Influence of Sheds Damage on the AC Pollution Flashover Performance of Different Voltage Class Composite Insulators

The damage of high-voltage terminal shed of composite insulators can influence the flashover performance of the insulators. The study was carried out experimentally to investigate the flashover characteristics of composite insulators with sheds damage at different voltage levels. The results show that sheds damage would decrease pollution flashover voltage, creepage distance flashover voltage gradient <inline-formula> <tex-math notation="LaTeX">$E_{L}$ </tex-math></inline-formula> and the dry arc distance flashover voltage gradient <inline-formula> <tex-math notation="LaTeX">$E_{H}$ </tex-math></inline-formula>, and such decrease range was proportional to damage degree. Besides, with the increase of composite insulator’s dry arc distance, effects of sheds damage on pollution flashover voltage, creepage distance flashover voltage gradient <inline-formula> <tex-math notation="LaTeX">$E_{L}$ </tex-math></inline-formula> and the dry arc distance flashover voltage gradient <inline-formula> <tex-math notation="LaTeX">$E_{H}$ </tex-math></inline-formula> were gradually decreasing. With the <italic>ESDD</italic> ranging from 0.1 mg/cm to 0.3 mg/cm<sup>2</sup>, AC pollution flashover voltage of 35 kV, 110 kV and 220 kV composite insulators decreased by up to 35.92%, 15.03%, 9.11% respectively; the creepage distance flashover voltage gradient <inline-formula> <tex-math notation="LaTeX">$E_{L}$ </tex-math></inline-formula> decreased by up to 19.15%, 6.05%, 1.34% respectively; the dry arc distance flashover voltage gradient <inline-formula> <tex-math notation="LaTeX">$E_{H}$ </tex-math></inline-formula> decreased by up to 35.92%, 15.03%,9.11% respectively.


I. INTRODUCTION
Composite insulators, which are extensively used in the power grid, have played an important role in reducing the pollution flashover accidents of power transmission and transformation equipment and ensuring the safety and reliability of the power grid [1]- [3].
In recent years, with the increase of environmental awareness and protection, the number of birds has in-creased dramatically, and the transmission line fault caused by bird activities has risen sharply. The bird-related fault has become the third largest fault only second to lightning stroke and external force damage, and the most dangerous type of bird-related fault has some relevance with birds' pecking composite insulators' sheds [4]- [7]. In Luoyang, Xinxiang and other areas of Henan Province, China, there have been The associate editor coordinating the review of this manuscript and approving it for publication was Boxue Du . many incidents of bird pecking composite insulator. According to statistics, the number of line trips caused by bird pecking composite insulator accounts for about 10% of the total line trips. [8].
Composite insulator sheds damage caused by birds pecking will not only reduce the creepage distance, but also cause electric field distortion [9] which will subsequently aggravate the uneven potential distribution and affect the pollution flashover characteristics of insulators as a result [10]. Up to now, the research on the pollution flashover characteristics of composite insulator at home and abroad mainly focuses on the effect of pollution types, pollution degree, wetting mode, shed profiles, altitude, temperature, hydrophobicity, etc. [11]- [19] on pollution flashover voltage and flashover process, while it has not been further studied and compared that the influence of sheds damage caused by bird pecking on the pollution flashover characteristics of composite insulators at different voltage levels. This paper aims to study the effect VOLUME 8, 2020 This work is licensed under a Creative Commons Attribution 4.0 License. For more information, see https://creativecommons.org/licenses/by/4.0/ of bird peck sheds damage on pollution flashover characteristics of composite insulators at different voltage levels by using 35kV, 110kV and 220kV composite insulators as test samples.

A. TEXT DEVICES
The experiments were performed in a large multi-functional artificial climate chamber where located in the State Key Laboratory of Power Transmission Equipment & System Security and New Technology. The voltage was applied by a 500kV/2000kVA test transformer with the rated current of 4A, maximum short-circuit current of 75A and short circuit impedance of less than 6%. Test wiring is shown in Figure 1.  Figure 2. The minimum nominal creepage distances of 35kV, 110kV and 220kV composite insulators are 1280mm, 3600mm and 6600mm respectively. JYH250-type grading ring is installed at the high voltage side of FXBW4-110/ 120 insulator; JYH305-type grading ring is installed at the high voltage side of FXBW4-220/ 100 insulator and JYH250-type grading ring is installed at the low voltage side. The parameters of the grading ring are shown in Table 1, and Figure 3 is its diagram. The ring diameter, fitting diameter, height of the joint, and pipe diameter of grading ring are represented as D, d, H, R respectively.

A. INVESTIGATION OF SHEDS DAMAGE
In this paper, 557 pieces of composite insulators pecked by birds were randomly sampled in Henan Province,   China. According to analysis based on statistical data above, the sheds damage caused by bird pecking accounted for 48.47% of the total number of bird-related damage. 84714 VOLUME 8, 2020

B. CHARACTERISTICS OF SHEDS DAMAGE 1) POSITION FEATURES
Statistics show that the composite insulator sheds damage is mainly in 1∼2 group sheds at the high voltage side, because the birds generally stand to peck at the connection part of the fittings or grading rings on the high voltage side.

2) SHAPE FEATURES
It is found that more than 80% of the shed damage caused by bird pecking are approximately circular, as shown in Figure 4. The reason is that birds generally swivel their beak and move to pick objects.

3) DEGREE FEATURES
Based on the defect area, the damage degree of the shed caused by bird pecking was analyzed. The results show that the maximum damage area of shed caused by bird pecking on a 110kV line is about 12% of the area of a healthy shed. For insulators with large and small sheds, when the damage degree of large shed exceeds the gap distance between two neighboring sheds as shown in Figure 5, the small shed will also be pecked by birds. The damage area and shape of the small shed can be approximately equivalent to the projection of the large damage shed on the small shed.

C. SHEDS DAMAGE SIMULATION 1) ARTIFICIAL SIMULATION METHOD
According to the analysis of the position and shape features of sheds, we selected 2 group of sheds on the high voltage side and the arc-shaped area to simulate defect artificially. In order to simplify the test, the damage degree of the first two groups of the high voltage side of insulators was set to be consistent.

2) DAMAGE PARAMETER SETTING UP
In this paper, 6% and 12% of the damage area were selected for research, and the case of the shed damage to the rod was also considered. For the 35kV, 110kV, 220kV composite insulator, when the damage reached the rod, the corresponding damage area was about 25%, 25%, 28% respectively. The simulation parameters are shown in Table 2, and the damage simulation diagram is shown in Figure 6.

D. TEST METHOD AND PROCEDURES
The test samples were made using the artificial simulation method. In addition, the artificial pollution tests were carried out based on the method recommended by IEC60507 and the whole test procedure is presented as follows.

1) POLLUTION PRETREATMENT
Before tests, all samples were carefully cleaned to ensure the removal of all traces of dirt and grease and then dried naturally. The surfaces of the samples were coated with a very thin layer of dry kieselguhr to destroy the hydrophobicity at the degree of WC4 or WC5. Because the layer of kieselguhr was very thin, the effect of the kieselguhr on non-soluble deposit density NSDD could be ignored.

2) ARTIFICIAL POLLUTING
The solid layer method was used to form the pollution layer on the samples, following the procedures prescribed in standard IEC 60507. Kieselguhr is used as inert materials, and industrial pure sodium chloride is used as conductive material, the ratio of non-soluble deposit density NSDD to equivalent soluble deposit density ESDD is 6:1. The ratio of pollution on upper and lower sheds surface of the insulator is 1:1.

3) WET TREATMENT OF TEST SAMPLE
The steam fog is applied to wet the pollution layer with the output rate of 0.06±0.01kg/h.m 3 . The spray direction of the fog is perpendicular to the axis direction of the insulator string.

4) FLASHOVER TEST
The flashover tests were carried out on three identical samples by rise-voltage method, with an interval of 5 minutes. Among the four flashover values of each sample, the lowest three flashover voltage values were taken. That is, the average value of nine flashover voltages of three samples was taken as the flashover voltage U f .

IV. TEST RESULTS AND ANALYSIS A. INFLUENCE OF SHEDS DAMAGE ON AC POLLUTION FLASHOVER VOLTAGE
In this paper, under the condition of ESDD of 0.1, 0.15, 0.2 and 0.3mg/cm 2 , the artificial AC pollution flashover characteristics of 35kV, 110kV and 220kV composite insulators with three kinds of damage (damage area of 6%, damage area of 12% and damage to root) were tested. The results of pollution flashover voltage test are shown in U f in Table 3. Due to the influence of the creepage distance on the pollution flashover voltage, the damage area is converted into the reduced creepage distance for analyzing the influence of the damage degree on the pollution flashover voltage. Take the change of the flashover voltage of the 35kV, 110kV, and 220kV composite insulators with the ESDD of 0.1mg/cm 2 as an example, shown in Figure 7.
It can be seen from Figure 7 that the shed damage results in the decrease of pollution flashover voltage of composite insulators. With the increase of damage degree, the decreasing range of pollution flashover voltage increases. The reason is that the reduced creepage distance increases with the increase of damage degree, and the pollution flashover voltage is positively related to the creepage distance by the Obenaus pollution flashover model theory. For example, for 110kV composite insulators, the reduced creepage distance under the condition of three damage degrees are 169.6mm, 253.6mm and 344.0mm respectively.
With the increase of insulator voltage level and dry arc distance, the influence of damage degree on flashover voltage gradually weakens. For example, when the ESDD is 0.1mg/cm 2 , the AC pollution flashover voltage of 35kV, 110kV and 220kV composite insulators with 12% damage area will be reduced by 27.02%, 11.17% and 5.03% respectively compared with those in the intact condition. The reason is that with the increase of the dry arc distance, the decreasing proportion of the composite insulator's creepage distance to dry arc distance ratio L/H decreases. For example, for a composite insulator at three voltage levels of 35kV, 110kV, and 220kV, L/H is reduced by 12.81%, 7.04%, and 4.24% respectively, when the damage area is 12%.

B. INFLUENCE OF SHEDS DAMAGE ON AC POLLUTION FLASHOVER VOLTAGE GRADIENT
The flashover voltage of insulator is related to the creepage distance L and the dry arc distance H . In this paper, the creepage distance flashover voltage gradient E L and the dry arc distance flashover voltage gradient E H are selected respectively to analyze the influence of the sheds damage on the pollution flash voltage gradient of the composite insulator. Calculation formula of E L and E H is as follows, and the unit of them is kV/cm .
where L is creepage distance, H is dry arc distance, U f is pollution flashover voltage. Based on the values of pollution flashover voltage of 35kV, 110kV and 220kV composite insulators and the creepage distance and dry arc distance of composite insulators with sheds damage, the creepage distance pollution flashover voltage gradient E L and the dry arc distance flashover voltage gradient E H can be calculated, as shown in Table 3.

1) THE INFLUENCE ON E L
For example, the variation of the 35kV, 110kV, and 220kV composite insulators' E L with the degree of contamination ESDD=0.1mg/cm 2 is shown in Figure 8.
It can be seen from Figure 8 that sheds damage leads to the decrease of composite insulator's E L and the extent increases with the increase of damage degree. For example, when ESDD=0.1mg/cm 2 and the creepage distances are 106.4mm, 164.0mm, and 265.6mm, the 35kV composite insulator's E L are 0.66kV/cm, 0.62kV/cm, and 0.60kV/cm, respectively, which are 0.08kV/cm, 0.12kV/cm, and 0.14kV/cm lower than those in the intact condition respectively. According to the Obenaus pollution flashover model theory, the pollution flashover voltage is positively related to the creepage distance. Besides, the decrease amplitude of pollution flashover voltage is larger than that of surface creepage distance, and the difference increases gradually with the increase of damage degree. For example, when ESDD is 0.1mg/cm 2 , the pollution flashover voltage of 35kV composite insulators with three degrees of damage decreases by 19.12%, 27.00% and 35.92%, and the creepage distance decreases by 8.31%, 12.81% and 20.75% respectively.
With the increase of insulator voltage level and dry arc distane, the influence of damage degree on E L gradually weakens. For example, when ESDD is 0.1mg/cm 2 , the E L of 35kV, 110kV and 220kV composite insulators with damage area of 12% are 0.62kV/cm, 0.54kV/cm and 0.45kV/cm respectively, which are 16.29%, 4.43% and 0.83% lower than those in the intact condition. The reason is that with the increase of insulator voltage level, the composite insulator's L/H decreases.

2) THE INFLUENCE ON E H
The variation of the 35kV, 110kV, and 220kV composite insulators' E H with the degree of contamination ESDD=0.1mg/cm 2 is shown in Figure 9.
It can be seen from Figure 9 that sheds damage has an effect on the composite insulator's E H , and the degree of influence is related to the reduced creepage distance and insulator voltage level. E H decreases with the increase of damage degree. For example, for 35kV composite insulators, VOLUME 8, 2020 when the ESDD is 0.1mg/cm 2 and E H at the reduced creepage distance of 106.4mm, 164.0mm, and 265.6mm are 1.71kV/cm, 1.54kV/cm, and 1.36kV/cm respectively. Compared with the intact condition, E H is reduced by 0.41kV/cm, 0.58kV/cm, and 0.76kV/cm, respectively. The reason is that the reduced creepage distance increases with the increase of damage degree, and the pollution flashover voltage is positively related to the creepage distance by the Obenaus pollution flashover model theory. Therefore, the decrease amplitude of E H increases due to the increase of pollution flashover voltage's decrease amplitude and the constant value of H .
With the increase of the insulator dry arc distance, the influence of damage degree on E H gradually weakens. For example, the dry arc distance flashover voltage gradient E H of the 35kV, 110kV, and 220kV composite insulators with a contamination degree of ESDD=0.1mg/cm 2 and damage area of 12% are 1.54kV/cm, 1.52kV/cm, and 1.41kV/cm respectively, which are 27.02%, 11.17%, and 5.03% lower than those in the intact condition. The reason is that the creepage height ratio L/H of the composite insulator decreases with the increase of dry arc distance.

V. CONCLUSION
Sheds damage results in the decrease of AC pollution flashover. With the increase of damage degree, the drop amplitude of pollution flashover voltage increases. As the insulator voltage level and dry arc distance increase, the influence on pollution flashover voltage decreases gradually. The contamination degree of ESDD is 0.1∼0.3mg/cm 2 , the pollution flashover voltages of 35kV,110kV and 220kV composite insulators decrease by up to 35.92%, 15.03% and 9.11% respectively under the condition of root damage.
The influence of sheds damage on the creepage distance flashover voltage gradient E L and the dry arc distance flashover voltage gradient E H of composite insulators is related to reduced creepage distance and voltage level. The E H and E L of the low voltage level insulator decreases as the damage degree increases. With the increase of insulator voltage level and dry arc distance, the influence on the E L and E H gradually decreases. When the contamination degree of ESDD is 0.1∼0.3mg/cm 2 , the E L of 35kV, 110kV and 220kV composite insulators decrease by 19.15%, 6.05% and 1.34% respectively, and the E H decreases by 35.92%, 15.03% and 9.11% respectively under the condition of root damage.