Research on Insulation Configuration of 220 kV Bushing in High-Altitude Polluted Areas

Abstract

The external insulation strength of the power equipment sleeve in the substation and converter station will be reduced with the increase in altitude and atmosphere pollution. The weakness of external insulation seriously threatens the safe operation of electrical equipment. At present, there are few studies and conclusions that focus on the flashover characteristics of power equipment bushing in high-altitude polluted areas. Therefore, this paper takes the 220 kV voltage level bushing as the research object and studies the Alternating Current (AC) pollution flashover characteristics and lightning and switching impulse flashover characteristics at low pressure. The insulation configuration at high-altitude polluted areas is analyzed. Finally, the insulation configuration scheme, which is suitable for high-altitude polluted areas, is proposed. The results show that the AC pollution flashover voltages of the bushing decrease with the decrease in the atmospheric pressure and the increase in the salt density as a power exponent function. The impulse flashover voltages also decrease with the decrease in the atmospheric pressure as a power exponent function. The atmospheric pressure/pollution impact characteristic index is related to the type of voltage, the value of the atmospheric pressure, and the value of the salt density. The test bushing does not meet the insulation configuration requirements in some high-altitude polluted areas. Through the analysis and calculation of the test results, the bushing insulation configuration scheme in high-altitude polluted areas is proposed. The research results can provide a reference for the external insulation design of bushings in substations and converter stations in high-altitude polluted areas.

1. Introduction

With China’s vigorous development of energy in the central and western regions, power transmission and transformation projects need to pass through high-altitude regions such as Southeast Tibet and West Mongolia. In Southeast Tibet, the potential sites for UHV converter stations are generally higher than 2000 m [1,2,3]. The increase in altitude will lead to a serious reduction in the strength of the external insulation of insulators, bushings, and other equipment in substations and converter stations [4,5,6]. Therefore, it is of great practical engineering significance to study the external insulation characteristics and design scheme of all kinds of power equipment bushings in substation and converter stations in high-altitude polluted areas.

At present, the research on the external insulation of power equipment in high-altitude polluted areas worldwide mainly focuses on suspension insulators used in transmission lines [7,8,9,10,11,12,13]. Literature [7] studied the impact of the shape of the shed of the suspended insulator on the DC pollution flashover voltages at an altitude of 50 m and 4300 m, respectively, and the study showed that the pollution flashover voltages increase with the rise in the shed overhang, and then decrease slightly. Literature [8] studied the flashover characteristics of composite suspended insulators under different voltage levels when the shed falls off the damage, and the results showed that the shed damage reduces the insulator pollution flashover voltages, creepage distance flashover voltage gradient, dry arc distance flashover voltage gradient, and the reduction range is proportional to the degree of damage. Literature [9] experimentally investigated the impact of various environmental factors on the saltwater deposition and flashover mechanism of composite suspension insulators in coastal areas, and the results showed that the rate of pollution accumulation on the insulator surface increases with the increase in wind speed and decreases with the increase in the distance from the coastline to the inland and that the flashover voltage of insulators is two to three times lower in rainfall than in a cold fog, and that the salt density affects the flashover voltage of the insulators, no matter what the wetting rate is. Literature [10] studied the impact of shed parameters of hollow porcelain insulators on pollution flashover characteristics in high-altitude areas through simulation and experiments and found that appropriately increasing the gap between sheds and reducing the overhang of large sheds could improve the surface electric field distribution of hollow porcelain insulators, thus improving their pollution flashover characteristics. Literature [11] studied the parameter optimization of the shed of high altitude DC UHV composite post insulator. The results show that with the increase in shed overhang, the partial discharge is more obvious, but with the increase in shed spacing, this phenomenon is getting weaker and weaker. The flashover voltage decreases with increasing core diameter but increases initially and then decreases with increasing average shed overhang or large shed spacing. According to the test results, the optimal value of the ratio of large shed overhang to large shed spacing is proposed. Literature [12] studied the insulation performance of creepage dielectric of post-insulators under the combined conditions of salt pollution, high humidity, and transient atmospheric pressure. The results showed that under the application of DC voltage, the flashover voltage of insulators decreases with the increase in salt pollution level, and the longer the creepage distance of insulators, the more the insulation characteristics are affected by environmental conditions (humidity, atmospheric pressure, salt pollution, etc.). Literature [13] studied the insulation performance of suspended SiR Insulators under extremely heavy pollution conditions. The results showed that the positive DC pollution flashover voltage of SiR Insulators is 4% higher than that of negative DC pollution flashover voltage. SiR Insulators with different shapes showed different sensitivities in terms of hydrophobicity. Specific leakage has the greatest impact on insulator pollution flashover voltage gradient.

To summarize, research focuses more on the external insulation characteristics of suspension insulators used in transmission lines at present. As an indispensable part of the power system, the research on the external insulation characteristics of power equipment bushing is less but equally important. Existing studies showed that the external insulation strength of transmission line insulators in high-altitude polluted areas decreases dramatically due to the complex environment. The complex environment has an impact on the external insulation characteristics of substations and converter stations bushing in high-altitude polluted areas. However, the study on the regularity of power equipment bushing impacted by the complex environment has not yet been clarified, and the study on the external insulation configuration scheme of power equipment bushing in high-altitude polluted areas has not been concluded. Therefore, this paper takes the porcelain bushing under 220 kV voltage level as the research object and carries out the tests of AC pollution flashover characteristics, lightning, and switching impulse flashover characteristics at high altitudes. This paper also analyzes the insulation configuration of the bushing in high-altitude polluted areas, according to the test results. Finally, this paper puts forward the recommended value of technical parameters applicable to 220 kV bushing in high-altitude polluted areas, which provides a reference for the design and selection of the bushing in high-altitude polluted areas.

2. Method of Obtaining Bushing Flashover Voltage

2.1. Test Sample

The test sample used in this paper is a porcelain bushing at 220 kV voltage level. The structure and parameters of the sample are shown in Figure 1 and

Table 1. Technical parameters of test bushing.

Parameters (mm)
H	h	L	P1/P2
2180	1963	7692	71/51

. The H, h, L, P₁, and P₂ are structural height, insulation height, creepage distance, large shed extension, and small shed extension, respectively. This paper mainly studies the impact of pollution degree and altitude on the external insulation characteristics of bushings by examining methods to enhance the electrical strength of the bushings by increasing the creepage distance. The bushing is filled with SF₆ gas to ensure that no flashover occurs inside the bushing during the test. Therefore, the test process is different from the actual operation of the bushing.

2.2. Test Equipment

The AC pollution flashover tests and impulse flashover tests of bushing in high-altitude polluted areas are completed in the multifunctional artificial climate chamber of Chongqing University [14]. The laboratory has a height of 11.6 m and a diameter of 7.8 m and can simulate the climatic conditions of the altitude of 7000 m and the following. Its minimum atmospheric pressure can be adjusted up to 30 kPa, the temperature adjustable range of −40–40 °C, and the relative humidity adjustable range is 10–100%. The laboratory is fitted with a fogging system that can supply fog for the AC pollution flashover tests, and the wiring principle is shown in Figure 2 and Figure 3.

In Figure 2, T₁, T₂, F₁, R₀, and B are the voltage stabilizer, transformer, capacitance divider, limiting resistor, and bushing. In Figure 3, T, F, R, r₁, r₂, C, and g are the transformer, capacitance divider, charging resistor, wave head resistor, wave tail resistor, charging capacitance, and ball gap. D₁ and D₂ are the silicon pile.

In this paper, the AC pollution flashover test voltages are provided by the AC test transformer with rated voltage of 500 kV, and the impulse flashover tests adopt the standard lightning wave with voltage waveform of 1.2/50 μs and the standard switching wave of 250/2500 μs, which is generated by the CDYH-3200 kV/320 kJ impulse generator, and measured by the FY-3200 kV weakly damped capacitance voltage divider. The CDYH-3200 kV/ 320 kJ impulse generator and the FY-3200 kV weakly damped capacitance voltage divider were manufactured by China Yangzhou Xinyuan Electric Co., Ltd. The bushing is arranged vertically during the AC pollution flashover tests and impulse flashover tests; the impulse flashover test electrode has positive and negative polarity. The high-voltage wire is connected to the top fittings of the test bushing, while the ground wire is connected to the bottom fittings of the test bushing, as shown in Figure 2 and Figure 3.

2.3. Test Methods

(1)

Pre-treatment of the sample. Wipe the sample clean before the tests, remove the pollution accumulated on the surface of the sample, and place it in a ventilated place to keep it dry;

(2)

Sample polluting. The tests in this paper adopt the solid coating method to pollute the test bushing. The salt density of the selected filth is 0.05, 0.10, and 0.15 mg/cm², respectively, and the fixed gray density is 0.5 mg/cm². NaCl is used to simulate the soluble conductive substances, and diatomaceous earth simulates the non-soluble substances;

(3)

Barometric simulation. The altitude simulated in this paper is divided into 232, 1000, 2000, 3000, and 4000 m, corresponding to the atmospheric pressure of 98.6, 89.8, 79.4, 70.1, and 61.5 kPa, and the tests adopt a vacuum pump to adjust the atmospheric pressure in order to ensure the stability of the atmospheric pressure.

(4)

Sample wetting. The fogging system generates a uniformly distributed fog in the climatic chamber and controls the concentration of the fog to make sure that the surface of the test sample is saturated and moist.

(5)

Test sample voltage applied. The AC pollution flashover tests and impulse flashover tests in this paper adopt the constant voltage up and down method stipulated in the IEC 60507:2013 standard [15]. In the AC pollution flashover tests, the test sample is in the same degree of pollution and atmospheric pressure for many repeated tests to ensure that the impactive number of test voltages is more than 10 times. In the impulse flashover tests, the impactive number of test voltages is between 20 and 30 times, and the time interval between two voltages applied is 3 to 5 min. The U₅₀ flashover voltage in tests can be obtained from the following equation [16].

$$U_{50}=\frac{{\displaystyle \sum _{j=1}^N{U}_j}}{N}$$

(1)

The relative standard deviation equation is as follows:

$$\sigma =\sqrt{\frac{{\displaystyle \sum _{j=1}^N{(U_j-U_{50})}^2}}{N-1}}\times \frac{100\%}{U_{50}}$$

(2)

In the above equation, U₅₀ is the 50% flashover voltage, kV. U_j is the j-th impactive flashover voltage, kV. N is the number of valid tests, and σ is the relative standard deviation.

3. Analysis of Flashover Characteristics of 220 kV Bushing in High-Altitude Polluted Areas

3.1. Analysis of AC Pollution Flashover Characteristics

According to the above test procedure, AC pollution flashover tests of the bushings were performed at different atmospheric pressures. In this paper, the temperature range of the AC pollution flashover test is 22–26 °C and the relative humidity is 100%. The altitude of the test site is 232 m and the atmospheric pressure value is 98.6 kPa, which is not much different from the standard atmospheric pressure value of 101.3 kPa. Therefore, only the AC pollution flashover test of the bushing under the natural pressure state is carried out for reference, and the test under the standard atmospheric pressure value is no longer carried out. The test results are shown in

Table 2. Results of bushing AC pollution flashover tests at different atmospheric pressures.

No.	P/kPa	P/P0	SDD/(mg/cm2)
			0.05		0.10		0.15
			U50	σ/%	U50	σ/%	U50	σ/%
1	98.6	0.97	213.4	2.3	178.1	0.9	156.0	0.6
2	89.8	0.89	200.1	3.8	167.5	2.1	146.6	5.3
3	79.4	0.78	180.3	1.2	154.6	0.6	135.8	2.4
4	70.1	0.69	167.4	2.8	143.4	1.9	127.0	6.4
5	61.5	0.61	153.1	1.0	133.2	2.1	117.0	3.8

. P₀ is the standard atmospheric pressure, P₀ = 101.3 kPa. SDD is the salt density of polluted bushing.

As can be seen from Table 2:

• The relative standard deviation values of AC pollution flashover voltages of the test bushing are all less than 7%, indicating that the test results have little dispersion and the test data are reasonable and impactive.
• Atmospheric pressure has an impact on the AC pollution flashover voltage of the bushing. Under the same salt density, the pollution flashover voltages of the bushing decrease with the decrease in atmospheric pressure. For example, when the salt density is 0.10 mg/cm², and the atmospheric pressure is 98.6, 89.8, 79.4, 70.1, and 61.5 kPa, respectively, the AC pollution flashover voltage of the bushing is 178.1, 167.5, 154.6, 143.4, and 133.2 kV, respectively; that is, when the atmospheric pressure is reduced from 98.6 to 61.5 kPa, the AC pollution flashover voltage of the bushing is decreased by 25.2%.
• Salt density also has a significant impact on the AC pollution flashover voltages of the bushing. Under the same atmospheric pressure, the AC pollution flashover voltages of the bushing decrease with the increase in salt density. For example, when the atmospheric pressure is 79.4 kPa and the salt density is 0.05, 0.10, and 0.15 mg/cm², respectively, the AC pollution flashover voltage of the bushing is 180.3, 154.6, and 135.8 kV, respectively; that is, when the salt density increases from 0.05 to 0.15 mg/cm², the pollution flashover voltage of the bushing decreases by 24.7%.

It can be seen from the above analysis that both atmospheric pressure and salt density have significant impacts on the AC pollution flashover voltages of the bushing; that is, with the increase in altitude and the increase in bushing pollution, the external insulation characteristics of the bushing will be seriously reduced. In order to further analyze the impact law of atmospheric pressure and salt density on the voltages of the bushing pollution, the data in Table 2 were fitted according to Equations (3) and (4) [17,18], and the results obtained were shown in Figure 4 and

Table 3. The fitting parameter values U₀, n, R² according to Equation (3).

Fitting Values	SDD (mg/cm2)
	0.05	0.10	0.15
U0	217.5	180.9	158.3
n	0.716	0.624	0.608
R2	0.998	0.999	0.998

, Figure 5, and

Table 4. The fitting parameter values A, a, and R² according to Equation (4).

Fitting Values	P(kPa)
	98.6	89.8	79.4	70.1	61.5
A	92.5	87.4	85.5	80.5	75.9
a	0.280	0.277	0.251	0.247	0.236
R2	0.997	0.996	0.991	0.995	0.985

, respectively.

$$U_{50}=U_0\times {\left(P/P_0\right)}^n$$

(3)

$$U_{50}=A\times {\left(SDD\right)}^{-a}$$

(4)

In Equation (3), P is the actual atmospheric pressure in the tests, kPa; P₀ is the standard atmospheric pressure, P₀ = 101.3 kPa; U₅₀ is the 50% flashover voltage under the test atmospheric pressure value, kV; U₀ is the 50% flashover voltage under the standard atmospheric pressure, kV; and n is the atmospheric pressure impact characteristic index. In Equation (4), SDD is the salt density value used in tests, mg/cm²; U₅₀ is the 50% flashover voltage under the test salt density value, kV; A is a constant related to the material of the bushing structure; a is the pollution impact characteristic index.

As can be seen in Figure 4 with Table 3:

• The fitted correlation coefficients, R², are all greater than 0.99, indicating that the deviation of each data point from the fitted curve is not large and there is no abnormal data. At the same time, it also shows that under the same SDD, the AC pollution flashover voltage of the test bushing decreases with the decrease in P/P₀.
• SDD has an impact on the value of n of the bushing. The value of n decreases with the increase in salt density.
• The value of n of the test bushing under each salt density condition does not differ much, ranging from 0.608 to 0.716, with an average value of 0.649, indicating that the atmospheric pressure under each salt density condition has a similar impact on the AC pollution flashover voltages of the bushing.

As can be seen in Figure 5 with Table 4:

• The fitted correlation coefficients, R², are all greater than 0.98, indicating that the test results are uniformly distributed around the fitted curves without abnormal data points. At the same time, it also shows that the AC pollution flashover voltages of the bushing decrease with the increase in salt density in a negative power exponential function.
• The atmospheric pressure has an impact on the value of a of the bushing. The value of a decreases with the decrease in atmospheric pressure.
• The value of a of the test bushing under each atmospheric pressure condition does not differ much, ranging from 0.236 to 0.280, with an average value of 0.258, indicating that the impact of salt density on the AC pollution flashover voltages of the bushing under each atmospheric pressure condition is similar.

3.2. Analysis of Lightning Impulse Flashover Characteristics

According to the procedure in Section 2.3, lightning impulse flashover tests were performed at different atmospheric pressures. According to the test procedure of external insulation configuration specified in IEC60071-2:2018 [19], only the lightning impulse flashover test of clean bushing under different atmospheric pressure levels is carried out. The temperature range of the lightning impulse flashover test is 23–27 °C, and the relative humidity range is 80–90%. The test results are shown in

Table 5. Results of lightning impulse flashover tests of bushing under different pressure conditions.

No.	P/kPa	P/P0	Positive		Negative
			U50	σ/%	U50	σ/%
1	98.6	0.97	1221.6	0.5	1646.9	0.5
2	89.8	0.89	1122.6	0.6	1504.5	0.4
3	79.4	0.78	1013.6	1.6	1349.0	1.0
4	70.1	0.69	903.7	1.1	1190.4	2.3
5	61.5	0.61	813.9	0.9	1062.5	0.7

.

As can be seen from Table 5:

• The relative standard deviation values of the lightning impulse flashover voltages of the bushing are all within 3%, indicating that the dispersion of the test results is small and the test data are reasonable and valid.
• Atmospheric pressure has a significant impact on the lightning impulse flashover voltages of the bushing. When the atmospheric pressure decreases, the lightning impulse flashover voltage decreases. For example, when the atmospheric pressure is 98.6, 89.8, 79.4, 70.1, and 61.5 kPa, the lightning impulse flashover voltage in positive polarity is 1221.6, 1122.6, 1013.6, 903.7, and 813.9 kV. In addition, the voltage in negative polarity is 1646.9, 1504.5, 1349.0, 1190.4, and 1062.5 kV. That means when the atmospheric pressure is lowered from 98.6 to 61.5 kPa, the lightning impulse flashover voltages of the bushing under positive polarity decrease by 33.4%, and those of the bushing under negative polarity decrease by 35.5%.
• The lightning impulse flashover voltages of the test bushing have an obvious polarity effect. The voltages in negative polarity are obviously larger than the positive polarity under the same atmospheric pressure.

From the above analysis, it can be seen that atmospheric pressure has a significant effect on the lightning impulse flashover voltages of bushings. In order to further analyze the impact law of atmospheric pressure, the data in Table 5 are fitted according to the following Equation (3), and the results obtained are shown in Figure 6 and

Table 6. Lightning impulse flashover test results fitting parameters U₀, n, R².

Fitting Values	Polarity
	Positive	Negative
U0	1250.7	1690.2
n	0.870	0.938
R2	0.999	0.999

.

As can be seen in Figure 6 and Table 6:

• The fitted correlation coefficients, R², are all greater than 0.99, indicating that the deviation of each data point from the fitted curve is not large and there is no anomalous data. At the same time, it also shows that the lightning impulse flashover voltages of the bushing decrease exponentially with the decrease in atmospheric pressure.
• The atmospheric pressure impact characteristic index of the test bushing at positive and negative polarity conditions are 0.870 and 0.938. The larger the value of n is, the faster the flashover voltage decreases. This shows that the lightning impulse flashover characteristics of the bushing under negative polarity conditions are more affected by atmospheric pressure than those under positive polarity conditions.
• The value of n of the atmospheric pressure impact characteristic index is similar to that of air gap discharge. It shows that the lightning impulse flashover characteristics of the test bushing are similar to the air gap discharge characteristics, which is similar to the conclusion obtained from the literature [20].

3.3. Analysis of Switching Impulse Flashover Characteristics

According to the procedure in Section 2.3, the test bushing was subjected to switching impulse flashover tests at different atmospheric pressures. This paper only conducts the switching impulse flashover test on the clean bushing under different pressure levels. The temperature range of the switching impulse flashover test is 24–28 °C, and the relative humidity range is 80–90%. The test results obtained are shown in

Table 7. Results of switching impulse flashover tests of bushing under different pressure conditions.

No.	P/kPa	P/P0	Positive		Negative
			U50	σ/%	U50	σ/%
1	98.6	0.97	879.6	1.0	1237.1	0.7
2	89.8	0.89	854.0	0.4	1124.0	0.9
3	79.4	0.78	764.8	1.1	1047.9	3.0
4	70.1	0.69	696.3	2.9	944.1	1.2
5	61.5	0.61	633.8	0.9	861.3	0.6

.

As can be seen from Table 7:

• The relative standard deviation values of the switching impulse flashover voltages of the bushing are all within 3%, indicating that the dispersion of the test results is small and the test data are reasonable and valid.
• Atmospheric pressure has a significant impact on the switching impulse flashover voltages of bushing. When the atmospheric pressure decreases, the switching impulse flashover voltage decreases. For example, when the atmospheric pressure is 98.6, 89.8, 79.4, 70.1, and 61.5 kPa, the switching impulse flashover voltage in positive polarity is 879.6, 854.0, 764.8, 696.3, and 633.8 kV. In addition, the voltage in negative polarity is 1237.1, 1124.0, 1047.9, 944.1, and 861.3 kV. That means when the atmospheric pressure is lowered from 98.6 to 61.5 kPa, the voltages in positive polarity of the bushing decrease by 27.9%, and in negative polarity decrease by 30.4%.
• The switching impulse flashover voltages of the test bushing have a significant polarity effect. That is, under the same pressure, the voltages in negative polarity are significantly larger than in positive polarity.
• Compared with the lightning impulse flashover voltages of the bushing, the switching impulse flashover voltages are lower under the same polarity and atmospheric pressure conditions, and the switching impulse flashover voltages are 24.3% lower than the lightning impulse flashover voltages on average under positive polarity and 22.4% lower than those under negative polarity.

In order to further analyze the law of the impact of atmospheric pressure on the switching impulse flashover characteristics of bushing, the data in Table 7 were fitted according to Equation (3), and the obtained results are shown in Figure 7 and

Table 8. Switching impulse flashover test results fitting parameters U₀, n, R².

Fitting Values	Polarity
	Positive	Negative
U0	912.2	1252.9
n	0.723	0.759
R2	0.990	0.991

.

As can be seen in Figure 7 and Table 8:

• The fitting correlation coefficients R² are all greater than 0.99, the test data points are evenly distributed on both sides of the fitting curve, and the fitting correlation is good. At the same time, it also shows that the switching impulse flashover voltages of the casing decrease with the decrease in the atmospheric pressure in a power exponential function.
• The atmospheric pressure impact characteristic index of the test bushing at positive and negative polarity conditions are 0.723 and 0.759. Both of them are smaller than the value n under lightning impulse flashover conditions. That means the switching impulse flashover characteristics of the bushing are less affected by atmospheric pressure. At the same time, it also indicates that the switching impulse flashover characteristics of the bushing under negative polarity are more affected by atmospheric pressure than by positive polarity.
• The value of n of the atmospheric pressure impact characteristic index is similar to that of air gap discharge. That means the switching impulse flashover characteristics are similar to the lightning impulse flashover characteristics, and both of them are similar to the air gap discharge characteristics.

4. Analysis of Bushing Insulation Level in High-Altitude Polluted Areas

Since converter stations are often constructed at high altitudes, it is particularly important to study the insulation level of the bushing of each power equipment in the converter station in different altitude pollution areas.

4.1. Calculation of Creepage Distance

In practical engineering, the pollution withstands voltage method can be used to design the external insulation of power equipment bushings in polluted areas at different altitudes. According to the pollution withstand voltage method, Equations (5) and (6) can be used to calculate the creepage distance of the bushing [21].

$$L=\frac{k_1{U}_w}{k_2{k}_3{U}_{50}(1-3\sigma )}{L}_0$$

(5)

$$U_w=\frac{U_m}{\sqrt{3}}$$

(6)

In Equations (5) and (6), U_w is the bushing pollution that withstands voltage, kV. U₅₀ is the 50% AC pollution flashover voltage of the test bushing used in the tests, kV. L₀ is the creepage distance of the test bushing used in the tests, L₀ = 7692 mm. L is the calculated creepage distance of the bushing under the application of AC voltage in pollution areas at different altitudes, mm; k₁ is the safety margin coefficient, generally 1.1; k₂ is the correction factor for the uneven distribution of pollution, and since this paper is uniform smearing, k₂ is 1; k₃ is the difference between the probability of single flashover and multiple parallel flashover, which is usually 0.92 according to mathematical statistics analysis method. σ is the relative standard deviation value of the test results. According to the test results, the maximum σ value is not more than 7%, so σ is 7%. U_m is the highest operating phase voltage of the system, kV.

The maximum operating phase voltage U_m of the system can be obtained from the following equation [10]:

$$U_m=(1+15\%)\times \frac{U_l}{\sqrt{3}}$$

(7)

In Equation (7), U_l is the system operating line voltage because the test bushing used in this paper is 220 kV voltage level, so U_l is 220 kV. U_l = 220 kV can be substituted into Equation (7) to get the highest operating phase voltage U_m = 146 kV.

A further simplified equation can be derived from Equations (5) and (6) as shown in Equation (8).

$$L=\frac{k_1{U}_m}{k_2{k}_3\sqrt{3}{U}_{50}(1-3\sigma )}{L}_0$$

(8)

Substitute the test results in Table 2 into Equation (8) to obtain the creepage distance of the bushing under different altitudes and different pollution levels calculated according to the pollution withstand voltage method. The results are shown in

Table 9. Calculated value of creepage distance of bushing under AC voltage (L).

Creepage	SDD (mg/cm2)	Altitude (m)
		1000	2000	3000	4000
L (mm)	0.05	4905	5443	5863	6410
	0.10	5859	6348	6844	7368
	0.15	6694	7227	7727	8388

.

In this paper, the creepage distance of the selected bushing is 7692 mm, which can be seen from Table 9 to meet the insulation requirements at various altitudes under the salt density of 0.05 and 0.10 mg/cm² pollution, but under the salt density of 0.15 mg/cm², it only meets the insulation requirements at altitudes of 2000 m and below and does not meet the insulation requirements in areas of 3000 m and above.

4.2. Calibration of Impulse Flashover Voltage

In order to determine the external insulation level of power transmission and transformation equipment in high altitude areas, the impact of lightning overvoltage, switching overvoltage, and working voltage under pollution conditions must be considered respectively, and the insulation level of external insulation of equipment, mainly depends on the pollution tolerance level under operating voltage. Section 4.1 of this paper shows the calculation of the insulation size of the test bushing under the operating voltage in the high-altitude polluted areas according to the pollution withstand voltage method, but it is still necessary to calibrate the external insulation withstand of the test bushing through lightning overvoltage and switching overvoltage.

4.2.1. Calibration of Lightning Impulse Flashover Voltage

Since the bushing used in the tests in this paper belongs to the 220 kV voltage level, the actual withstand voltage under the lightning impulse voltage should be higher than 850 kV according to the rated lightning impulse withstand voltage value of the bushing standard specified in the standard IEC60071-1: 2019 [22] to ensure the safe operation of the electrical equipment.

In order to obtain the required value of the bushing creepage distance at each altitude under the application of lightning impulse voltage, the voltage value per unit creepage distance at each altitude can be calculated according to the results of the positive lightning impulse flashover tests in Table 5, and then the 850 kV voltage required in the IEC standard is divided by the voltage per unit creepage distance. The required creepage distance of bushing at each altitude can be calculated.

Therefore, based on the test results of the positive polarity lightning impulse flashover tests of the bushing at different altitudes in Table 5, the calculated values of creepage distances under each altitude region can be derived, as shown in

Table 10. Calculated value of creepage distance of bushing under lightning impulse voltage (L).

Creepage	Altitude (m)
	1000	2000	3000	4000
L (mm)	5825	6451	7235	8034

.

As the creepage distance of the test bushing in this paper is 7692 mm, according to the calculation results in Table 10, it can be seen that under the application of lightning impulse voltage to meet the insulation requirements of the altitude of 3000 m and the following areas, does not meet the insulation requirements of the altitude of 4000 m and above.

4.2.2. Calibration of Switching Impulse Flashover Voltage

After the calibration of the lightning impulse flashover voltage, it is still necessary to carry out the calibration of the switching impulse flashover voltage, and the calibration method is the same as the calibration method of lightning impulse flashover voltage in Section 4.2.1.

Referring to the provisions in the standard IEC 60071-2:2018, the safe operation of electrical equipment can be ensured when the actual withstand voltage of the bushing at 220 kV voltage level under the application of switching impulse voltage is higher than 531 kV [19].

Therefore, based on the calibration method in Section 4.2.1 and the test results of the positive polarity switching impulse flashover tests of the bushing at different altitudes in Table 7, the calculated values of creepage distances under each altitude region can be derived, as shown in

Table 11. Calculated value of creepage distance of bushing under switching impulse voltage (L).

Creepage	Altitude (m)
	1000	2000	3000	4000
L (mm)	4783	5341	5866	6445

.

As the creepage distance of the test bushing in this paper is 7692 mm, according to the calculation results in Table 11, it can be seen that the test bushing meets the insulation requirements at various altitudes under the application of switching impulse voltage.

4.3. Recommended Values for Creepage Distances of Bushing in High-Altitude Polluted Areas

After the calibration calculation of the pollution withstand voltage method and lightning/switching impulse voltage, it can be seen that the test bushing meets most of the insulation requirements of the high-altitude polluted areas but does not meet some of the insulation requirements of the high-altitude heavy polluted areas.

Table 9, Table 10 and Table 11, respectively, calculate the creepage distance requirements of the bushing at different high-altitude polluted areas under the application of AC voltage, lightning impulse voltage, and switching impulse voltage. In order to meet the insulation requirements of the bushing under the application of the three types of voltages, the values from Table 9, Table 10 and Table 11 are compared, and the maximum value is taken as the recommended value of the creepage distance of the bushing, as shown in

Table 12. Recommended value for creepage distance of bushing (L).

Creepage	SDD (mg/cm2)	Altitude (m)
		1000	2000	3000	4000
L (mm)	0.05	\	\	\	8034
	0.10	\	\	\	8034
	0.15	\	\	7727	8388

.

According to the above analysis and calculation of the test results, the creepage distance of 7692 mm of the 220 kV porcelain bushing used in the test meets the insulation requirements of the areas with an altitude of 2000 m and below under each pollution level. At the same time, it also meets the insulation requirements of the areas with pollution levels of 0.05 mg/cm² and 0.10 mg/cm², but at an altitude of 3000 m. However, it does not meet the insulation requirements of the areas with a pollution level of 0.15 mg/cm² and an altitude of 3000 m, nor does it meet the insulation requirements of the areas with an altitude of 4000 m and above under each pollution level. Therefore, it is necessary to extend the creepage distance of the test bushing to meet the insulation requirements of ultra-high altitude heavy pollution areas. The recommended creepage distance of the bushing is shown in Table 12.

5. Conclusions

The AC pollution flashover characteristics and lightning and impulse flashover characteristics of 220 kV bushing under low pressure are studied. The conclusions obtained are as follows:

• The AC pollution flashover voltages of the bushing are significantly impacted by atmospheric pressure and salt density and decrease as a power exponential function with the decrease in atmospheric pressure and the increase in salt density.
• Under the application of lightning impulse and switching impulse voltage, the flashover voltages of the bushing are also impacted by the atmospheric pressure. With the decrease in atmospheric pressure, a power exponential function decreases. In addition, the flashover voltages have an obvious polarity effect.
• According to the AC pollution flashover test results, the value of n of the atmospheric pressure impact characteristic index of the test bushing ranges from 0.608 to 0.716, and the value of a of the pollution impact characteristic index ranges from 0.236 to 0.280. According to the lightning impulse flashover test results, the values of n of the atmospheric pressure impact characteristic index of the test bushing in positive and negative polarity are 0.870 and 0.938. According to the results of the switching impulse flashover tests, the values of n of the atmospheric pressure impact characteristic index of the test bushing in positive and negative polarity are 0.723 and 0.759.
• When the test bushing works in high-altitude polluted areas, its external insulation level does not meet the requirements of insulation configuration. This paper proposes a new external insulation configuration scheme for the test bushing under high-altitude polluted areas according to the results of AC pollution flashover tests and the calibration results of lightning and switching impulse flashover voltage of the test bushing.