with a radius of the Thévenin?s impedance. For normal operation the Zthevis smaller than Zload (i.e. it is outside the circle) and the system operates on the upper part (or the stable region) of a conventional P-V curve [2].
However, when ZthevexceedsZloadthe system operates on the lower part (or unstable region) of the P-V curve, indicating that voltage collapse has already occurred. At the maximum power point, the load impedance becomes same as the Thévenin?s (ZL?Zthev). Therefore, for a given load impedance (Zload), the difference between
Zthevand Zloadcan be considered as a safety margin. Hence the voltage as given in an IEEE survey, which described (111) schemes from (17) different countries [8].
Fig. 6. Load actions to prevent from voltage instability
2.3. Advantages of the proposed VIP algorithm
By incorporating the measurements from other load buses (Figure 3), the proposed VIP algorithm achieves a more accurate value of Zload . The on-line tracking of Zthev is used to track system changes.
The proposed improvements in the VIP algorithm will result in better control action for power system voltage stability enhancement. The control measures are normally shunt reactor disconnection, shunt capacitor connection, shunt VAR compensation by means of SVC?s and synchrouns condensers, starting of gas turbines, low priority load disconnection, and shedding of low-priority load [8]. Figure 6 shows the most commonly used remedial actions .
3. Conclusions
An improved Voltage Instability Predictor (VIP) algorithm for improving the voltage stability is proposed in this paper. The previous VIP method [7] used measurements only from the bus where the relay is connected. The new method uses measurements from other load buses as well. The voltage instability margin not only depends on the present state of the system but also on future changes.
Therefore, the proposed algorithm uses an on-line tracking Thévenin?s equivalent for tracking the system trajectory. The algorithm is simple and easy to implement in a numerical relay. The information obtained by the relay can be used for load shedding activation at the bus or VAR compensation. In addition, the signal may be transmitted to the control centre,where coordinated system-wide control action can be undertaken. The algorithm is currently being investigated on an IEEE 30 bus system and results using the improved VIP algorithm will be reported in a future publication.
References
[1] M.H.Haque, “On line monitoring of maximum permissible loading of a power system within voltage stability limits”, IEE proc. Gener. Transms. Distrib.,Vol. 150, No. 1, PP. 107-112, January, 2003
[2] V. Balamourougan, T.S. Sidhu and M.S. Sachdev, “Technique for online prediction of voltage collapse”, IEE Proc.Gener.Transm. Distrib., Vol.151, No. 4, PP. 453-460, July, 2004 [3] C.A. Anizares, “On bifurcations voltage collapse and load modeling “IEEE Trans. Power System, Vol. 10, No. 1, PP. 512-522, February, 1995
[4] T.J Overbye and S.J Demarco, “Improved Technique for Power System voltage stability assessment using energy methods“, IEEE Trans. Power Syst., Vol. 6, No. 4, PP. 1446-1452, November, 1991
[5] P.A Smed Loof. T. Andersson, G. Hill and D.J,”Fast calculation of voltage stability index”, IEEE Trans. Power Syst. Vol. 7, No. 1, PP. 54-64, February, 1992
[6] K. Ohtsuka ,” An equivalent of multi- machine power system and its identification for on-line application to decentralized stabilizers”, IEEE Trans. Power Syst., Vol. 4 No. 2, PP. 687-693, May, 1989
[7] Khoi Vu, Miroslav M Begovic, Damir Novosel, Murari Mohan Saha, “ Use of local
Measurements to estimate voltage – stability margin “ IEEE Trans. Power syst. Vol. 14, No. 3, PP. 1029-1035, August, 1999
[8] G.Verbic and F. Gubina “Fast voltage-collapse line protection algorithm based on local phasors”, IEE Proc.Gener.Transm. Distrib., Vol. 150, No. 4, PP. 482-486, July, 2003
译文：
一种特殊的预防电压波动的保护方案
摘要
电压的波动与输电线路的最大负载能力密切相关。输电系统中电能的传输依赖于输电线路的拓扑结构,发电和负载,以及无功电源的出处。一种用于分析电压波动的方法是电压波动的预测(VIP)。由继电器测量变电所连接到线路上的电路的电流和电压。根据测量结果,借助戴维南定理估算出输送到变电所线路和从变电所提供的负载的阻抗。本文描述了一个测量相邻系统母线并考虑到的负荷预期变化的扩展的VIP技术。
关键词:最大负载能力;电压波动;VIP算法。
1.简介
宽松的政策迫使发电企业要更好地利用电力系统中的输电。这导致了输电量的增加,降低了输电利润和减小了电压安全裕度。
操作一个有足够安全裕度的电力系统,在系统的使用信息中估算当前操作点的最大允许负载是必要的。一个电力系统的最大负载不是一个固定的值而是取决于各种各样的因素,比如输电线路的拓扑、无功电源的出处和他们的位置等等。决定最大允许负载,在电压稳定极限内,在电力系统运行和规划研究中已成为一个非常重要的问题。常见的P-V或V-Q曲线通常当作一个评估电压稳定的依据,进而为在电力系统电压崩溃端寻找最大负载提供依据[1]。这些曲线常规的方法是在大量负载流运行使用的情况下产生的。虽然这样的过程已经可以自动化,但它们是耗时的,在发现稳定性问题的起因时不易提供一些有用的信息[2]。
为了克服上述缺点的多个方法已经在文献上提到,比如分叉理论[3],能量法
[4]
、本征值法[5],多个负载流解法[6]等。
参考[7]提出了一个简单的方法,它不需要离线的模拟和训练。电压指标预测
方法(VIP)[7]是在本地测量值(电压和电流)的基础上,产生一个连接到母线上估算优点和缺点的输电系统,并将它与当地的需求对比。估算出最接近本地需求的输电量,更为紧迫的是电压波动。该方法的主要缺点是在戴维南定理的估算,
它在不同时刻获得两个测量值。对于一个更精确的估值,一般需要两个不同
的负荷测量值。
本文提出了一种提高稳定性算法的算法,包括周围负载母线的额外的测量值外也考虑到相邻总线之间局部的负载变化。
2.提出的方法
VIP算法在本文中提到在负载母线和互连线(Z12 ,Z13)的假设阻抗在已知的情况下使用电压和电流测量 ,如下所示(图1)。发电机负载母线的电流被用来估计戴维南等效的输电方向。类似于用从其他负载母线(图2)的电流来估计戴维南等效的其他方向。这个结果在以下方程式(图3)。注意在输电线路上来自第二负载母线的电流被排除在最初的估算(VIP)算法。
V1(ZL1?Zth1)?V2(Z12)?Eth1(Zth1) [1] V2(ZL2?Zth2)?V1(Z12)?Eth2(Zth2) [2] Eth1(Zth1)?V1(Zth1)?IE1 [3] Eth2(Zth2)?V2(Zth2)?IE2 [4]
由戴维南定理得来自第一和第二母线的电流IE1和IE2。方程(1)-(4)可以组合为一个矩阵形式:
?1?1?ZL1?1?Z12?1?Zth1?1?V?Z12?Zth10?1???-1?1?1?1?1?Z12ZL2?Z12?Zth20?Zth2??V2???? ??*??1?1?Zth10Zth10???Eth1????1?1??E0?Zth20Zth2??th2???1?1?1?1?1?1?1?1?1?1?1?1?0??0???[5] ?IE1????IE2?使用第一行系统方程(1)-(4)中的2,在母线1和2上的电压可以发现如以下方程式(6)所示。从方程式(6)中我们可以看到,电压是一个阻抗的函数。请注意这个方法是假定所有戴维南的参数是常数时的估算。
?1?1?1?1??Eth1Zth1?1??Z12?V1??ZL1?Zth1?Z12? [6] ?????*??1?1?1?1?1?Z12ZL2?Zth2?Z12??V2??????Eth2Zth2???1 在 y1?ZL1 y12?Z12 和 y2?ZL2 中
系统等效理解为母线1如图3所示。图4(a)显示了负载通道(y1和y2) 和母线1电压之间的关系。电力输送到母线1是(S1),它是一个(ZL1,ZL2).S1?V1*yL1的
2?1?1?1函数。 [7]
方程式7如图4(b)“形象化”绘制并且最大负载点取决于系统轨迹”超过顶点”。
图1.3母线系统连接 图2.1母线模型
图3.系统等效为被提议的VIP转接到母线#1(母线#2模型)
电气工程及其自动化电压波动中英文对照外文翻译文献
