交直流混合系统静态电压稳定分析与应用
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摘要
长久以来,电力系统界的专家学者所关注的稳定问题主要是指功角稳定问题,但20世纪70年代,世界范围内发生了多起电压不稳定事故,造成了重大的经济损失和长时间大面积停电,电压稳定性研究也逐渐成为电力工程界的学者和工程师们密切关注的问题,有关电压稳定的研究成果也不断出现,但与电力系统功角稳定的研究相比,电压稳定的研究仍然不够深入。
随着电力工业的迅速发展,电力系统的结构日趋复杂,高压直流输电的输送容量占系统总容量的比例也越来越大,但高压直流输电一个很大的缺陷就是换流站需要消耗大量的无功,约为其所输送有功的40%-60%,这使得与直流相连的交流端的电压稳定问题变得日趋严重,尤其是对受端较弱的交流系统,微小的扰动或故障都可能引起换流站交流母线电压的崩溃,因此对交直流混合系统的电压稳定性进行研究是很有必要的。
由于直流系统运行和控制方式的多样性,分析交直流混合系统电压稳定的方法比较复杂,目前一般都是把纯交流系统的电压稳定分析方法推广到交直流混合系统中去,但推广的难点在于：随着负荷的加重,换流站交流母线电压不断下降,需要不断地考虑直流系统变量是否越限及控制方式是否需要调整。
通常是将交直流混合系统分为交流系统和直流系统两部分分别进行建模分析,由于直流系统的控制方式一旦确定,注入任意一个换流站的功率只与该换流站的交流母线电压幅值和该换流站所处直流系统的其它换流站交流母线电压幅值有关,即直流系统被等效掉了,这样就把交直流混合系统简化为纯交流系统,从而可以利用纯交流系统的方法对其进行各种计算。
由于常规连续潮流算法一般是用标准牛拉法进行潮流计算,计算速度较慢,本文提出了一种改进方案,改进方案在初始阶段采用快速解耦法进行计算,能够大大加快计算速度,而且考虑到λ-Ⅴ曲线的拟二次性,在预测环节使用拉格朗日二次插值方法且在校正环节使用局部参数法,这种自动变步长的方法可以解决步长选择问题,能更精确、快速地求得系统的电压稳定极限和网络节点的λ-Ⅴ曲线。实践证明,上述改进方案是有效可行的。
最后将改进的连续潮流方法应用于Visual C++6.0开发的一款电力系统电压稳定性分析软件VSAS,针对山东电网运用VSAS进行了简单的电压稳定分析,给出某一重负荷情况下的电压稳定裕度。分析结果表明,济南、潍坊和青岛地区是山东电网电压稳定的薄弱区域,虽然宁东直流输电工程的投运在一定程度上缓解了东部电网的能源紧张问题,但山东电网西电东送的济南-淄博-潍坊-崂山北传输路径的电压稳定情况仍然比较薄弱,所以在坚持电源合理分散、平衡布局原则的基础上,山东电网仍需要大规模的西电东送。
For a long time, the focus of experts in power system stability is the power angle stability, but seventies in the 20th century, there were several power utilities related to voltage instability in the world, which resulted in large economic loss and long time blackout. After that, the study of voltage stability has gradually become experts and scholars'concerns, but compared to the study of power angle stability, the study of voltage stability is still not enough depth.
With the rapid development of the power industry, the structure of power grid is becoming more and more complex. The proportion of HVDC in power system is becoming larger and larger, But one of the problems of HVDC is converters'spending lot's of reactive power, which is 40%-60%of the DC active power, and this makes the voltage stability problem of DC system side connecting to AC system becomes more serious, Especially for AC systems whose receiving end is weak, small disturbances or failures can easily lead to collapse of AC system bus voltage of converter, so the study of voltage stability of AC/DC systems is necessary.
Because of the diversity of DC system operation and control, the analysis of AC/ DC system voltage stability is very complex, many voltage stability analysis methods used for AC/DC system are resulting from AC system, but the difficulty is as the increase of load, the voltage of converter AC bus declines, so the variable of DC system and adjustments of the operation mode need to be constantly considered
The AC/DC power system can be divided into AC system and DC system two parts and analyzed separately, since the control of DC system is established, the injection power into a converter is decided only by the AC bus voltage magnitude of the converter and the rest of converters in DC system, then the DC system is equivalent to a AC system, so in this way put the flow calculation of an AC/DC system is simplified into a pure AC system flow calculation.
Because conventional continuation power flow algorithms are carried out using standard N-R power flow calculation, the calculation speed is very slow, so this paper proposed an improved scheme, we used fast decoupled power flow calculation in the initial stage, which can greatly accelerate the calculation speed, and taking into account the similarly quadratic characteristic of theλ-V curve, we used Lagrange quadratic interpolation in the prediction step, and local parameter method in the correct step, and in this way the step can vary automatically, which solves the difficult of selecting steps. We can quickly and accurately obtain the static stable limit andλ-V curves of nodes through the improved CPF. And the result of test system verified the feasibility and effectiveness of the improvements.
Finally we put the improved CPF into use through a software named VSAS developed by Visual C++. We used VSAS to analyze the voltage stability of Shandong Power Grid and got the static stable limit under heavy load. The results show that, Jinan, Weifang and Qingdao are weak areas of voltage stability in Shandong power grid. Although the Ningdong HVDC project eases the energy shortage in eastern Shandong power grid, the north power transmission path of Jinan-Zibo-Weifang-Laoshan is still very weak, so based on the principle of the power being laid out balanced and reasonably, Shandong power grid still needs large-scale power transmission from west to east.
随着电力工业的迅速发展,电力系统的结构日趋复杂,高压直流输电的输送容量占系统总容量的比例也越来越大,但高压直流输电一个很大的缺陷就是换流站需要消耗大量的无功,约为其所输送有功的40%-60%,这使得与直流相连的交流端的电压稳定问题变得日趋严重,尤其是对受端较弱的交流系统,微小的扰动或故障都可能引起换流站交流母线电压的崩溃,因此对交直流混合系统的电压稳定性进行研究是很有必要的。
由于直流系统运行和控制方式的多样性,分析交直流混合系统电压稳定的方法比较复杂,目前一般都是把纯交流系统的电压稳定分析方法推广到交直流混合系统中去,但推广的难点在于：随着负荷的加重,换流站交流母线电压不断下降,需要不断地考虑直流系统变量是否越限及控制方式是否需要调整。
通常是将交直流混合系统分为交流系统和直流系统两部分分别进行建模分析,由于直流系统的控制方式一旦确定,注入任意一个换流站的功率只与该换流站的交流母线电压幅值和该换流站所处直流系统的其它换流站交流母线电压幅值有关,即直流系统被等效掉了,这样就把交直流混合系统简化为纯交流系统,从而可以利用纯交流系统的方法对其进行各种计算。
由于常规连续潮流算法一般是用标准牛拉法进行潮流计算,计算速度较慢,本文提出了一种改进方案,改进方案在初始阶段采用快速解耦法进行计算,能够大大加快计算速度,而且考虑到λ-Ⅴ曲线的拟二次性,在预测环节使用拉格朗日二次插值方法且在校正环节使用局部参数法,这种自动变步长的方法可以解决步长选择问题,能更精确、快速地求得系统的电压稳定极限和网络节点的λ-Ⅴ曲线。实践证明,上述改进方案是有效可行的。
最后将改进的连续潮流方法应用于Visual C++6.0开发的一款电力系统电压稳定性分析软件VSAS,针对山东电网运用VSAS进行了简单的电压稳定分析,给出某一重负荷情况下的电压稳定裕度。分析结果表明,济南、潍坊和青岛地区是山东电网电压稳定的薄弱区域,虽然宁东直流输电工程的投运在一定程度上缓解了东部电网的能源紧张问题,但山东电网西电东送的济南-淄博-潍坊-崂山北传输路径的电压稳定情况仍然比较薄弱,所以在坚持电源合理分散、平衡布局原则的基础上,山东电网仍需要大规模的西电东送。
For a long time, the focus of experts in power system stability is the power angle stability, but seventies in the 20th century, there were several power utilities related to voltage instability in the world, which resulted in large economic loss and long time blackout. After that, the study of voltage stability has gradually become experts and scholars'concerns, but compared to the study of power angle stability, the study of voltage stability is still not enough depth.
With the rapid development of the power industry, the structure of power grid is becoming more and more complex. The proportion of HVDC in power system is becoming larger and larger, But one of the problems of HVDC is converters'spending lot's of reactive power, which is 40%-60%of the DC active power, and this makes the voltage stability problem of DC system side connecting to AC system becomes more serious, Especially for AC systems whose receiving end is weak, small disturbances or failures can easily lead to collapse of AC system bus voltage of converter, so the study of voltage stability of AC/DC systems is necessary.
Because of the diversity of DC system operation and control, the analysis of AC/ DC system voltage stability is very complex, many voltage stability analysis methods used for AC/DC system are resulting from AC system, but the difficulty is as the increase of load, the voltage of converter AC bus declines, so the variable of DC system and adjustments of the operation mode need to be constantly considered
The AC/DC power system can be divided into AC system and DC system two parts and analyzed separately, since the control of DC system is established, the injection power into a converter is decided only by the AC bus voltage magnitude of the converter and the rest of converters in DC system, then the DC system is equivalent to a AC system, so in this way put the flow calculation of an AC/DC system is simplified into a pure AC system flow calculation.
Because conventional continuation power flow algorithms are carried out using standard N-R power flow calculation, the calculation speed is very slow, so this paper proposed an improved scheme, we used fast decoupled power flow calculation in the initial stage, which can greatly accelerate the calculation speed, and taking into account the similarly quadratic characteristic of theλ-V curve, we used Lagrange quadratic interpolation in the prediction step, and local parameter method in the correct step, and in this way the step can vary automatically, which solves the difficult of selecting steps. We can quickly and accurately obtain the static stable limit andλ-V curves of nodes through the improved CPF. And the result of test system verified the feasibility and effectiveness of the improvements.
Finally we put the improved CPF into use through a software named VSAS developed by Visual C++. We used VSAS to analyze the voltage stability of Shandong Power Grid and got the static stable limit under heavy load. The results show that, Jinan, Weifang and Qingdao are weak areas of voltage stability in Shandong power grid. Although the Ningdong HVDC project eases the energy shortage in eastern Shandong power grid, the north power transmission path of Jinan-Zibo-Weifang-Laoshan is still very weak, so based on the principle of the power being laid out balanced and reasonably, Shandong power grid still needs large-scale power transmission from west to east.
引文
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[2]周双喜等.电力系统电压稳定性及其控制[M].北京：中国电力出版社,2004：14,171-174
[3]王正风.电力系统电压稳定的综述[J].电气时代,2007,12(1)：103-104.
[4]Y.Mansour,ed. Voltage stability of power systems:Concepts, analytical tools and industry experience, IEEE Publication 90TH0358-2-PWR,1990.
[5]GIGRE Task Force 38.02.10, "Cigre technical brochure:Modeling of voltage collapse including dynamic phenomena", ELECTRA No.147 April 1993.
[6]GIGRE Working Group 34.08, "Protection against voltage collapse", Edited by D.Karlsson, GIGRE,21 Rued'Artois-F-75008 PARIS.
[7]IEEE/CIGRE Joint Task Force, Definition and classification of power system stability, CIGRE,2003.
[8]IEEE/CIGRE joint task force on stability terms and definitions, Definition and classification of power system stability, IEEE,200419(2):1387-1401.
[9]孙华东,汤涌,马世英.电力系统稳定的定义与分类述评[J].电网技术,2006,30(17)：31-35.
[10]中华人民共和国电力行业标准,电力系统安全稳定导则(DL 755-2001),中国电力出版社,2001.
[11]苏永春,程时杰,文劲宇,刘岂.电力系统电压稳定性及其研究现状(一).电力自动化设备[J],2006,26(6)：97-101.
[12]陈鸿煜.基于分岔理论的交直流输电系统电压稳定性研究[D].上海大学硕士论文,2008：3.
[13]N. Flatabo, R. Ognedel, T. Carlsen. Voltage stability condition in a power transmission system calculated by sensitivity methods, IEEE Trans, on PWRS, 5(4),1999:1286-1293.
[14]Y.Tamura, H. Mori, S. Iwamoto, "Relationship between Voltage Instability and Multiple Load Flow Solutions in Electric Power Systems", IEEE Trans on PAS, 102(5),1983:1115-1125.
[15]Haque M H. Determination of steady-state voltage stability limit using P-Q curve. IEEE Power Engineering Review,2002,22(4):71-72.
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