Discussion on the Problem of Pollution Flashover of Power Transmission and Transformation Equipment

Since the 1990s, China has made great progress in the prevention of pollution flashover of power transmission and transformation equipment. All regions have adjusted the creepage distance of power transmission and transformation equipment to varying degrees, and have successively drawn and completed the distribution map of pollution areas in the power system and implemented it. The national prevention of pollution flashover has gradually entered a standardized track. It is precisely because of the development of these works that China has not caused major economic losses and social impacts in many periodic pollution flashover accidents. However, China's pollution flashover prevention work has not been able to completely eliminate the occurrence of large-scale pollution flashover accidents. What is the fundamental reason? How can we prevent and eliminate the occurrence of large-scale pollution flashover accidents? It is worth discussing.

1 The main characteristics and causes of large-scale pollution flashover accidents

Since the 1990s, large-scale pollution flashover accidents have occurred in Northeast China, Northwest China, North China, Central China, East China and South China. The main characteristics and causes can be summarized as low insulation configuration of some lines, bad weather, large-scale environmental pollution that reduces the external insulation strength, and low cleaning quality. The equipment that has pollution flashover at different times and places is also very different. For example, in the large-scale pollution flashover accident in North China in 1990, the transmission lines mainly occurred on the suspended strings. The failure of substation equipment mostly occurred on the post insulators such as busbars, disconnectors, and surge arresters, or on the equipment that was not coated with RTV, installed with creepage skirts, or not washed with water in time. In the large-scale pollution flashover accident in 2001, the Liaoshen area was mainly concentrated in the pollution areas of level I-II; North China and Henan were mainly distributed in the pollution areas of level II-III; Beijing-Tianjin-Tangshan, Hebei, Henan and Liaoning power grids, where composite insulators were used throughout the line, almost no pollution flashover occurred. Compared with the large-scale pollution flashover in North China in 1990, the line pollution flashover had more tension strings. The pollution flashover of substation equipment mainly occurred on the post insulators (accounting for 78.0% of the total flashovers), especially the double post insulators in the heavy pollution area.

2 Problem

2.1 Limitations of cleaning

With the construction and transformation of urban and rural power grids, the Three Gorges Project, the West-to-East Power Transmission, and the construction of cross-regional power grids across the country, we must face up to the fact that it is becoming increasingly difficult to clean the current operating lines every year, especially for lines crossing mountainous areas, especially 500kV lines. The problem is that the current standards GB/T16434-1996, JB/T5895-1991 and GB5582-1993 are based on cleaning for the classification of pollution levels and the selection of external insulation. Although cleaning is the most effective anti-pollution flashover measure for the insulator string to restore the insulation strength, objective facts require that the design of polluted insulation should no longer be based on cleaning, especially for new or to be built projects.

2.2 Problems with the original pollution area distribution map

The salt density for dividing pollution levels in the current pollution area distribution map refers to the measured value on the suspension string composed of ordinary suspension insulators XP-70 (X-4.5) and XP-160. China's current lines have used about 45 million glass insulators (of which 9 million were produced by Nanjing domestic lines) and about 4 million composite insulators. The natural pollution accumulation characteristics of insulators of different materials and types are different from those of XP-70 and XP-160, and the pollution accumulation characteristics of different string structures are also different, and there is no systematic research, which will obviously cause a large deviation in the determination of pollution levels. In addition, the measured salt density values ​​are mostly the maximum salt density after one year of operation. The above problems may lead to the actual insulation configuration often not being in place. In the future anti-pollution management work, it is necessary to fundamentally adjust the salt density measurement and pollution level division methods, and re-formulate the principles for drawing pollution area distribution maps.

2.3 The main cause of pollution flashover

The basic data of the pollution withstand voltage of insulators currently used is obtained from the pollution test of short strings. Since the artificial pollution voltage flashover gradient is not strictly linear with the length of the insulator string. Therefore, when using the pollution withstand voltage method for pollution design, extrapolating the short string results to the long string will bring a large deviation. The test results of long strings show that the pollution withstand voltage value of a single piece is 40% lower than the value determined by the short string. The pollution resistance characteristics of different types of porcelain and glass insulators do not improve linearly with the increase of creepage distance. For insulators with poor umbrella shape, although the creepage distance increases greatly, the pollution withstand voltage does not increase significantly, and some even decrease. Although the creepage distance increases greatly, the local creepage distance is easily short-circuited by air gap discharge under the two conditions of pollution and moisture, which fully demonstrates that the effectiveness of the creepage distance has a great influence on the pollution withstand voltage. Appendix D of GB/T16434--1996 and Article 6 of JB/T5895-1991 both clearly point out that the creepage distance effectiveness coefficient must be considered when using the creepage ratio method to design pollution insulation. The effectiveness of creepage distance has not yet been systematically studied in China. The above reasons will undoubtedly lead to a low configuration of pollution insulation or a small margin.

3 Ideas for solving the problem of pollution flashover

The solution to the problem of pollution flashover is mainly to re-understand the design of pollution insulation.

3.1 Problems in determining the number of insulator strings according to creepage distance ratio.

At present, all countries divide the pollution level according to the pollution level and stipulate the creepage distance ratio corresponding to each pollution level. Only the former Soviet Union and China determine the number of insulator strings according to the creepage distance ratio method. The design of the former Soviet Union is different from that of China. It not only systematically considers the creepage distance effectiveness coefficient (generally 1.1-1.2), but also stipulates the 50% artificial pollution withstand voltage value under different pollution levels, that is, 220kV and below voltage levels are the corresponding rated voltage values, 330kV and 500kV are respectively stipulated as 315kV and 410kV, and only according to GB/T16434-1996 for external insulation design, which is undoubtedly lower than that of the former Soviet Union.

3.2 Problems in determining the number of insulator strings according to pollution withstand voltage

The United States, Japan and Wuhan High Voltage Research Institute of China mainly design external insulation according to pollution withstand voltage, and the pollution withstand voltage is determined by long string true type test. The design principles of pollution insulation in different countries are the same, only the design parameter values ​​are different.

According to the literature [1], the number of insulator strings N is the ratio of the pollution design target voltage value UΦmax to the maximum withstand voltage Umax of a single insulator, and the maximum withstand voltage Umax of a single insulator is a function of σ and k. The larger the σ and k, the smaller the Umax and the smaller the N, and vice versa. After σ and k are fixed, the larger the correction factor k1 considered according to the importance of the system, the larger the N, that is, the greater the pollution margin of the insulator string. The σ value is generally determined by a 50% artificial pollution withstand voltage test. As can be seen from Table 1, the values ​​of pollution insulation design parameters in different countries are different. The difference in σ values ​​is mainly caused by the equivalence of different pollution test laboratories, while the k value is mainly determined by the line design flashover probability value. If the single string flashover probability value is too high, k is undoubtedly too low and Umax is too high; if the k1 value is too low, UΦmax is too low; if both p and k1 values ​​are too low, N is too low. The values ​​of p and k1 in my country are relatively low compared with those in the former Soviet Union, the United States and Japan. It can be seen that the N value is small, the insulation configuration of the insulator string is low, or the margin is small. With the pollution of the environment, if the pollution level develops from level I (0.025mg/cm2) to level III (0.1mg/cm2), the Umax value of different types of insulators can decrease by 32.2%-44.0%. The real test results of the long string of XP-160 insulators show that the Umax value (11.81kV) of level I (0.03mg/cm2) decreases by 29.2% compared with the Umax value (8.36kV) of level III (0.1mg/cm2). There is no doubt that the number of insulator strings will increase by 31.1%-22.7% or 34.2% accordingly. Due to the height limit of the tower, it is inevitable that it cannot be adjusted. The margin should be left to the operation department when designing infrastructure.