1

M. A. Salam a,*

, Q. M. Rahman b, S. P. Ang

a, F. S. Wen

a, H. A. Hadi

a, M. Fadil

a, S. Hassan

b, W. Voon

c

a Institut Teknologi Brunei, Brunei Darussalam b University of Western Ontario, London, ON, N6A 5B9, Canada c

Berakas Power Management Company, Brunei Darussalam

* Corresponding author at: M. A. Salam, Institut Teknologi Brunei, Brunei Darussalam,. Tel: 00673-2461020 Ext 1315

E-mail address: abdus.salam@itb.edu.bn (M. A. Salam)

Abstract:

For an extended period of time, long-rod porcelain and silicone rubber insulators are being used in transmission lines

by two power utility companies, the Department of Electric Services (DES) and the Berakas Power Management

Company (BPMC), in Brunei Darussalam. Over time, these insulators have got polluted by sand, sodium chloride

(salt), coastal dust and seawater. The pollution levels in terms of the equivalent salt deposit density (ESDD) of those

insulators were measured and the results reported in this paper. It is found that the pollution levels fall in the range of

0.273-0.301 mg/cm2 at the bottom surface and 0.227-0.261 mg/cm2 at the top surface of the porcelain insulators.

Whereas the pollution level for long-rod silicone insulators is found to be 0.363-0.372 mg/cm2 at the bottom and

0.319-0.329 mg/cm2 at the top of the silicone rubber insulators. In addition, simulations of electric potential and field

distributions of these insulators were carried out under clean, uniform and non-uniform pollution, and broken surface

conditions using the COMSOL Multiphysics software platform.

Article Information:

Keywords:

Pollution layers

Long-rod porcelain

Long-rod silicone rubber insulator

Electric potential distribution

Electric field distribution

Submitted: 15 Aug 2015

Revised form: 24 Oct 2015

Accepted: 27 Oct 2015

Available Online: 27 Oct 2015

1. Introduction DES and BPMC are the two power utility companies in Brunei Darussalam.

These two companies transfer power from substations to Tutong and Kuala

Belait districts through double circuited transmission lines, respectively.

These transmission lines pass near the South China Sea and forest.

Insulators exposed to environments in these areas are polluted by sand,

sodium chloride (salt), coastal dust and seawater. These pollutants are

accumulated on the surface of the insulators by natural wind, and a dry

pollution layer is finally formed. This dry pollution layer does not affect the

insulator performance in sunny days. However, the pollution produces a

conducting layer in the presence of light rain or dews [1]. Under this

condition, the small arcs appear on the surface which may cause insulator

flashover and hence disturbance on transmission networks [2]. The

resulting electric field distributions due to the above mentioned condition

were analyzed for an 11 kV composite insulator using the COMSOL

Multiphysics software platform in [3]. Based on this platform, C. Volta [4]

applied volume and surface approaches to determine the voltage

distribution of a 28 kV dead-end thermoplastic elastomeric (TPE) insulator.

The potential and electric field distributions were calculated inside and

around I, II and V cap-and-pin insulators in 400 kV transmission lines with

the COMSOL Multiphysics 3.5 software platform [5]. It was also

mentioned that these distributions were affected by the tower, voltage

magnitude, corona, contamination and environmental conditions. A

comparative study between 2D axisymmetric and 3D modeling of an extra

high voltage (EHV) post insulator equipped with a standard corona ring

was presented in [6], and it was demonstrated by the results that the

creation of a complete electrical link between the grading ring and the high

voltage (HV) electrode could result in a reduction of the electric field

strength in the vicinity of a HV electrode.

T. Doshi et al [7] calculated the electric field distribution for composite

insulators up to 1200 kV using a 3D software package based on the

Boundary Element Method, and the impacts of corona and grading rings,

single and bundled conductors, insulator orientation (dead-end and

suspension), single and double units, and surface conditions (dry and wet)

on the electric field distribution were analyzed. Suat Ilhan et al [8] studied

the AC and transient electric field distributions along a 380 kV V-string

insulator using the COMSOL Multiphysics software by considering a 2-mm

thick uniform pollution layer. However, the pollution layer is seldom

uniform in practice. H. Akkal et al [9] presented some solutions, using

grading rings, for improving the performance of an EHV post station

insulator under severe icing conditions. In addition, the COMSOL

Multiphysics software was used for simulating electric fields and voltage

distributions. The voltage distribution of the insulator was examined for

situations polluted with the combination of NaCl, CuSo4 and distilled water

in [10].

The performance of an insulator in terms of withstanding voltage and

electric field distributions, is different for different types of coating under

pollution conditions [11]. Imre Sebestyenbthe [12] determined the electric

stresses acting around and inside the insulator considering the interactions

with the three-dimensional environment using a domain decomposition

approach for the finite element method.

Given the above mentioned background, measurements of pollution levels

of aged long rod porcelain and silicon rubber insulators are reported in this

work. Also, the voltage and electric field distributions are investigated

under clean, polluted and broken surface conditions.

2. Measurement of Pollution Levels

Two heavily polluted, and approximately 30 years old insulators were

brought in from DES, for the pollution measurement experiment. Some

parts of these insulators were found to be cracked. However, extreme

cautions were taken during the collection process of the pollutants. The

insulators were brought to the laboratory, and the pollutants were collected

by brushing them off; later on, pollutants were mixed with distilled water

Characteristics of Aged Long-rod Porcelain and Silicone Rubber Insulators under Pollution Conditions

An Open Access Journal

www.measpublishing.co.uk/BJRE

British Journal of Renewable Energy

British Journal of Renewable Energy 01(01) 01-08 (2015)

2

for making salt-solution. These two insulators are long-rod porcelain and

long-rod silicone rubber (SiR) ones. The long-rod porcelain insulator has 15

layers, ‘a’ to ‘o’, where ‘o’ and ‘a’ have the highest and lowest potentials,

respectively. The long rod silicone insulator has 7 layers, ‘a’ to ‘g’, where

‘a’ and ‘g’ have the highest and lowest potentials, respectively. The HV

electrode is connected to the top layer of the silicone insulator, while for the

porcelain insulator it is connected to the bottom layer.

The equipment YSI Model 63 was used to measure the solution

temperatures, PHs and conductivities. These measured conductivities at

different temperatures were then converted to the corrected conductivities

at 20oC by the following formula [13]:

𝜎20 = 𝜎𝜃(1 − b(θ − 20), (1)

In addition, the salinity value, Sa, of the solution can be expressed by the

following empirical formula:

𝑆𝑎 = (5.7𝜎20𝜇𝑆

𝑐𝑚

∗ 10−4)1.03, (2)

Finally, the Equivalent Salt Deposit Density (ESDD) is determined by

using the following expression,

𝐸𝑆𝑆𝐷 = (𝑆𝑛 ∗ 𝑉)/𝐴 , (3)

The pollution levels in terms of ESDD at the bottom and top surfaces of

both the long-rod porcelain and silicone rubber insulators are shown in

Table 1.

Table 1 ESDD of long-rod porcelain and SiR insulators

Layer from

insulator-

pin side

Long rod porcelain Long rod SiR

ESDD (mg/cm2)

Top Bottom Top Bottom

a 0.227 0.274 0.319 0.364

b 0.227 0.273 0.328 0.363

c 0.233 0.28 0.328 0.365

d 0.236 0.28 0.327 0.366

e 0.235 0.28 0.329 0.367

f 0.252 0.284 0.329 0.367

g 0.261 0.286 0.329 0.372

h 0.24 0.295

i 0.24 0.282

j 0.233 0.282

k 0.233 0.286

l 0.249 0.286

m 0.254 0.292

n 0.246 0.288

o 0.243 0.301

3. Model Design

The electric filed under the static condition is equal to the negative gradient

of the scalar electric potential, and can be written as [14],

𝐸 = −∇𝑉, (4)

The electric displacement is related to the electric field and electric

polarization, and can be expressed as,

𝐷 = 𝜀𝐸 + 𝑃, (5)

Substituting (4) into (5) yields,

𝐷 = 𝜀(−∇𝑉) + 𝑃, (6)

The Gauss law is defined as,

𝜌 = ∇. 𝐷, (7)

Substituting (6) into (7) yields,

𝜌 = ∇. (𝜀(−∇𝑉) + 𝑃), (8)

𝜌 = −∇. (𝜀∇𝑉 − 𝑃), (9)

Where

𝜀 is the permittivity of the medium, F/m,

P is the electric polarization vector, C/m2,

𝜌 is a space charge density, C/m3.

The dimensions of the original long-rod porcelain and silicone rubber

insulators were measured for constructing the shapes of these insulators in

the COMSOL software platform. Figures 1 and 2 are the designed outputs

for the long-rod porcelain and silicone rubber insulators, respectively. For

the designed model in COMSOL, the space dimension must be selected

either in 1D, 2D or 3D. After selecting the space dimension, physics can be

added to the model. In this case, the electrostatics interface is added to the

model to show the electric potential between layers of an insulator. The

model is completed by selecting the geometry. In the next step, a stationary

or time dependent parameter is added into the model. The material of each

insulator is defined before computing the electric potential and electric field

distributions. Afterwards the electric potential and electric field

distributions are plotted under uniform and non-uniform pollution

conditions.

Fig. 1: Long-rod porcelain insulator

Fig. 2: Long-rod SiR insulator

4. Results and Discussions

4.1. Uniform and Non-uniform Pollutions

From the experiment results, it is found that more pollutants are

accumulated on the top and bottom sheds of the insulator, and the pollution

thickness decreases towards the middle of the insulator and finally results in

uneven pollution layers. In practical scenarios, non-uniform pollution

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distributions are more popular in an insulator. Both uniform and non-

uniform pollution layers are considered here. As the insulators were

exposed to the atmosphere near hills and sea beaches, the pollutants such as

seawater (SW), sand, sodium chloride and coastal dust (CD) are considered.

A thin conductive layer (in mm) produced by the pollution on an insulator

is considered. In case of the uniform pollution distribution, the thickness of

the pollution layer is considered to be 1 mm. The permittivities for different

pollutions and insulator materials are shown in Table 2. The thickness of a

non-uniform pollution layer is classified into five groups: very heavy (3

mm), heavy (2.5 mm), medium (2 mm), light (1.5 mm) and very light (1

mm). For the porcelain insulator, layers 1, 2, and 15 are considered very

heavy, layers 3, 4, 6, 13 and 14 heavy, layers 5, 7 and 12 medium, layers 8

and 11 light, layers 9 and 10 very light. For the silicone rubber insulator,

layers 1, 2 and 7 are considered heavy, layers 3 and 4 medium, layers 5 and

6 light. The top and bottom parts of the insulator are specified by the

potentials of 0 kV and 66 kV, respectively. Under clean conditions, the

variation of the electric potential in long-rod porcelain and silicone rubber

insulators are shown in Figures 3 and 4, respectively. The red color

indicates the highest potential while the blue color indicates the lowest

potential.

Figures 5 and 6 show the electric potential distributions of the porcelain

insulator under uniform and non-uniform distributions of pollutions. In this

case, it is observed that the electric potential distribution with seawater is

more or less linear, and this kind of pollution is a little bit lower than other

pollutions. The electric potential distributions of uniform and non-uniform

pollutions in the long-rod silicone rubber insulator are shown in Figures 7

and 8.

The voltage variation in this case is from capacitive to resistive. The

electric field distributions in both insulators are shown in Figures 9, 10, 11

and 12. It can be seen that the highest electric field occurs at the highest

potential side, i. e. at the junction of the high voltage and the insulator part.

The electric field distributions at the middle layers are almost constant due

to the distances of these layers from the source.

Table 2 Permittivities of different kinds of material

Material Relative permittivity,

Air 1

Silicon rubber 4

Porcelain 5

Sand 5

Sodium chloride (salt) 6.1

Coastal dust 10

Seawater 82

Fig. 3: Electric potential in a long-rod porcelain insulator under clean

condition

Fig. 4: Electric potential in a long-rod SiR insulator under clean condition

Fig. 5: Electric potential in a porcelain insulator under the uniform

pollution

4

Fig. 6: Electric potential of a porcelain insulator under the non-uniform

pollution

Fig. 7: Electric potential in a SiR insulator under the uniform pollution

Fig. 8: Electric potential in a SiR insulator under the non-uniform pollution

Fig. 9: Electric field in a porcelain insulator under the uniform pollution

Fig. 10: Electric field in a porcelain insulator under the non-uniform

pollution

5

Fig. 11: Electric field in a SiR insulator under the uniform pollution

Fig. 12: Electric field in a SiR insulator under the non-uniform pollution

4.2. Breakdown Surface Behavior

The insulator surface can be broken if a flashover occurs. In this situation,

the electric potential and electric field distributions change abruptly. The

top surface of each layer is considered to be broken due to high pollution

accumulation which is confirmed by the experiment. On the broken surface,

the characteristics of the electric potential in the silicon rubber insulator

with undersea water (SW) and coastal dust (CD) pollutants are shown in

Figures 13 and 14, respectively. In this case, the highest potential is

observed at the top and the potential varies from the highest to the lowest.

The electric potential distribution in the presence of seawater pollutant is

higher than that of the coastal dust under the broken surface condition, as

can be seen in Figure 15.

Fig. 13: Electrical potential in a SiR insulator with the broken surface in the

presence of the seawater pollution

Fig. 14: Electrical potential in a SiR insulator with the broken surface in the presence of coastal dust pollution

The electric field distributions in long-rod silicone rubber insulators under

the broken surface condition with seawater and coastal dust pollutants are

shown in Figures 16 and 17, respectively. In Figure 16, the highest electric

field is found to be 4 MV/m at layer 1 which is close to the highest

potential. In addition, it is also found that the electric filed distribution

varies from a higher spike to a lower spike.

The broken surface electric field distributions in the SiR insulator under

seawater and coastal dust pollution conditions are plotted as shown in

Figure 18. From this figure, it is found that the electric field with the coastal

dust pollution is higher than that with the seawater one. The voltage

distributions in the long-rod porcelain insulator with different pollutions

and broken surface conditions are shown in Figures 19 to 21. From these

figures, it is seen that the voltage varies initially as a pulse and moves

linearly except for a few initial layers under different pollution conditions.

6

Fig. 15: Electrical potential in a SiR insulator with the broken surface in the

presence of different kinds of pollutions

Electric potential distributions at layers 1, 8 and 15 with seawater pollution

are found to be higher than the other layers with coastal dust pollution, as

can be seen in Figure 21.

Fig. 16: Electric field in a SiR insulator with the broken surface in the

presence of seawater pollution

Fig. 17: Electric field in SiR insulator with broken surface in the presence

of coastal dust pollution

Fig. 18: Electric field in a SiR insulator with the broken surface under CD

and SW pollutions

Fig. 19: Voltage distribution in a porcelain insulator with the broken

surface in the presence of SW

7

Fig. 20: Voltage distribution in a porcelain insulator with the broken

surface in the presence of CD

The electric field distributions in the long-rod porcelain insulator under

seawater, coastal dust and broken surface conditions are shown in Figures

22 to 24. From Figure 22, it is seen that the electric field distribution is less

than 2 MV/m up to a creepage distance of 1.5 m. Initially, this value of the

electric field increases slowly with the increase of the creepage distance,

and at a distance of 2.25 m (in layer 15) it rises sharply to 12.22 MV/m. A

comparative analysis for electric field distributions due to seawater and

coastal dust pollutions in porcelain insulators is depicted in Figure 24,

where the electric field due to the coastal dust pollution is seen to be higher

than the seawater pollution in layer 15.

Fig. 21: Comparisons of voltage distributions in a porcelain insulator with

the broken surface under different kinds of pollutants

Fig. 22: Electrical field distributions in a porcelain insulator with the broken surface in the presence of seawater

Fig. 23: Electrical field distributions in a porcelain insulator with the

broken surface in the presence of CD

Fig. 24: Comparative analysis of electrical field distributions in a porcelain

insulator with the broken surface under different kinds of pollutants

5. Conclusions

The pollution levels of used long rod porcelain and silicone rubber

insulators are measured. The pollution range for a long-rod porcelain

insulator at the bottom and top surfaces are found to be in the range of

8

0.273-0.301 mg/cm2 and 0.227-0.261 mg/cm2, respectively. Whereas these

levels for a long-rod silicone insulator are found to be in the range of 0.363-

0.372 mg/cm2 and 0.319-0.329 mg/cm2, respectively. The measured

pollution levels are compared with the pollution standards, and found to be

outside the acceptable range. Electric potential and field distributions are

carried out under clean, uniform and non-uniform pollutions and broken

surface conditions using the COMSOL Multiphysics software platform. It

is found that the potential distributions with the seawater pollution are

higher than those with other pollutions for SiR insulators. Whereas in the

porcelain insulators, electric field and potential distributions are higher due

to the coastal dust pollution.

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