Effects of Non-uniform Pollution on the AC Flashover Performance of Suspension Insulators

Abstract

The non-uniform distribution of contamination on insulator surface has appreciable effects on flashover voltage, and corresponding researches are valuable for the better selection of outdoor insulation. In this paper, two typical types of porcelain and glass insulators which are widely used in ac lines were taken as the research subjects, and their corrections of AC flashover voltage under non-uniform pollution were studied. Besides, their flashover characteristics under different ratio (T/B) of top to bottom surface salt deposit density (SDD) were investigated, including the analysis of flashover voltage, surface pollution layer conductivity and critical leakage current. Test results gave the modified formulas for predicting flashover voltage of the two samples, which can be directly applied in the transmission line design. Also, the analysis delivered that, the basic reason why the flashover voltage increases with the decrease of T/B, is due to the decrease of equivalent surface conductivity of the whole surface and the decrease of critical leakage current. This research will be of certain value in providing references for outdoor insulation selection, as well as in proposing more information for revealing pollution flashover mechanism.

1. Introduction

In recent years, along with the rapid development of industry and economy, the air quality is getting worse, and the accident caused by pollution flashover occurs from time to time in China and around the world. These pollution flashovers may cause large-scale blackouts accident of the grid system [1, 2]. Given this, plenty of studies on the pollution flashover performance and mechanism have been done in many countries [3].

The pollution accumulation experiments of field operating insulators delivered that the contamination on insulators in service is always non-uniform. Thus some researches were conducted to discuss the performance of non-uniform contamination on top and bottom surfaces, as well as its influence on insulator flashover [4-16].

For example, according to the dc operation experience in China, researchers in [8] found that, for porcelain and glass insulator, the contamination ratio (T/B) of top to bottom surface of porcelain and glass insulators is generally in range of 1:5 - 1:10; test results in [10] indicated that, under non-uniform pollution on top and bottom surface, the pollution withstand voltage increases by 30% and 50% respectively when T/B is 1:5 and 1:10; EPRI [11] raised a formula,

for the correction of dc flashover voltage under non-uniform pollution, and got that the correction coefficient (C) was in the range of 0.29-0.47; in [12], another research found that the formula of EPRI is also applicable in ac case and the value of C was obtained as 0.31.

Some works of the non-uniform pollution have been done around the world, and the related data in specific to certain kinds of insulators were referable in outdoor insulation design. However, their results are of some discrepancy. For example, in [10], the value of C for ceramic insulators ranges from 0.24 to 0.29 and for glass insulators it ranges from 0.18 to 0.20, while in [12], the correction coefficient C for ceramic insulators is in the range of 0.21-0.37. Therefore, more tests were needed to provide detailed information for the selection of correction coefficient.

In previous work, few works have focused on the effect of the surface pollution layer conductivity and the critical leakage current on the flashover characteristic. For example, in [1], the relationship between surface layer conductivity and equivalent salt deposit density on the uniform condition was studied, while in [2], the influence of atmospheric parameters, such as air density humidity and temperature, on the dielectric strength of insulators was presented. Whereas, the data of surface pollution layer conductivity as well as the critical leakage current, are necessary for better understand insulator flashover performance under non-uniform pollution. In this paper, the effects of non-uniform pollution on the AC flashover performance of two typical types of insulators was revealed through analyzing the surface layer conductivity and critical leakage current.

Given this, ac pollution flashover performance of two typical types of porcelain and glass insulators, which are mostly used in the 110 kV, 220 kV and 500 kV ac transmission lines in China, were studied in this paper, and the influence of non-uniform pollution distribution was systematically analyzed. Modified formulas for the two typical insulators were proposed, which are directly referable for outdoor insulation design. Research results are of certain value in providing references for engineering practice, as well as in proposing more information for revealing pollution flashover mechanism.

 

2. Insulator Samples, Experimental Setup and Procedures

2.1 Insulator Samples

The samples were two typical types of suspension insulators. The technical parameters and profiles of the samples are shown in Table 1 and Fig. 1, in which H is the configuration height, L is the leakage distance and D is the diameter of insulators.

Table 1.Technical parameters of the samples

Fig. 1.Structure of the two samples: (a) Type A; (b) Type B.

2.2 Experimental setups

The tests were carried out in the multi-function artificial climate chamber. The artificial climate chamber, with a diameter of 7.8 m and a height of 11.6 m, can simulate complex atmospheric environments such as fog, rain, ice and high altitude [13-16]. The power was supplied by the AC voltage test set (YDTW) - 500 kV/ 2000 kVA pollution test transformer, of which the maximum short current is 75 A. The applied voltage on test samples was supplied by a 50 Hz AC power. The test circuit is shown in Fig. 2. In the test circuit, T1 is the 10kV/2000 kVA voltage regulator, T2 is the 500 kV/ 2000 kVA ac testing transformer, C1 (150pF) and C2 (1.5μF) are the capacitors of the capacitor divider F (10000:1), H is a wall bushing (330 kV), R0 (10kΩ) is a current limiting resistor, G is a protective discharge tube (voltage rating 5V), r (1Ω) is a current sampling resistor, E is the artificial climate chamber. The setups meet the requirements of pollution flashover test [17-18].

Fig. 2.AC pollution flashover test circuit.

2.3 Test Procedures

2.3.1 Preparation

Before the tests, all the samples were carefully cleaned by Na2PO3 solution so that all traces of dirt and grease were removed. Then the samples were thoroughly rinsed with tap water, and let to dry naturally indoor to avoid dust or other pollution.

2.3.2 Pollution

The soluble contaminants were calculated by soluble deposit density (SDD). SDD represents the weight of soluble materials per unit area of insulator, in ‘mg/cm2’. And non-soluble materials were still calculated by non-soluble deposit density (NSDD). SDD was selected as 0.06, 0.10 and 0.25 mg/cm2 to represent three different levels of pollution. The ratio of NSDD to SDD was 3.5 in all the tests. The contamination ratio (T/B) of top to bottom surface of porcelain and glass insulators was generally selected for 1: 1, 1: 3, 1: 5, 1: 8 and 1: 15 to represent five different degrees of pollution.

The insulators were polluted by solid layer method using brush. The tests used NaCl to represent soluble contaminant, and Kieselguhr to represent non-soluble contaminant. The mathematic relationship between soluble deposit density of top and bottom surface (SDDT/SDDB) and the average soluble deposit density (SDD) of the porcelain and glass insulators can be expressed as follows:

where ST, SB are the area of top and bottom surface of the porcelain and glass insulators.

2.3.3 Wetting

Natural drying of the samples was ensured to be sufficient. Then the samples were suspended into the climate chamber. The polluted insulators were wetted by steam fog which was generated by a 1.5 t/h boiler. The fog input rate was 0.05 ± 0.01 kg/h•m3 , and the temperature in the chamber was controlled between 30 ℃ and 35 ℃ through the refrigeration system and the atmospheric pressure is 98.6 kPa in all the experiments.

2.3.4 Flashover test

The flashover tests were carried out right after the pollution layer was completely wet. In the tests, up and down method was adopted [17-18]. Each contaminated sample was subjected to at least 15 “valid” individual tests. The voltage step was approximately 5% of the expected U50. The first “valid” individual test was selected as being the first one that yields a result different from the preceding ones. Only the individual test and at least 14 following individual tests were taken as useful tests to determine U50. The U50 and relative standard deviation error (σ) are calculated as follows:

where Ui is an applied voltage level, ni is the number of tests carried out at the same applied voltage Ui , and N is the total number of “valid” tests.

For example, the scatter plot below demonstrate the flashover result of type-B insulator when T/B is 1:1, SDD is 0.1 mg/cm2. As the figure showed that the second test result is different from the first one, so it can be classified as valid results as well as the 18 results after it. Then U50 and relative standard deviation error (σ) can be calculated through those 19 valid results using Eq. (3), (4).

 

3. Test Results and Analysis

3.1 AC Flashover voltage results

Following the procedures above, ac flashover tests of 7-unit insulator strings polluted under different SDD and contamination ratio (T/B) of top to bottom surface were carried out. The results are shown in Table 2 and Table 3.

Table 2.Test results of 7-unit A-type insulator string

Table 3.Test results of 7-unit B-type insulator string

From the test results, conclusions can be made as follows:

(1) The relative standard deviations of these results are all less than 8%, which means that the dispersion of the test results is slight.

(2) The flashover voltage of insulator string decreases with the increase of SDD under a certain value of T/B. Take B-type insulator string for example, when T/B = 1:3, the value of SDD is 0.06, 0.10 and 0.25 mg/cm2, the corresponding U50 is 116.5 kV, 93.2 kV and 69.2 kV respectively, which means that the voltage decreases by 20.0% and 40.6% when the SDD increases from 0.06 to 0.10 and 0.25 mg/cm2 correspondingly.

(3) U50 was remarkably affected by the contamination ratio (T/B) of top to bottom surface of insulator, and the lower the T/B ratio, the higher the U50 of insulator strings. Take A-type insulator string for example: when SDD is 0.10 mg/cm2 , and the T/B ratio changes from 1/1, 1/3, 1/5, 1/8 to 1/15 respectively, the U50 is 90.8 kV, 98.9 kV, 105.7 kV, 110.3 kV and 118.1 kV correspondingly. The data shows that when the T/B ratio decreases from 1/1 to 1/3, 1/5, 1/8 and 1/15, the U50 will increase by 7.7%, 15.1%, 20.2% and 28.6% correspondingly.

3.2 Relationship between T/B and SPLC

The non-uniformity T/B on the top and bottom surface of insulators affects its pollution flashover voltage, and the relationship between them is shown in Fig. 4.

Fig. 4.Relationship between the flashover voltage and T/B for A-type insulator.

It can be seen from Fig. 4 that the AC flashover voltage grows with the decreases of T/B.

Eq. (2) shows that, when T/B < 1, SDDB > SDD and SDDT

The relationship between the shape factor of insulator ( f ), the conductivity of pollution layer (γ), the surface conductance (G) and the resistance of pollution layer (R) satisfy [19]:

The resistance of the whole surface pollution layer consists of the resistance of the top surface in series with that of the bottom surface. Thus from Eq. (5), the equivalent conductivity of the whole insulator surface (γeq_non) can be obtained:

where γT is the conductivity of top surface and γB is that of bottom surface. The shape factor (f) and the profile of the insulators can be expressed by [20]:

where L is the insulator surface creepage distance, dl is the increment of creepage distance, D(l) represents the diameter at distance dl, fT and fB is the shape factor of the top and bottom surface of insulator respectively, f is the total shape factor of insulator.

Since the conductivity of pollution layer (γ) is proportional to SDD, according to Eq. (2), (5), (7) and (8), the equivalent conductivity ratio (K) of the whole insulator surface under non-uniform pollution distribution to that of uniform pollution can be expressed by the function of SDD, SDDT and SDDB:

Following Standard [20] and the insulator structure in Table 1, the related technical parameters of the samples can be calculated. For A-type insulator, f, fT and fB are 0.748, 0.203 and 0.545 respectively, while For B-type insulator, they are 0.702, 0.210 and 0.492 correspondingly.

With values of f, fT and fB, the ratio K can be calculated using Eq. (8) as shown in Table 4. It can be seen that the mean pollution surface conductivity along the whole surface of insulator will get smaller if the non-uniformity of the pollution distribution between the top and bottom surfaces increases. For example, when T/B is 1/3, 1/5, 1/8 and 1/15 respectively, the K of A-type insulator is 0.859, 0.680, 0.509 and 0.318 correspondingly, which means that the comprehensive function of non-uniform pollution is to make the mean conductivity of the whole surface pollution layer decrease with the decrease of T/B. Therefore, under the same applied voltage, the leakage current may decrease with the decrease of T/B, making it more difficult for dry band to appear on the pollution layer.

Table 4.Value K of the two samples in different T/B.

3.3 Relationship between T/B and ICR

Leakage current is an important parameter of electrical property test, which contains the information of insulators operational status. In this paper, the leakage current just before flashover, namely the critical leakage current ICR, was selected as the characteristic parameter of discharge process, and the influence of T/B on ICR was analyzed. Fig. 5 shows the waveform of leakage current during the flashover process when SDD is 0.1 mg/cm2 , T/B is 1:1 of type-B insulator. Generally, the peak value at the first half cycle before flashover is defined as critical leakage current ICR, as is marked in Fig. 5. During the test, ICR of each flashover test were recorded, and the mean values corresponding to each pollution condition were calculated and shown in Fig. 6.

Fig. 5.Waveform of leakage current along the surface of insulator.

Fig. 6.Relationship between ICR and T/B of for B-type insulator string.

It can be indicated from Fig. 6 that, under a certain SDD, ICR decreases with the decrease of T/B. Take A-type insulator for example, when SDD = 0.06 mg/cm2, T/B= 1:1, the critical leakage current values is 0.595 mA, 0.526 mA, 0.507 mA, 0.489 mA and 0.478 mA respectively when T/B decreases from 1/1, 1/3, 1/5, 1/8 to 1/15. The change of ICR is also due to the influence of non-uniform pollution layer on the mean conductivity of the whole surface pollution layer.

From the mathematic flashover model in [21], the basic equation to maintain the AC arc along the polluted insulator can be expressed as follows:

where Um and Im are the peak value of the applied voltage and the leakage current; Uarc_m is the voltage on the arc; Up_m is the voltage on the residual pollution resistance. A and n are the arc constant; L is the total creepage distance; x is the length of the arc; ra is the residual pollution resistance per unit length.

Moreover, the partial arc propagation criterion is [22]:

where Ep is voltage gradient of the residual contaminated parts, Earc is arc gradient. Ep and Earc can be expressed as [22]:

According to the Eq. (11) and (12), the lower value of leakage current, the harder the Inequality (10) can be satisfied. It makes the partial arc propagation on the surface of polluted insulator difficult. Therefore for the insulator with lower T/B, the arc propagation criterion is hard to be satisfied, the applied voltage should be increased to increase the leakage current and satisfy the arc propagation criterion.

 

4. Flashover Voltage Correction under Non-uniform Pollution

The relationship between U50 and SDD under different T/B can be indicated from the test data, as is shown in Fig. 7. This figure shows that under a certain T/B, insulator strings flashover voltage U50 decreases with the increase of SDD.

Fig. 7.Relationship between U50 and SDD: (a) A-type insulator; (b) B-type insulator.

Insulator flashover voltage and salt deposit density meet negative exponent function:

where a is a coefficient associated with insulator profile and environment conditions, b is characteristic exponent characterizing the influence of SDD on U50.

Therefore, through fitting the curves in Fig. 7 by Eq. (13), the coefficient a, the influence characteristic exponent b and the fitting degree R2 of each sample in each T/B condition can be obtained, as shown in Table 5 and Table 6.

Table 5.Results of a and b of A-type insulator string in different T/B

Table 6.Values of a and b of B-type insulator string in different T/B

The fitting results reveal that the influence of T/B and SDD on ac flashover voltage should be independent. Take B-type insulator for example, the b values are 0.369, 0.360, 0.369, 0.358 and 0.369 respectively when T/B are 1/1, 1/3, 1/5, 1/8 to 1/15 correspondingly, and the mean value of b is 0.365. The relative errors between the b values and its mean value are just within 1.10% and - 1.92%, which are very small, so the influence of T/B on b is not obvious. In other words, b can be treated as a constant while T/B is changing. Therefore, the functions of T/B and SDD on U50 can be seen to be independent. According to Eq. (1) and (13), the calculation of U50 under non-uniform pollution can be expressed as follows:

Some mathematical methods and the fitting analysis based on Eq. (14) were adopted for the test data in Table 2 and 3, and then the equations for predicting the U50 of A-type and B-type insulators can be got:

Define the calculating error as:

where UfT is the test value of U50 while UfC is its calculated value using Eq. (15) and (16). ΔU (%) corresponding to each pollution condition can be calculated, as shown in Table 7.

Table 7.Calculating errors between the test values and the calculated values

It can be inferred from the table that by using Eq. (15) and (16) to calculate the U50, the relative error are all within ±3%, which suggests that the two equations for predicting flashover voltage under different T/B and SDD values are acceptable.

 

5. Conclusion

In this paper, the flashover performance of typical type insulators under non-uniform pollution was studied. Through analysis the following conclusions can be obtained:

(1) Both salt deposit density SDD and pollution non-uniformity T/B of insulator have obvious effects on flashover voltage, and their effects are independent from each other.

(2) The relationship among the ac pollution flashover voltage (U50), SDD and T/B of insulator string meets:

U50 = a·SDD−b[1−C·log(T / B)]

For the two typical types of porcelain and glass insulators, the prediction of U50 can be made by:

U50 = 46.98·SDD−0.280[1−0.271·log(T / B)] TypeA

U50 = 33.46·SDD−0.365[1−0.501·log(T / B)] TypeB

(3) The non-uniformly distribution of pollution layer on top and bottom surfaces of insulator string causes the decrease of equivalent conductivity of the whole surface. The more uneven the pollution distribution on the top and bottom surface of insulators, the smaller the mean pollution surface conductivity along the whole surface of insulators. In that case, the leakage current is lowered, which restricts the propagation of partial arc and finally causes the flashover voltage to rise.