提高銀東直流輸電線路外絕緣性能研究

劉　輝，王洪川，沈　浩，韓正新，王　進（.國網山東省電力公司電力科學研究院，濟南　5000；.國網山東省電力公司檢修公司，濟南50000；.廣西大學，南寧　50000）

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提高銀東直流輸電線路外絕緣性能研究

劉輝1，王洪川2，沈浩3，韓正新2，王進2
（1.國網山東省電力公司電力科學研究院，濟南250003；
2.國網山東省電力公司檢修公司，濟南250000；3.廣西大學，南寧530000）

摘要：±660 kV銀東直流線路的電壓等級是我國新的直流輸電電壓等級，在世界范圍內尚屬建設與運行先例。自2010年投運以來銀東直流線路多次發生因線路異常放電導致的降壓運行。基于銀東直流1894號塔現場異常放電現象，同時結合該塔所處區域的污穢度研究，綜合考慮其地理位置及環境因素，對直流線路放電原因進行分析研究。同時結合該塔的污穢度研究，綜合考慮其他地理位置及環境因素，對該塔的異常放電原因進行分析研究。并以此為基礎提出1894號塔及整個銀東直流線路異常放電現象的對策。

關鍵詞：銀東直流；±600 kV；異常放電

0　Introduction

Recently，with economic development，imbalance between regional distribution and use of power resources is increasingly prominent．Power requirement of long-distance and large-capacity is proposed in response to this imbalance，in the context of which HVDC showing booming trend［1-2］．Since first directcurrent power transmission line project - Ningbo to Zhoushan HVDC project started in 1984［3］，research on DC transmission line is always committed［1-2］．

As the first and only power transmission level in the world，±660 kV DC power transmission demonstration project from Yinchuan to Jiaodong starts from Yinchuan east converter station in Lingwu to Jiaodong converter station in Qingdao through Ningxia，Shaanxi，Shanxi，Hebei，Shandong，connecting the Northwest Power Grid to North China Power Grid，whose path length is 1 335 km，including 3．3 km crossing the Yellow River in Shandong［4-5］．It enriches the standard sequence of DC voltage in the world and plays a more and more important role in solving the short of power supply．

According to statistics，since November 2010，Yinchuan-Jiaodong DC line run with lower voltage five times due to the occurrence of abnormal dis-charge．Yinchuan -Jiaodong DC line running with lower voltage would result in economic loss，stress and other issues of regional power，while the frequent occurrence of abnormal discharge in the region increases the risk of flashover discharge．Once massive power outages accident occurs，consequences will be incalculable．Typically flashover accident with wide-ranging，long power outages and other characteristics are serious threats to the safe operation of power systems and power transmission equipment，resulting in significant economic losses．

At 18∶50 on March 21，2012，No．1894 strain tower pole I line appeared abnormal discharge phenomenon，accompanied by a strong sound，with live weather rainy．Pole I line small-side had serious abnormal discharge level with four strong and stable discharge phenomenon occurring，threating to security and stable operation of the line seriously．Considering the weather conditions，voltage step -down was operated to ensure security and stability of the DC transmission．

Figure 1 shows that the insulator surface has many white spots whose peripheral surface accumulates filth，and the spot position is in a dry area where partial discharge formed，signifying degree of abnormal discharge severity．

Figure 1　The insulator of No. 1894 after discharging

On January 14，2013，during night patrol composite insulators on the left side of V-shaped string of the No．1894 tower poleⅠlarge-side were found about twelve discharge points，accompanied by a large discharge sound，with haze weather．Discharge mitigated after voltage lowered by twenty percent．

On January 20，2013，during night patrol，discharge on the left side of poleⅠlarge -side V -shaped string composite insulator of the No．1894 tower was found．At the same time，composite insulators on both poles of No．1893 tower had discharge，too．The discharge arcing disappeared after voltage lowered by twenty percent at twenty -two fifty．

On January 31 and February 2，2014，No．1894 tower，No．1893 tower，No．1899 tower，No．1900 and No．2326 occurred insulator discharge，with frog or haze．The discharge disappeared after voltage lowered by thirty percent．

In summary，abnormal discharge problem of Yinchuan-Jiaodong DC transmission line should be taken seriously enough．In this paper，based on Yinchuan -Jiaodong DC transmission line abnormal discharge phenomenon，combined with the degree of contamination，considering its geographical location and environmental factors，the cause of abnormal discharge is researched．On this basis，countermeasures are propose to deal with Yinchuan -Jiaodong DC transmission line abnormal discharge．

1　Abnormal Discharge Cause

1.1Characteristics of the environment and filth accumulating

In recent years，with the rapid development of the industrial and agricultural production in Shandong province as well as the rise and development of township enterprises，transmission lines in the farmland and suburb which have distance with the original source of pollution in the early time have been polluted as the result of the increase of traffic volume，the increasing usage in fertilizers and pesticides as well as the expanding influence brought by sea fog．The air pollution and particulates from afar have become a very serious threat to the safe operation of line with the pollution levels around increasingly intensifying and the scope of contami-nation expanding．Meanwhile，local sources also increase year by year，and only in 2014 there are 193 newly increased sources of pollution shown in Figure 2．

Figure 2　Distribution of increased pollution sites of Shandong province in 2014

Air pollution in most parts of Shandong province is quite serious．The content of sulfur dioxide and nitrogen oxides is particularly high in Jinan，Zibo，Weifang，Qingdao，Jining，Laiwu，Zaozhuang，Binzhou，because of the booming of heavy industry such as metallurgical，chemical，electric power，chemical fertilizer，pesticide and so on，all of which pollute the air seriously．The electrical conductivity of precipitation is higher in some areas such as Qingdao，Jinan，Dongying and Yantai．The high conductivity of precipitation in Qingdao，Yantai and other coastal areas partly results from the nearer distance with the sea，while mainly caused by air pollution．

PH values of precipitation are higher or lower in some areas such as Jinan，Zibo，Weifang，Qingdao，Taian，Liaocheng and Heze while Qingdao has the lowest value and Tai’an holds the highest，easy to form acid or alkaline rain．In recent winter and spring，severe fog and haze occurs in Shandong Province．Among the 391 contamination samples tested by Shandong company in 2014，43 measuring points had their contamination level increased mostly concentrated in Dezhou，Heze，Binzhou，Jinan，Laiwu，Liaocheng，Weifang and Linyi．

1.2Insulation configuration of Yinchuan-Jiaodong DC line

Towers ranged from No．1884 to No．2807 of Yinchuan-Jiaodong DC line cross Shandong province from west to east．Among all the towers，No．1884～2119 towers are in the area with polluted class C （medium polluted area with designed salt density value of 0．08 mg/cm2，the number of strain insulator string is 60，the height of jumper composite insulator string is 9 200 mm，the height of linear composite insulator string is 8 500 mm）．Others are in the area with polluted class D（heavy polluted area with designed salt density value of 0．15 mg/cm2，the number of strain insulator string is 72，the height of jumper and linear composite insulator string are both 9 200 mm）．Detailed parameters of the strain porcelain insulators and composite insulators of Yinchuan-Jiaodong DC line are shown in Table 1 and Table 2．Table 3 shows designed salt density value the value of Yinchuan-Jiaodong DC line．

Table 1 The strain porcelain insulator configuration values

Notes:①and②are length of composite insulator（m）/creepage distance（m）．

Table 3 Polluted area salt density design values

1.3 Long tim e of filth accumulating

The climate of Shandong province is monsoon climate with warm temperate，which is characterized by relatively concentrated rainfall and high temperature in summer and low-temperature drying in winter with the annual average temperature 11～14℃．The rainfall is quite imbalanced with 300～600 mm concentrated from June to August，100～200 mm from September to November，15～50 mm from December to February and no more than 100 mm from March to May．Table 4 shows the statistical data of annual precipitation in northern Shandong from which we can conclude that the rainfall around No．1894 tower is often short．As the rainfall concentrated in summer，there are no effective precipitation in the area of No．1894 tower which causes continuously accumulation of filth on the insulator surface contamination aggravating the abnormal discharge level to some extent．

Table 4 The statistical data of annual precipitation

1.4 High speed of filth accumulating

During annual maintenance for Yinchuan-Jiaodong DC line in April 2012，for No．1894 contamination of tower pole I side insulators and composite insulators of No．1894 tower was sampling analyzed．Results showed that the average salt density of porcelain insulators was 0．12 mg/cm2，ash density was 0．6～0．7mg/cm2，the highest salt density at the lower surface had reached 0．231 mg/cm2．The average salt density of composite insulators was 0．167 mg/cm2，the average ash density was 1．0 mg/cm2，the highest salt density at the lower surface had reached 0．277 mg/cm2．Figure 3 shows the reference of insulator contamination situation of No．1894 tower．

Obviously the speed of filth accumulating is quite fast．At the same time the speed is higher in DC line than AC because of the DC electrostatic precipitation．DC voltage charged particles subjected to a constant force in the direction of the electric field and are attracted to the insulator surface which results in the higher speed of accumulation and more amount of the dirt compared to AC voltage under the same conditions．The contamination of DC insulators is twice as much as that of AC which further exacerbated the abnormal discharge of No．1894 tower．

Figure 3　The reference of insulator contam ination situation of No. 1894 tower

1.5The stability of DC arc

Compared with the AC arc，DC current at constant DC voltage will never reach 0 at any time，which results in higher stability and longer duration than AC．As the increase of the degree of contamination，DC flashover voltage decreases faster than AC．DC flashover voltage is lower by 15%to 30% than AC at the same level of contamination．

1.6Insulator sufficient moisture

Through the four accidents of No．1894 mentioned above，we could find out that abnormal discharge usually occurs when the weather conditions are relatively poor such as rain，fog，haze and snow．In such weather，the insulator surface will gradually damp as well as the filth on the surface which contains conductive material which could greatly reduce the insulation strength of the insulator surface，resulting in abnormal discharge．

2　Countermeasures to the Abnormal Discharge

2.1Brushing RTV coating

Measures to decrease the pollution flashover in power system are mainly as below，adjustment insulator creepage distance，insulator cleaning，using of composite insulator and brushing RTV coating．Taking things considered，it is found brushing RTV （PRTV）coating is the most economical and effective measure．

During annual maintenance for Yinchuan-Jiaodong DC line in 2012（to be confirmed），No．1894 tower of porcelain insulators was brushed with RTV coating which weaken the discharge to a certain extent，but not completely suppressed discharge．Figure 4 shows the strain porcelain insulators of No．1894 covered with RTV coating in Yinchuan-Jiaodong DC line．

Figure 4　Porcelain insulator coated w ith RTV

2.2 The usage of floral umbrella type composite insulator

Figure 5 shows a type of large floral umbrella flower±660 kV DC composite insulator，including a ring-shaped metal connector at the top marked as 1，seal ring at the end of the mandrel marked as 2，grading ring connected to the ground marked as 3，the epoxy glass fiber rods located in the center and the sheath coated marked as 4，some large umbrella skirt on the surface of the sheath marked as 5，some large umbrella skirt on the surface of the sheath marked as 6，some small umbrella skirt on the surface of the sheath marked as 7 and the small grading ring at the high voltage marked as 8．

The ring-shaped metal connector 1 at the top suffered a hot -dip galvanizing process．Both the grading ring connected to the ground and high voltage are made of aluminum alloy material．The number of super large umbrella skirt is 13，the number of large umbrella skirt is 94 and the number of small umbrella skirt is 214．The structure is two small umbrella skirt with one large umbrella skirt performing as big-small-small form．The original composite insulators have 107 large umbrella，now using 13 large umbrella，94 large umbrella instead，changing the large umbrella counted by the cross arm side with number as 1，9，17，25，33，41，49，57，65，73，81，89，97 to the super large umbrella．The number and position of small umbrella maintains unchanged．The position of large umbrella is easy to adjust．The use of super large umbrella increases the±660 kV DC composite insulators creepage distance from the original 38 400 mm to 40 400 mm，which improves electric field distribution on the±660 kV DC insulator surface，reduces the local field strength，suppresses the occurrence of partial discharge，increases the flashover voltage and finally reduces the probability of occurrence of flashover．

Figure 5　Construction of the floral umbrella type com posite insulator

2.3 Ad justment on the creepage distance of insulators

At the beginning of Yinchuan-Jiaodong DC line put into operation in 2011，the length of insulator on strain tower of No．1894 of was 60 porcelain insulators．Due to the serious abnormal discharge and the limited effect after brushing RTV，the length was adjusted to 75 in 2014 after which noanomalous discharge has happened yet．The reason was that the increase of the insulator sheet decrease the voltage shared by each sheet which reducing the likelihood of occurrence of discharge．Figure 6 shows the electric field distribution simulation before and after the adjustment on the creepage distance．It is obvious that the electric field strength has declined after the adjustment．

Figure 6　The electric field distribution simulation of each sheet of porcelain insulator

3　Conclusions

During annual maintenance for Yinchuan-Jiaodong DC line for No．1894，the result of analysis suggests that DC line has higher speed of filth accumulating for its electrostatic precipitation．As the DC current never reach 0，the DC arc is more stable resulting in lower flashover voltage．

The air pollution and the imbalance of the precipitation in northern Shandong enhance the accumulation of contamination as well as the bad weather such as rain，fog，haze and snow．

Countermeasures to deal with the abnormal discharge of No．1894 tower and other towers in this area include brushing RTV coating，using floral umbrella type composite insulator and adjusting the creepage distance of insulator．

Reference

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［2］ZHANG Wenliang，YU Yong-qing，LI Guang-fan，et al．Researches on UHVDC Technology［J］．Proceedings of the CSEE，2007，27（22）：1-7．

［3］BAI Jimin，ZHANG Baolin，HE Dayu，et al．The Performances of Weak Receiving System of Zhoushan and Its Improving Measures［J］．Proceedings of the CSEE，1988，8（6）：39-44．

［4］LIU Hongzheng，LIU Kai，MENG Hailei，et al．Experimental Research of Live Working on±660 kV Double Circuit DC Transmission Line on the Same Tower［J］．High Voltage Engineering，2011，37（12）：2997-3005．

［5］ZHANG Wenliang，LIAO Weiming，DING Yujian，et al．Selection of the Minimum Air Clearance for±660 kV Doublecircuit DC Transmission Line Tower in Different Altitude Areas［J］．Proceedings of the CSEE，2008，28（34）：1-6． Accepted data：2015-12-15

LIU Hui（1986）received Master degree from Tsinghua University in 2011．He is now an engineer in State Grid Shandong Electric Power Research Institute．His research interests include transmission line and pollution flashover．

中圖分類號：TM726．1

文獻標志碼：A

文章編號：1007-9904（2016）02-0021-06