Jaser 120017

Int. Journal of Applied Sciences and Engineering Research, Vol. 1, No. 2, 2012 www.ijaser.com

© 2012 by the authors – Licensee IJASER- Under Creative Commons License 3.0 [email protected]

Research article ISSN 2277 – 9442

�� 165

*Corresponding author (e-mail: [email protected])

Received on, Feb, 2012; Accepted on Feb. 2012; Published on Feb 24, 2012

Simulation and measurement of the voltage distribution on high

voltage suspension Porcelain insulator string under pollution

condition Mohammad Bagher Asadpoor, Mohammad Mirzaie

Babol University of Technology, Department of Electrical & Computer Engineering, P. O. Box 484,

Babol, Iran

doi:10.6088/ijaser.0020101017

Abstract: Satisfactory operation of suspension insulator strings is intimately related to the voltage

distribution of the string. At operational voltage, the voltage distribution along insulator string is affected

by stray capacitances, which causes a non-uniform voltage distribution. In addition, the performance of

insulator changes by the accumulation of environmental pollution over its surface that will deteriorate by

absorption of moisture sharply. In order to explain this dilemma clearly, a full equivalent circuit, which

takes into account the insulator material properties and the stray capacitances, is derived from the Finite

Element Method Based Software, which is implemented in the EMTP-ATP package to calculate the

voltage distribution, power loss and the leakage current, flowing through the string with and without

pollution under power frequency voltage. Finally, the simulation results of voltage distribution have

been compared with experimental results, which were obtained in the High Voltage Laboratory. In

experimental procedure, an attempt to form the pollution on the surface of the insulator has been carried

out. For this investigation the dry pollutant on the insulator surface has been approached according to

IEC60507, which has led to the resistance of pollutants. Then, the voltage distribution of clean and

polluted insulator strings has been measured by sphere gap method.

Keywords: Insulator, Pollution, FEM, EMTP-ATP, Sphere Gap.

Nomenclature

C Capacitance

EMTP-ATP Electromagnetic transient modeling software for power systems- Alternative

transients program

FEM Finite element method

IEC International Electrotechnical Commission

Megger Mega ohm meter

Pi Percentage of voltage across the i-insulator unit

Rp Pollution resistance

Ud Breakdown voltage of sphere gap

Uti Applied voltage to the insulator string for i-insulator

ρ Electric charge density

ε Dielectric constant

1. Introduction

Insulator’s capability to insulate the power lines as well as their function in bearing the weight of

the line conductor, makes it to be of significant importance in the overhead transmission lines. The

knowledge of the voltage distribution and electric field within and around high voltage insulators is of

Simulation and measurement of the voltage distribution on high voltage suspension Porcelain insulator string under pollution

condition

Mohammad Bagher Asadpoor, Mohammad Mirzaie

Int. Journal of Applied Sciences and Engineering Research, Vol. 1, No. 1, 2012 166

paramount importance for the engineer involved in the design of power lines insulation. For instance,

voltage distribution is helpful for the detection of punctured insulators in the string. Corona, radio/TV

interference and premature aging of insulation are the results of high level electric fields. Ideally, the

potential distribution along the insulator string should be uniform. The capacitance of each insulator can

be determined approximately by the geometry and permittivity of each consisting element, while

neglecting the impact of stray fields. However, for a more accurate computation of the potential

distribution, a finite-element package is required to compute the insulator string capacitances (including

stray capacitances).

Contamination level and the design of the fittings, conductors and tower are eminent facts that

influence the voltage distribution over an insulator string. The line insulators are often covered with

contaminations, especially in industrial and coastal regions due to the long time exposure in the air. In a

condition of high humidity, the salt in the contamination will be dissolved by the moisture because of

rain, fog or dew. Therefore, the conductivity of the surface pollution increases, leading to a possible

flashover accident.

To compute the electric fields and potentials along a polluted insulator, numerous methods have

been used such as boundary element method (QueW, 2002; Rasolonjanahary, 1992), finite difference

method (Morales et al, 2001) and finite element method (Asenjo, 1997; Ashouri et al, 2010; Faisal,

2011). Potential distribution along a suspension insulator string have been determined by FEM

(Kontargyri et al, 2005) and theoretical model which considered the contamination effect in (Dhalaan

2003 a; Dhalaan 2003 b). In experimental part, reference (Pattanadech, 2004) have been measured the

voltage distribution along three insulators string by sphere gaps method. Also, this method is used as a

reliable method in (Kontargyri et al, 2005) to calculate the voltage distribution on glass insulator string.

However, no remarkable endeavor regarding the polluted condition and its impacts upon the voltage

distribution in test procedure has been made so far.

In the present paper, a full electric circuit of insulator string, considering the insulation geometry,

permittivity and stray capacitances, which were derived from a FEM-based software, have been

presented. The circuit was simulated in ATP-EMTP in order to calculate the voltage distribution and

leakage current following throw the string under various pollution levels. To verify the simulation results,

insulators are subjected to the pollution of different severity, according to IEC 60507 and the simulation

results of voltage distribution for clean and polluted insulators, have been compared with experimental

results using sphere gap method which have an acceptable accordance.

2. Simulation process in FEM-based software

2.1. FEM procedure

In electrostatic field problem for isotropic, linear and equalizing dielectric, potential v can be

determined according to the Poisson equation :

(1)

The finite element method is one of numerical process based on the variation approach and has been

widely used in electric and magnetic field analyses which have illustrated widely in (He et al, 2009). In this

investigation electrostatic problem should be solved to compute the voltage distribution of insulator string.

The boundary problem of the electrostatic field, by supposing that the domain under consideration does not

contain any space and surface charges, is turned to evaluate the functional equation:

2vρ

∇ = −ε


condition



22 2

x y zD

1 v v vF(v) dxdydz

2 x y z

∂ ∂ ∂ = ε + ε + ε ∂ ∂ ∂

∫ (2)

In case of isotropic material, dielectric constant distribution is ( x y zε = ε = ε = ε ).

Whole domains of the solution divide into many triangles, which equation (2) must be applied to

them. The calculation of the electric potential at every knot in the total network composed of many

triangle elements was carried out by minimizing the functional F(v) , that is,

i

i

F(v )0 ; i 1,2,3,....n

v

∂= =

∂ (3)

Where n is the total number of knots in the solution region.

2.2. Insulator string simulation

In this study, disk shaped porcelain insulator was selected to simulate potential distributions. The

basic design of this insulator is as follows; porcelainc with the relative dielectric constant of 6, attached

with two alumina with 14 dielectric constant, used as fixing material to the cap and pin. Surrounding of

the insulator string is air having relative dielectric constant 1.0. Also, the numbers of cap and pin type

insulators, in the string are 13 units which are used for 230 kV overhead transmission lines. The

geometrical characteristics of each insulator unit are depicted in figure 1.

Figure 1: Profile of a cap and pin insulator


condition



2.3 Calculation of the equivalence circuit parameters for insulator string

Experimental investigating the behavior of an insulator, we can see that, when it is clean and has no

pollution on its surface, there is practically no leakage and the insulator behaves as a capacitor. As

pollution is deposited on the insulator surface, without humidity, it behaves as a capacitance in parallel

with a high resistance. When, the humidity level of the pollution on the insulator surface rises because of,

for instance, hoar-frost, mist, drizzle, and frost, and then an electrolytic solution is formed. This results

in reduction of insulator pollution resistance, leading to the development of leakage current. The

equivalent circuit of a polluted insulator is shown in Figure 2, where C and Rp are the capacitance and

pollution resistance of the insulator, respectively.

Figure 2: Simplified equivalent circuit of a polluted insulator

For the simulation of insulator string in different pollution condition, equivalent circuit parameters

shall be determined. In order to calculate the self capacitance of each insulator unit and stray

capacitances between caps and ground, caps and energized conductor and also each insulator cap and

other caps, the insulator string structure is drawn in a FEM-based software. The complete part of a

transmission line; including insulator string, tower and conductor and its equivalent circuit are shown in

Figure 3.

a. b.

Figure 3 a. Complete structure of insulator string in FEM-based software b. Equivalent electric circuit


condition



First, the voltage and field stress distributions on the insulator string were computed. Then

computed electric field values are used to obtain the electrical energy (We) stored in various parts of the

model. These energies values combine with the computed potentials (V) to allow determination of the

various capacitances of the equivalent circuit. It should be mentioned that, pollution level for all of the

insulators in the string are the same ( RpRpRp === ...21

). In addition, the pollution resistances are

measured by Megger test that will be represented in the next section.

2.4 Measuring the insulator resistance in different pollution levels

In order to measure the insulator resistance, the Megger test has been conducted. Relative humidity

during the test was 45-50% (ambient humidity) which provides a dry pollution on insulator surface.

2.5 Insulators contamination

In order to artificially contaminate the surface of insulators, solid layer method has been chosen

(IEC 60507, 1991). This method can produce an approximately uniform contamination layer and can

also be performed rapidly, so that the results will be satisfying. To prepare the contamination solution,

according to the IEC 60507, some kaolin and salt was poured in one liter distilled water and have

sprayed on insulators surface. The amount of salt has a direct effect on the electrical conductance of

contamination slurry, so three different types of solutions was prepared to contaminate the insulators in

different stages of pollution based on Table 1. In the next stage, insulators were suspended vertically to

dry out. Figure 4 shows a contaminated insulator.

Table 1: Electrical conductance and ESDD in 20 oC

Ins. No. Kaolin /Salt

(gr/lit)

Measured electrical

Conductance in 20oC

(S/m)

Salinity (Sa) ESDD Pollution

level

1 40/10 0.0168 0.089 0.028 Light

2 40/10 0.0155 0.082 0.026 Light

3 40/10 0.0144 0.076 0.024 Light

1 40/40 0.0885 0.494 0.156 Moderate

2 40/40 0.0845 0.471 0.149 Moderate

3 40/40 0.0989 0.554 0.175 Moderate

1 40/70 0.198 1.133 0.358 Heavy

2 40/70 0.181 1.031 0.326 Heavy

3 40/70 0.212 1.215 0.384 Heavy

2.6 Data measurement

In order to measure the conductance of pollution layer for each insulator the value of the leakage

current was recorded in thirty minutes by Megger. For every pollution level three insulators have been


condition



tested, that the average resistance of them are shown in Figure 5. As it mentioned, the higher pollution

degree exist, the lower pollution resistance occurred. For the simulation, the average value of resistance

in thirtieth minute has been considered.

Figure 4: Contaminated insulator.

0 10 20 300

1

2

3

4

5

6

7x 10

4

Time (min)

Insu

lato

r R

esis

tan

ce (

Mo

hm

)

0 10 20 30

2000

2500

3000

3500

4000

Time (min)

Insu

lato

r R

esis

tan

ce (

Mo

hm

)

0 10 20 30

1000

1250

1500

1750

2000

Time (min)

Insu

lato

r R

esis

tan

ce (

Mo

hm

)

0 10 20 30450

500

550

600

650

700

750

800

Time (min)

Insu

lato

r R

esis

tan

ce (

Mo

hm

)

a. b. c. d.

Figure 5 Average resistance of Polluted Insulators a. Without pollution b. Moderate pollution c. Light

pollution d. Heavy pollution

3. Simulation results

Since getting the leakage current throw insulator string and lower running time, ATP-EMTP has

been used. The lumped equivalence circuit in Figure 3.b is running in ATP-EMTP and results have been

obtained.


condition



3.1 Performance of insulator string under clean condition

Figure 6 demonstrates the voltage distribution of insulator string under clean condition. The first

insulator unit in this simulation is placed near the tower and the thirteenth is placed near the energized

conductor. Due to the stray capacitances existing between the discs, conductor and ground, the

distribution of the voltage along the string is not uniform, and the discs around the conductor being more

highly stressed. So the rate of puncture of these insulators is more reported in transmission lines. Also,

the leakage current in this condition is capacitive and its value is very low. The amplitude of the leakage

current is 0.5923 mA.

0 2 4 6 8 10 12 142

4

6

8

10

12

14

16

18

20

Insulator number

Vo

lta

ge

dis

trib

uti

on

(%

)

Figure 6: Voltage distribution along insulator string under clean condition

3.2 Performance of insulator string under different pollution condition

To investigate the performance of insulator string under pollution condition, as it mentioned, a

resistance with higher value is paralleled with each unit. In Figure 7, voltage distribution of insulator

string in different pollution levels has been represented. As it can be depicted, any significant changes

haven’t occurred on voltage distribution, since the value of pollution resistance under dry condition isn’t

considerable in comparison with the impedance of each unit. The same conclusion can be getting for

leakage current, as is represented in Table. 2. In addition string loss for different pollution levels are

shown in this table.

Table 2: Maximum leakage current passing through insulator string with different pollution levels and

string loss

Pollution level Maximum leakage current (mA) String loss (Watt)

Without pollution 0.5923 0.031

Light pollution 0.5923 0.463

Moderate pollution 0.5924 0.925

Heavy pollution 0.5934 2.467


condition



1 2 3 4 5 6 7 8 9 10 11 12 130

5

10

15

20

Insulator number

Volt

ag

e d

istr

ibu

tio

n (

%)

Without pollution

Light pollution

Moderate pollution

Heavy pollution

Figure 7: Voltage distribution along insulator string with different pollution levels

4. Experimental procedure

To measure the voltage distribution of an insulator string, sphere gap method has been used in this

paper. The required facilities for the test are shown in Figure 8.

0-220 v 0-100 kv

Electrostatic

voltmeter

220 v V1

HV

Transformer

Self

Transformer

Figure 8: Experimental set-up used to measure of the voltage distribution

The voltage is measured by using an electrostatic voltmeter, measuring the high voltage in the

secondary part of the transformer (IEC 60060-1, 1989). Distance between two spheres is fixed during

the voltage measurement. The sphere gap is installed parallel with each insulator. The capacitance of the

sphere gap is very small compared to the capacitance of the insulator, so it can be neglected. The applied

voltage to the insulator string is up turned until the spark over of the sphere gap at the critical voltage

(Ud) occurred. Ten tests for each of the insulators has been carried out to obtain the accurate results and

the average of them have been considered as Ut. Then the percentage of voltage across the i-insulator

unit Pi , is calculated by:


condition



di

ti

UP 100%

U= × (4)

By moving the sphere gap along the insulator string, rate Pi for every insulator will achieve until

thirteenth. Ut for each insulator varies, since the existence of stray capacitances between the cap and pin

of the insulators, tower and conductor. Totally, the critical voltage of the sphere gap is calculated by the

equation:

13 13

i d

i 1 i 1 ti

1P U 1

U= =

= =∑ ∑ (5)

As a result, by calculating Ud, the voltage percentage of each insulator will obtain. The

experimental and simulation results for clean and dry insulator string are shown in Figure 9.

0 2 4 6 8 10 12 142

4

6

8

10

12

14

16

18

20

Insulator number

Vo

lta

ge

dis

trib

uti

on

(%

)

Simulation Results

Exprimental Results

Figure 9: Comparison between simulation and the experimental results for clean and dry insulator string

0 2 4 6 8 10 12 142

4

6

8

10

12

14

16

18

20

Insulator number

Vo

lta

ge

dis

trib

uti

on

(%

)

Simulation Results

Exprimental Results

Figure 10 Comparison between simulation and the experimental results for heavy pollution level


condition



It shows that there should be more consideration to the insulators near the conductor in

comparison with other insulators in the string. As it can be seen, there is an acceptable concord between

the results. In this comparison the root mean square and maximum value of errors for all units are 2.7

and 5. Also all of the insulators in the string are contaminated according to the IEC60507 and suspended

in the chamber. The same experimental procedure for voltage measurement is conducted in the ambient

humidity and heavy pollution level and results are shown in figure 10. The experimental results admit

the simulation ones that show, dry pollution doesn’t affect the voltage distribution of the string.

5. Conclusion In this work, a full electric circuit of insulator string, which takes into account the insulator material

properties and the stray capacitances, was derived. The capacitances values are calculated using a

finite-element package. The circuit was implemented in the EMTP-ATP package in order to simulate the

voltage distribution and the leakage currents flowing through the insulator string under various scenarios

of pollution. Also, indoor laboratory experiments for the investigation of pollution severity on potential

distribution along a 230 kV I-string insulator have been carried out in low humidity that made a dry

pollution on insulator surface. A very satisfactory agreement has been ascertained when comparing

experimental results with results from simulations. The results show the voltage distribution and leakage

current on pollution condition didn’t change remarkably in comparison with the clean condition. As can

be concluded, when pollution level increased to heavy, the electric potential over the unit nearest to the

line conductor decreased from 19.98 % to 19.9 %, which had a reduction of about 0.4%. Although

power loss on insulator string in clean and pollution condition is very low, it changes from 0.031 W in

clean condition to 2.467 W in heavy pollution condition.

6 References

1. Asenjo. E, Morales N, Valdenegro., 1997. Solution of low frequency complex fields in

polluted insulators by means of the finite element method, IEEE Transactions on

Dielectrics and Electrical Insulation. doi:10.1109/94.590856.

2. Ashouri. M, Mirzaie. M, Gholami. A, 2010. Calculation of Voltage Distribution along

Porcelain Suspension Insulators Based on Finite Element Method, Electric Power

Component System. doi: 10.1080/15325000903489694.

3. Dhalaan, S.M.A.; Elhirbawy, M.A., 2003. Investigation on the characteristics of a string

of insulator due to the effect of dirt, Transmission and Distribution Conference and Exposition,

IEEE PES; September., 3, 915 – 920. doi: 10.1109/TDC.2003.1335059.

4. Dhalaan, S.M.A. Elhirbawy., M.A. 2003. Simulation of voltage distribution calculation methods

over a string of suspension insulators, Transmission and Distribution Conference and

Exposition, IEEE, PES; 3, 909 – 914. doi: 10.1109/TDC.2003.1335058.

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8. IEC Standard 60060-1, 1989, High voltage test technique, Part 1: General Definitions and test


condition



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9. Kontargyri. V.T, Plati, L.N, Gonos, I.F, Stathopulos., I.A; 2005. Measurement and

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10. Morales. N, Asenjo E, Valdenegro., 2001. Field solution in polluted insulators with non-

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