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Conference Proceedings of ISEIM 2014

Study of Electric Field Distribution on 22kV Insulator under Three Phase Energisation

Mu Liang1, and K. L. Wong 1* 1 RMIT University

*E-mail: [email protected]

AbstractSimulation of the electric field and potential distribution is widely performed in the design of high voltage composite insulators. This paper introduces and analysis of electric field distribution around the insulator under a 22kV high voltage AC (HVAC) transmission line. A two-dimensional model is built to calculate the electric field distribution in single phase system and another 2-D model is used to calculate the distribution under a three-phase HVAC transmission line. The effects of the metal ground pin are considered in the model and a comparison of the single phase system and three-phase system is given in the conclusion. The results show that electric field strength in a three-phase system is significant larger than a single phase system. The electric field fluxes are also affected by the ground pin and other two phases in a three-phase system.

Keywords: Electric field calculation, finite element method, composite insulators, three-phase system.

I. INTRODUCTION Insulators are extensively used in power transmission

system. First porcelain insulators were born in 1850s for telephone lines. At the end of 19th century, porcelain insulators were first used at a 15-kV voltage level in Germany [1]. These days, porcelain insulators are being replaced more by those composite insulators. The first composite insulators were installed in Germany in 1967[2]. With more than 40 years of development history in manufacturing and using composite insulators, the material of composite insulators is continuously improving and now became various and reliable. Comparing with old ceramic insulators, newly composite insulators have light weight, high mechanical strength and convenient maintenance. Indeed, the development of composite insulators is providing us a relatively reliable insulation system with favorable electrical and mechanical properties[3]. No matter how the insulators develop, aging problems are still one of the main issues in the utilizing of composite insulator. The role of corona from water drops on transmission line conductors is well known. Even in the clean and dry circumstance, the corona discharge of the composite insulator surface is also an important cause of the aging of composite insulators.

The calculation of electric field distribution on composite insulators is helpful to the study of partial discharge occurs on the surface of composite insulators and relating surface aging problems and is also helpful for the insulation design, operation and maintenance of composite insulators. Many researches have been taken on the electric field analysis of composite insulators[4-8]. In modern days, the distribution on composite insulators can be studied by numerical techniques. Finite Element Method (FEM) is one of the numerical techniques which solve the governing differential equations directly. It is commonly used in modern simulation tools.

The purpose of this paper is to study the electric field distribution of a 22kV insulator under three phase

energisation. With the simulation results of single phase system and three phase system, the effect of three phase energisation on the electric field can be studied.

II. NUMERICAL MODEL OF INSULATORWith the analysis software COMSOL based on the FEM,

the 2-D model of a composite insulator and its ground pin is created. As shown in Fig. 1, the composite insulator is made of epoxy and designed for a 22kV transmission system. It has 5 sheds and 27cm in height with a 24cm length metal ground pin in the bottom. About 7cm metal ground pin is insert into the insulator. Three larger sheds have a diameter of 15cm and two smaller sheds have a diameter of 12cm. A single-phase model with one insulator and a three-phase model with three insulators are built to study the electric field changes in different systems. The insulator is assumed to be located in a clean and dry condition. The air has an electrical conductivity of 1 x 10-11 S/m. Refers to other studies, the relative permittivity of epoxy is between 4 and 5 [9]. In this model, the relative permittivity of the epoxy insulator is set to 4.

For a 22kV transmission system, the line voltage is kVVl 7.12=3/22000= . For transient analysis, the voltage

at the zero phase transmission line is expressed as a function of time:

)250sin(212700= tVl V (1) For a three phase system, the other two phases have a 120

phase angle between each other. The voltage expressions at the other two phases are (2) and (3):

)3

2+250sin(212700= tVl V (2)

)3

4+250sin(212700= tVl V (3)

In the three-phase transmission system model, each

Figure 1. 22kV epoxy insulator sample.

F2

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insulator is located 1m away from each other. The middle one has a zero phase shift, the left one has a delay angle of 120 and the right one has a delay angle of 240. In both models, the insulators are considered to be clean and defect-free.

III. COMPUTATION RESULTS Computer software COMSOL based on the finite-element

method was used for computation of electric field strength and potential distribution. The simulation uses transient analysis to get the electric field variation over time. All of the simulations in this study run for 2 power-frequency cycles. A. Effect of Metal Ground Pin

To study the effect of ground pin of the pin-type insualtors to the electric field distribution of insulators, two models with different configuration were calculated: (1) without a ground pin, (2) with a ground pin and grounded. Both models are built under a single phase system.

Fig. 2 shows the electric field strength along the top surface of insulator for the two models. Arc length is the horizontal distance from the tip of the top surface. The position of measuring point is closer to the center when the arc length is larger. The metal ground pin has a large effect on the electric field intensity around the insulators. At the time t = 0.005s, with a grounded metal pin, the values of electric field mainly varied from 0.6 x 105 V/m to 0.8 x 105 V/m and sharply went up when approaching to the center of insulator. Compare with the values around 0.5 x 105 V/m without a ground pin, the metal ground pin significant raise the electric field intensity around the insulator. The closer to the center,

the larger the influence will be. B. Electric Field Distribution in Single Phase System

Fig. 3 shows the electric field distribution around the insulator with a metal ground pin in a single phase system. Apparently, the influence of ground pin cannot be ignored. Without taking into account of the effect of the metal ground pin leads to grossly inaccurate estimates of electric field distribution for the insulators. Although the metal ground pin significant raise the electric field strength around the insulator, its still necessary to support the insulator in industry. In the model of three phase system, the effect metal ground pins are also considered.

C. Electric Field Distribution in Three Phase System Fig. 4 shows the electric field distribution at a particular

time t = 0.0064s. It can be observed that at t = 0.0064s, the electric field around the insulator in the middle slants to the left side, which is caused by the high voltage magnitude at the transmission line in the left. The whole distribution became asymmetric. This kind of asymmetric electric field is easier to lead to corona discharge on the surface of insulator. Especially in bad climate situation like rainy or sand-dust whether, the electric field strength on some parts of the insulator may be extremely high and the corona discharge will easily and frequently occur at these areas.

(a)

(b) Figure 2. Electric field intensity along the surface without a

ground pin (a), electric field intensity along the surface with a ground pin and grounded (b).

Figure 3. Electric field distribution around the insulator with a metal ground pin in single phase system.

Figure 4. Electric field distribution around the insulator in three phase system.

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D. Electric Field Variation in Three Phase System Fig. 5 shows the electric field variation from t = 0s to t =

0.02s, the time interval between each graph is 0.002s. The graphs clearly show the changes of electric field distribution in one cycle. The contour stands for electric field strength. In Fig. 5, the electric field strength of each insulator reaches its maximum value every half cycle (0.01 second). For the insulator in the middle, that is time t = 0.005 + 0.01(n) where n = 1,2,3. The calculation also shows that the electric field strength around the insulator in the middle is slightly larger than the electric field strength around the insulators aside.

IV. ANALYSIS In the three phase system, the electric field around

different phases interacted with each other. The effects of other two phases would not only warp the distribution of electric field but also enhance it.

Electric Field Enhancement in Three Phase System Fig. 6 shows the electric field distribution around the

insulator in the middle at time t = 0.064s and the position of four measuring points in the model. The electric field intensity reaches the maximum value at time t=0.5. The maximum electric field intensity at four measuring points is calculated and shown in Table 1. Point A, top of the insulator, has an increase from 1.9 x 105 V/m to 1.97 x 105 V/m which is a percentage of 3.6%. Point B, on the first large plate near the middle, has an increase from 7.5 x 105 V/m to 8.4 x 105 V/m, a percentage of 12%. Point C, at the tips of the shed, has an increase from 4.5 x 105 V/m to 5.9 x 105 V/m, a percentage of 31%. Point D, at the tip of the top surface, has an increase from 0.8 x x105 V/m to 1.2 x 105 V/m, a percentage of 50%.

From the calculation above, the electric field intensity of middle phase is obviously enhanced by the other two phases. The electric field in some parts of surface on the middle phase insulator in three-phase system can increase up to 50% comparing with the situation of single-phase system. The surfaces near the tips of the sheds are commonly suffering a larger enhancement of electric field in three-phase system.

TABLE I. ELECTRIC FIELD INTENSITY AT FOUR MEASURING POINTS

Measuring points

Electric Field Intensity (V/m) Single phase system Three phase system

A 190,000 197,000

B 75,000 84,000

C 45,000 59,000

D 80,000 120,000

V. CONCLUSION Using a two-dimensional model of composite insulator in

22kV AC transmission lines, the electric field distribution were calculated with finite element method in this paper. In order to demonstrate the influences of metal ground pin on the electric field along the insulators. Modeling of the insulator alone, without taking into account the effect of ground pin, the whole electric field intensity significantly decreased. The metal ground pin raises the electric field around the insulator significantly at the same time. In horizontal direction, the closer to the center of insulator, the larger the influence will be. In vertical direction, the ground pin can not only affect the electric field at the lower side of the insulator but also influence the electric field at the conductor side.

The electric field distribution of insulator in three-phase system is asymmetric. The other two phases warps the distribution of middle phase, the electric field intensity is enhanced in some area, which is easier to lead to corona discharge on the surface of insulator.

On the surface of the insulator, the electric field intensity at the tips of the sheds is higher than the electric field intensity on the curvature surface of the sheds. For the aging problems, the dangerous places are at the tips of the sheds and the surface near the transmission line. Comparing with single-phase system, at some parts of the surface of the insulator in three-phase system, the electric field intensity has an increase of up to 50%. The surfaces near the edge of the insulator are com monly suffering a larger enhancement of electric field intensity in three-phase system.

ACKNOWLEDGMENT

Figure 6: Three measuring points in the model (A: top of insulator, B:curvature on top sheds, C: tip of shed, D: tip of the top surface )

Figure 5. Electric field variation from time t = 0 to t = 0.018s in a

step of 0.002s.

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First of all, I would like to thank my supervisor A/Prof. Alan K.L. Wong for his guidance and encouragement during the research project. I am also grateful to my fellows in RMIT University and old schoolmates at Xian Jiaotong University, thanks for their friendship and help throughout this work.

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