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32ELECTRICITY 2010.3IntroductionIntroductionThe technical and economic advantages enjoyed by HVDC power transmission systems enable them to play an important role in long-distance and large-capacity power transmission, asynchronous interconnection between AC systems, as well as submarine cable power transmission 1. In recent years, the State Grid Corporation of China (SGCC) has conducted in-depth research on primary energy distribution and characteristics of power energy consumption in China, put forward the strategy of “one ultra and four larges,” namely, “to construct a strong smart grid with the ultra-high voltage power grid as the backbone, and to promote the intensive exploitation of large-scale hydropower, coal power, nuclear power and renewable energy,” and promoted the construction of HVDC power transmission systems on the basis of the Three Gorges project. By April 2010, eight HVDC power transmission systems had been put into operation with the power transmission capacity of 15 810 MW, and the total line length of 4 468 km. Five HVDC power transmission projects are under construction with a power transmission capacity of 23 600 MW, and the total line length of 7 226 km. The basic information of HVDC power transmission systems of SGCC is shown in Table 1. By Xu Lingling, Ye Tinglu and Zhang Qiping State Grid Corporation of ChinaStatistics and Analysis on Reliability of HVDC Transmission Systems of SGCCAbstract: Reliability level of HVDC power transmission systems becomes an important factor impacting the entire power grid. The author analyzes the reliability of HVDC power transmission systems owned by SGCC since 2003 in respect of forced outage times, forced energy unavailability, scheduled energy unavailability and energy utilization effi ciency. The results show that the reliability level of HVDC power transmission systems owned by SGCC is improving. By analyzing different reliability indices of HVDC power transmission system, the maximum asset benefi ts of power grid can be achieved through building a scientifi c and reasonable reliability evaluation system.Keywords: HVDC power transmission system; reliability; forced outage times; forced energy unavailability; scheduled energy unavailability; energy utilization effi ciency332010.3 ELECTRICITYTable 1 The basic information of the HVDC power transmission systems of SGCCNo.Power transmission systemVoltage grade/kVTransmission capacity/MWLine length/kmOperation time1GezhoubaNanqiao HVDC power transmission system500Bipolar 1 2001 04619892009199020082LongquanZhengping HVDC power transmission system 500Bipolar 3 000860200320063Jiangcheng HVDC power transmission system500Bipolar 3 000941200420064Lingbao back-to-back converter station120360200520075Yihua HVDC power transmission system500Bipolar 3 0001 047200620126Gaoling back-to-back converter station125Double unit 1 500200820117Lingbao back-to-back expansion project167750200920128DeyangBaoji HVDC power transmission system 500Bipolar 3 00057420092012201020149XiangjiabaShanghai UHVDC power transmission system800Bipolar 6 4001 916Under construction10 Jinpingsouth of Jiangsu UHVDC power transmission system800Bipolar 7 2002 090Under construction11 NingdongShandong HVDC power transmission system660Bipolar 4 0001 335Under construction12 HulunbeierLiaoning HVDC power transmission system500Bipolar 3 000908Under construction13 JingmenFengjing HVDC power transmission system500Bipolar 3 000977Under constructionsingle DC system is becoming larger. For example, the UHV Xiangjiaba-Shanghai and Jinping Projects which will soon be put into operation will reach the rated capacity of 6 400 MW and 7 200 MW, respectively. For HVDC systems with large capacity or ultra large capacity, any monopolar or bipolar forced outage will exert an impact on the stable operation of the sending-receiving end system. To ensure the safe operation of the system and reduce the occurrence of uncontrollable factors, the forced outage times are considered as the most important reliability index of the HVDC system by SGCC.The statistical data of forced outage times of HVDC power transmission system in SGCC during 20032009, is shown in Table 2. During this period of time, the annual average forced bipolar outage times of HVDC systems are 8 the maximum, and 0 the mininum. Jiangcheng HVDC power transmission system realized zero-time forced outage in 2006, which is extremely rare in the history of large-capacity and long-distance HVDC power With the development of technologies, the transmission capacity of HVDC system is becoming larger. The reliability level of HVDC power transmission system has become a main factor to affect the entire power grid. The major reliability evaluation indices of HVDC power transmission system include forced outage times, forced energy unavailability, scheduled energy unavailability and energy utilization efficiency4-17. In this paper, the reliability indices of HVDC power transmission system owned by SGCC during 20032009 are listed and analyzed in detail.Forced outage timesForced outage timesThe index of forced outage times reflects HVDC power transmission systems security level of controllability. The HVDC power transmission systems under the jurisdiction of SGCC are mainly used for electricity sending in large capacity. Moreover, as the technology improves, the transmission capacity of a Table 2 Forced outage times of HVDC transmission systems of SGCC during 20032009YearGezhoubaNanqiaoLongquanZhengpingJiangchengYihuaGaolingLingbaoAnnual average forced outage timesbipolarmonopolarbipolarmonopolar bipolar monopolar bipolarmonopolar Double cell Single cell monopolarbipolarmonopolar200336081.507.0020041707160.676.67200502171610.674.00200616110020.672.2520071601010710.253.2020080604150110.253.402009120105010210.202.0034ELECTRICITY 2010.3of HVDC transmission lines, the back-to-back HVDC system can always achieve better reliability indices. Meanwhile, compared with large-capacity HVDC transmission systems, the small-capacity HVDC transmission systems are easier to acquire good reliability indices in consideration of its relatively simple technology and matured equipment. Fig. 3 Annual average forced energy unavailability of HVDC power transmission systems of SGCC during 20032009(Red bar: contributed by converter station only)In recent years, by controlling the forced outage times, the forced energy unavailability shows a year-by-year downward trend. In 2009, the average forced outage unavailability reached the lowest level in history.Scheduled energy unavailabilityScheduled energy unavailabilityScheduled energy unavailability is the reduction ratio of transmission capability of HVDC system caused by scheduled outages, mainly including the system unavailability caused by yearly maintenance and vacancy of daily plans. The scheduled energy unavailability index has little infl uence on system operation, because planned outages are usually carried out when system load is low or power transmission demand is small. As the operation, maintenance and overhaul arrangement vary in different companies in the world, it makes the scheduled energy unavailability to vary greatly among different HVDC systems 18.The statistical data of scheduled energy unavailability of SGCC is shown in Table 5 and Fig.4.In 2004, 2005 and 2009, the scheduled energy unavailability of GezhoubaNanqiao HVDC project was high, which is owing to the retrofit of control and protection system and the reconstruction of DC lines. The HVDC power transmission systems worked in normal state in 2006 and 2007, with the indices tending to be stabilized. In 2008, infl uenced by the rare ice disaster, the scheduled energy unavailability of each HVDC power transmission transmission system in the world. Fig. 1 and Fig. 2 are the annual average forced bipolar and monopolar outage times of the HVDC power transmission systems, respectively. We can find from the figures that the annual average forced monopolar and bipolar outage times are declining year by year, and reached the historical lowest level in 2009 with the annual average forced monopolar outage times of 2 times, far lower than the design level of 1012 times a year.Fig. 1 Annual average forced bipolar outage times of HVDC power transmission systems of SGCC during 20032009Fig. 2 Annual average forced monopolar outage times of HVDC power transmission systems of SGCC during 20032009Forced energy unavailabilityForced energy unavailabilityForced energy unavailability is the reduction ratio of transmission capacity of the HVDC systems caused by forced outages and derating operation. In actual operation, forced energy unavailability is determined mainly by the recovery time of the forced outage system and reduced-voltage operation time. The statistical results of forced energy unavailability of SGCC during 20032009 is shown in Fig.3, Table 3 and Table 4, while Table 4 is the forced energy unavailability contributed by converter stations only.According to the data between 2007 and 2009, the forced energy unavailability caused by converter stations and DC lines are more or less the same, accounting for about 50%, respectively. Because of the absence 352010.3 ELECTRICITYsystem was on the high side. In 2009, anti-ice retrofit was carried out on HVDC lines, which made a big impact on scheduled energy unavailability. According to the condition-based maintenance schedule in SGCC, when there is no large-scale reconstruction or no special natural disasters, the scheduled energy unavailability of HVDC power transmission systems under the jurisdiction of SGCC can be controlled at around 3.5%.However, compared with other HVDC power transmission systems with large capacity and in long distance in the world, the scheduled energy unavailability of SGCCs HVDC power transmission systems is relatively high. Through the statistics and research on the indices of other projects in the world, it is found that those projects with good scheduled energy unavailability indices always have more forced outage times. To a certain extent, the two indicesscheduled energy unavailability and forced outage times, are in inverse proportion. Sufficient repair and maintenance work, although may lower the availability of the system, to some extent, it plays a role of “grinding a chopper will not delay the work of cutting fi rewood.”As for SGCC, HVDC power transmission systems which are mostly used for hydropower transmission, such as LongquanZhengping, Jiangcheng, Yihua, GezhoubaNanqiao HVDC power transmission systems and UHVDC XiangjiabaShanghai, Jinping transmission systems which are going to be put into operation, are restricted by water infl ow. In the dry season in winter, the system basically transmits with the lowest power. Sometimes, it is even in a stop and stand-by state due to little power transmission demand. While in the high water season in summer, the system works in full-load operation state for a long time, with high power transmission pressure. If monopolar or bipolar forced outages occur in this season, it would signifi cantly infl uence the power of transmission, even cause load shedding at the receiving end. Based on this, SGCC defi nes its maintenance strategy as follows: carry out scientifi c, reasonable, comprehensive and thorough examination and maintenance to the HVDC power transmission system during the low water period to ensure the equipment is in good condition; during the high water period, keep the HVDC power transmission system Table 3 Forced energy unavailability of the HVDC power transformation systems of SGCC during 20032009YearGezhoubaNanqiao/%LongquanZhengping /%Jiangcheng/%Yihua/%Gaoling/%Lingbao/%Corporation weighted average/%20030.460.530.51 20040.480.9221.30 20050.221.261.860.621.30 20060.40.150.70.430.42 20070.70.011.191.492.690.9320082.130.091.540.820.120.94 20090.230.100.190.010.220.040.13Table 5 Scheduled energy unavailability of the HVDC power transmission systems of SGCC during 20032009YearGezhoubaNanqiao/%LongquanZhengping /%Jiangcheng/%Yihua/%Gaoling/%Lingbao/%Corporation weighted average/%200313.440.894.48200416.986.221.406.01200534.413.154.257.278.7420065.223.223.774.973.8420078.692.934.506.922.835.1620082.065.1520.9513.382.1211.52 200933.415.4310.417.042.9211.859.73Table 4 Forced energy unavailability of HVDC power transformation systems of SGCC during 20072009(contributed by converter station only)YearGezhoubaNanqiao/%LongquanZhengping /%Jiangcheng /%Yihua/%Gaoling/%Lingbao/%Corporation weighted average/%20070.5500.041.342.690.5520081.520.070.160.680.120.4420090.210.100.030.010.220.040.08Fig. 4 Annual average scheduled energy unavailability of HVDC power transmission systems of SGCC during 2003200936ELECTRICITY 2010.3operating steadily, reduce the monopolar forced outage times as far as possible, put an end to the bipolar forced outage, and fi nally alleviate the infl uence on the sending-receiving end system caused by HVDC non-planned forced outage.Energy utilization effi ciencyEnergy utilization effi ciencyEnergy utilization efficiency shows the amount of actual transmission capacity by using HVDC power transmission system. This index is related with the system planning and power sending demand, and varies among different HVDC power transmission systems in the world. The statistical results of energy utilization efficiency of the HVDC power transmission systems of SGCC are shown in Table 6. The energy utilization efficiency of Lingbao back-to-back and GezhoubaNanqiao power transmission systems is relatively high. In 2008, the energy utilization efficiency of Lingbao back-to-back system was as high as 98.12%, almost running with the rated power around the whole year. The energy utilization effi ciency of GezhoubaNanqiao amounted to 84.23% in 2008, reaching a record high. The energy utilization effi ciency of Longzheng, Jiangcheng and Yihua systems is depended on the constraints of infl ows from the Three Gorges and its power distribution.Statistics and analysis on the causes of Statistics and analysis on the causes of monopolar and bipolar outages monopolar and bipolar outages The causes of HVDC power transmission systems monopolar and bipolar forced outages are divided into the following six categories: AC and auxiliary equipment, converter valve, control and protection system, DC yard equipment, DC lines and others. During 20032009, 116 monopolar and bipolar forced outages occurred in the HVDC power transmission systems of SGCC, most in the DC lines and control and protection systems.Fig.5 shows the causes of monopolar and bipolar forced outages during 20032009, among which the DC line accounts for 32%, while the control and protection system accounts for 28%. Fig.6 shows the cause of monopolar and bipolar forced outages in 2009, among which the outages caused by DC lines account for 31%, close to the average amount; the outages caused by control and protection system reduced to 15%. It shows that the control and protection system is tending to run steadily after regulation. The outages caused by AC and auxiliary equipment account for 31%, increasing a little, which was mainly caused by failure of power consumption in converter station.Fig. 5 Cause analysis of the HVDC power transmission systems monopolar and bipolar forced outage of SGCC during 20032009ConclusionsConclusions1) Through reinforced management, the reliability indices of HVDC power transmission system under the jurisdiction of SGCC increases year by year, reaching the best record in 2009.2) For large-capacity and ultra-large-capacity HVDC power transmission systems, from the view of security level of controllability, in control and under control, forced outage times is the most important index among all the reliability indices of the HVDC system. 3) Scheduled energy unavailability and forced outage times are in certain Table 6 Energy utilization effi ciency of the HVDC power transmission systems of SGCC during 20032009YearGezhoubaNanqiao/%LongquanZhengping/%Jiangcheng/%Yihua/%Gaoling/%Lingbao/%Corporation weighted average/%200356.360.6959.44200460.278.1358.7367.06200558.6177.4169.6473.171.14200667.4963.6274.3694.5969.97200771.9658.7468.1637.0993.9757.97200884.2352.9454.4254.398.1258.84200959.3659.5656.4558.9353.6185.8658.65372010.3 ELECTRICITYinverse proportion. By seeking the optimal relationship between scheduled energy unavailability and forced outage times, SGCC stipulates reasonable overhaul strategies to realize the maximum power grid asset benefi ts for SGCC.4)Through statistical analysis on all the monopolar and bipolar forced outages occurred in the past years, the equipments with weakness will be found out. Furthermore, by taking measures in an orderly way and improving the defi ciency, the forced outage times will be further reduced for SGCCs equipments, and the system reliability will be consequently enhanced. References:1 Su H T, Qi X, Wu Y. Study on market demand of UHVDC power transmission in ChinaJ. Power System Technology, 2005, 29(24): 1-5. 2 Zhou J Q, Chen W J, Xie K G, et al. A sensitivity analysis model of hvdc transmission system reliability evaluationJ. Power System Technology, 2007, 31(19): 18-23. 3 Liu Y, Wang M X. Reliability analysis on redundant configuration of protective relayings for HVDC power transmission systemJ. Power System Technology, 2008, 32(5): 51-54, 65. 4 Zhao C Y, Sun Y, Li G Ki. Control strategy of VSC-HVDC in dual-infeed HVDC systemsJ. Proceedings of the CSEE, 2008, 28(7): 97-103. 5 Ding M, Wang J J, Song Q. Reliability modeling and redundancy analysis of converter valves for VSC-HVDC power transmission system based on k-out-of-n:G modelJ. Power System Technology, 2008, 32(21): 32-36,41. 6 Wang S, Ren Z, Jiang J L. Application of hybrid reliability evaluation method for HVDC transmission systemsJ. Power System Technology, 2007, 31(12): 42-46. 7 Shen W H, Li X P, Hu M, et al. DC power transmission system reliability specifications and measures of increasing reliability J
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