor Bundle 300 2500


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-500000

0

500000

1000000

1500000

0 10 20 30 40

Voltage Surge Distortion Due to Corona

Electrotek Concepts® TOP, The Output Pro

Vol

tage

(V

)

Time (us)

V@ 3.0 km V@ 4.0 km V@ 0.5 km V@ 1.0 km V@ 2.0 km

0.5 km1.0 km

2.0 km3.0 km

4.0 km

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Back flashover


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Lightning currents

Shielding failure: 4 kA or greater at 89% probability: 20 kA or greater at 80% probability.

Back flashover: 20 kA or greater at probability of 80%; 90 kA or greater at probability of 5%

Median current: 32 kA

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Station insulation coordination: determine the representative incoming lightning overvoltage.

Two approaches are possible:

1. Comprehensive approach: requires a computer study because of the complexity of the calculations.

2. Deterministic approach: a more general approach but does take into account the lightning stress severity of the station under study.


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Comprehensive approach flow diagram:

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Comprehensive approach - need to consider:

• Statistical (stochastic) variation of the lightning flash and stroke parameters

• Dependence of the flash strike point on these parameters• Response of the line to the lightning flash including the tower

footing impedance current dependence• Propagation of the surge from the strike point to the station

and the deformation effect of corona


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Station insulation coordination: deterministic approach

1. Determine the lightning performance of incoming transmission line based on parameters discussed earlier –ground flash density, line height etc.

2. Determine steepness of incoming surge based on the above and system fault tolerance.

3. Determine the protective zones based on selected arrester locations; may need a number of iterations and this type of study is usually done using EMTP simulations.

4. Transformer protection tends to get most attention but open breaker protection is also a consideration

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130

Insulation Coordination 101

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Basic case: arrester at transformer onlyCASE1>B-B1 (Type 1)

0.03300 0.03400 0.03500 0.03600 -200000

0

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Time (mS)

Voltage (V)

VB-B1V-CBV-JTN VSA-1 VTX


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33 35 37 39 41

Entrance Bus Voltages : Normal, Wih Ccvt, With SA2 & Ccvt

Electrotek Concepts® TOP, The Output Processor®

Vol

tage

(V)

Time (us)

Vb: normal Vb: With Ccvt Vb: With SA2 & Ccvt

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Transformer Voltages: Normal, With Ccvt, With SA2 & CCVT


Vol

tage

(V)

Time (us)

Vtxc: normal Vtx: With Ccvt Vtx: With SA2 & Ccvt


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Vsa : Normal, With Ccvt, With SA2 & Ccvt


Vol

tage

(V)

Time (us)

Vsa: normal Vsa: With Ccvt Vsa: With SA2 & Ccvt

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2000

4000

6000

8000

10000

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SA Currents: Normal, With Ccvt, With SA2 & Ccvt


Cur

rent

(A)

Time (us)

Isa: normal Isa: Ccvt Isa: SA2 & Ccvt


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Surge arrester application:

• Arrester selection – recap

• Arrester location relative to protected equipment

• Clearances associated with arresters

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Arrester selection based on:

• system voltage and grounding

• temporary overvoltages

• desired safety factor relative to the equipment withstand voltage

but its physical location relative to the protected equipment is also a consideration (why?)


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whereUpl = lightning impulse protective level of the

arrester (kV)S = steepness of incoming surge (kV/µs)L = d1 + dA + d2 + d (m)c = velocity of light (300 m/µs)

Simple estimation of protective distance:

cSL2UU plrp +=

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Example: 245 kV transformer, LIWL 850 kV192 kV rated surge arrester, Upl 440 kV, 1000 kV/µs

L = d1 + dA + d2 + d= 2 + 2.5 + 2.6 + d= 7.1 + d

Urp =

= 440 + 6.67 (7.1 + d)( )d1.7

30010002440 +⋅+


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0

100

200

300

400

500

600

700

800

900

0 10 20 30 40 50Distance d (m)

Urp

(kV)

Voltage attransformer850*0.85

850*0.6375

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Let’s take a look at what is really happening when a surge approaches a transformer protected at a certain distance by an arrester:

•Incoming surge steepness 1000 kV/μs

•Arrester with protective level of 800 kV installed at distance of 75 m from the transformer

•Remember the questions: how does traveling wave deal with arresters and their voltage limiting effect and how does the transformer know there is an arrester ahead of it?


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Arrester at distance 75 m and surge 1000 kV/µs

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APIC 2014

EMTP study result same case

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What conclusions can we draw?

Urp = Upl + 2ST for Upl > or = 2ST

Urp = 2Upl for Upl < 2ST

where T is the travel time of the lightning surge L/c.

Actual voltage at the transformer oscillates due the ‘arrival’ and ‘departure’ of the traveling waves but the maximum value is Urp.

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Disadvantages of simplified method:

1. Does not take the capacitance of transformers or reactors into account

2. Real VI characteristic of the arrester is not used

3. Does not take the effect of the power frequency voltage into account


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Switching surges within stations:

• Are ‘man-made’ events and generally more an issue for lines than for stations; however, station equipment and clearances must be capable of withstanding the applicable rated switching surge withstand voltages and surge arresters must provide the corresponding overvoltage protection. Equipment standards fix a certain ratio between rated lightning and switching impulse voltages as we have seen earlier.

• Surge arresters are applied to limit switching overvoltages within stations for the switching of shunt capacitor banks. This is a special case usually requiring a computer study to determine energy requirements in particular but some generalization is possible. Key parameters are discussed in the following.

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Clearances: two types of clearances to consider

• Station air clearances

• Surge arrester clearances

to ground

to energized equipment on same phase


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Statistical nature of electrical breakdown

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Statistical nature of electrical breakdown (cont/d)

• Applied voltage must exceed critical value (field strength) where cumulative ionization is possible

• Statistical time lag ts from application of voltage to the time of creation of the first free electron – time lag decreases with increasing voltage

• Once electron found must further generate a streamer, then streamer converts to a spark – formative time lag tf


150

Statistical nature of electrical breakdown (cont/d)

• Vs must be high enough for ionization to occur and sustained for duration longer than the total time lag

• Total time lag ts+tf not the same for each voltage application – same voltage waveform may or may not cause breakdown and therefore it is a probabilistic event

• Question: what does this mean for lightning impulses and switching surges?

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Influencing factors on breakdown:

• Waveshape: breakdown voltage dependent on impulse voltage and its profile over time

• Gap configuration: more of an influence on switching impulses than lightning impulses

• Polarity: breakdown voltage lowest for positive impulses


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Influence of waveshape: lightning impulses:

• Lightning impulses represented by a standard1.2/50 µs waveshape

• Duration of voltage around the peak does not give enough time for leaders to develop and breakdown is dependent on streamers only

• Breakdown (aka sparkover) voltage Vs given by:

Vs = Esd

where Es is the electric field gradient and d is the gap length

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• Rod-plane gaps have the lowest breakdown voltages for all gap configurations; for positive standard lightning impulses for such gaps 1 to10 m (IEC 60071-2):

U50RP = 530d

U50RP is the 50% probability of flashover voltage in kV crest

d is the gap spacing in m


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• For other gap configurations such as rod-rod, U50 can be as high as 700d (rod-plane and rod-rod gaps tend to represent two extremes)

• For rod-plane gaps up to 6 m and negative standard lightning impulses:

U50RP = 950d0.8

and can be as low as 700d0.8 for other gaps

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0

1000

2000

3000

4000

5000

6000

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0 2 4 6 8 10

Gap length (m)

U50

flas

hove

r val

ue (k

V cr

est)

Rod-plane positiveRod-rod positiveRod-plane negative


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Important notes for lightning impulses:1. Gap factors (to be discussed for switching impulses) are

generally not directly applicable

2. Because positive impulses give the lowest breakdown values does not mean that negative impulses can be ignored; most lightning surges have negative polarity and internal insulation (e.g. in a transformer) has lower withstand for negative impulses

3. Above-noted equations apply at sea level

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Influence of waveshape: switching impulses• Switching impulses represented by a standard rise time of

250 µs and a time to half-value of 2500 µs

• Breakdown is dependent on gap configuration and hence the use of gap factor

• Breakdown also dependent on voltage rise time giving so-called U-curves


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Rod-plane gaps: K = 1

0

500

1000

1500

2000

2500

3000

3500

4000

0 5 10 15 20 25

Gap length (m)

U50

flas

hove

r val

ue (k

V cr

est)

EdF

H arbec-M enemeniis

IEC 60071

M eek-C raggs

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So what is the gap factor? The gap factor represents the influence of the electrode configuration

* dependent on length of grounded rod (5 m in this case)

Configuration Gap factor

Rod-plane 1

Conductor-plane 1.15

Vertical rod-rod 1.4*


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* IEC 60071-2

Configuration α = 0.5 α = 0.33Ring-ring 1.8 1.7

Crossed conductors 1.65 1.53Conductor-conductor 1.62 1.52

Supported busbars (including fittings) 1.5 1.4

Gap factors for phase-to-phase configurations*:

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Lightning versus switching impulse: rod-plane and rod-rod gaps:

0

500

1000

1500

2000

2500

0 1 2 3 4 5

Gap length (m)

U50

flas

hove

r val

ues

(kV

cres

t)

IEC 60071

Meek-CraggsIEC*1.4Meek-Craggs*1.4Lightning


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0

500

1000

1500

2000

2500

0 0.5 1 1.5 2 2.5 3

Gap length (m)

U50

flas

hove

r val

ues

(kV

cres

t)

IEC 60071Meek-CraggsIEC*1.4Meek-Craggs*1.4

Lightning

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Arrester minimum clearances phase to ground:

• Because arresters are self-protecting lower minimum clearances than station minimum clearances can be used

• Consult the appropriate manufacturer catalog or instruction manual for their recommended minimum clearances


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Arrester rated voltage(kV rms)

Minimum clearance phase-to-ground (mm)for

Mfr 1 Mfr 2 Mfr 3 Mfr 4 Mfr 512 92 115 216 127 9215 118 115 216 147 12221 168 165 229 190 15424 168 165 279 216 18260 448 505 483 407 41666 499 505 - 457 45872 549 505 508 508 500

120 829 870 914 813 832132 905 1140 965 889 916144 981 1140 1168 991 998192 1270 1345 1448 1334 1198216 1473 1465 1854 1611 1292258 1599 1925 2337 1842 1612396 - 2270 3530 2921 2475

Surge Arrester ManufacturerRecommended Minimum Clearances Phase-to-Ground

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Surge arrester clearances to energized equipment on the same phase:

• Energized equipment on the same phase includes transformer bushings (arrester on an outrigger), CVTs or PTs, post insulators and so on

• Arrester voltage distribution determined by grading rings and proximity to other equipment on same phase really only an issue where pollution is a consideration; however what about tight fit scenarios?


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Distribution system arrester practices:

• On overhead systems arresters applied at the cable riser pole from substation and possibly on pole-top transformers in very lightning prone areas

• On underground systems, arresters applied at the cable riser pole and at open points in the underground circuit; other measures may include paralleling arresters at the riser pole or applying a surge arrester at next pole with or without a groundwire

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TWENTY-ONE QUESTIONS

1. What are the voltages considered in insulation coordination?

2. Which overvoltage determines the rated voltage of the surge arrester to be used?

3. Metal oxide material consists of zinc oxide plus various additives: which additive gives the non-linear VI characteristic?


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4. Which four points are used to define the VI characteristic of the required surge arrester?

5. Which quantities define arrester energy handling capability?

6. Which test defines the relationship between arrester rated voltage and rated thermal energy?

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7. What determines the withstand capability of the arrester housing?

8. How is the required creepage distance for the arrester housing selected?

9. How should the required pressure relief capability for the arrester be selected?


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10. With respect to lightning, what representative quantity is used for station insulation coordination?

11. If the required protective levels are not met, what additional measures can be applied to resolve it?

12. Why is there no distance effect for switching surges?

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13. What determines the required clearances for surge arrester installation?

14. Why is the separation distance arrester to transformer so important?

15. What is the effect on added capacitance on incoming surges?


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16. What are the characteristic waveshapes for lightning and switching surges?

17. What is a gap factor?

18. Which electrode configuration has the lowest switching surge withstand capability?

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19. Rated lightning withstand capability for a transformer is deterministic: what is the expected probability of withstand?

20. Rated lightning withstand capability for a circuit breaker is probabilistic: what is the expected probability of failure?

21. More questions?


Thanks for listening!

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