摘要
电力系统的电压稳定问题是非常复杂的。此前提出了许多电压稳定性指标,但由于系统非常复杂且状态量之间的耦合性,使这些指标在线应用的效果并不理想,如常用的线路局部信息电压稳定指标L_P、L_q、F_(VSI)、L_(PN)、L_(QN)、L_(PP)和L_(QP)等,实际运行表明,指标结果与P - V曲线不相符。这些指标的推导,都是将功率方程进行多种形式的变换后,基于二次型方程有解的条件而得到的。通过对这些指标数学推导过程的分析,发现二次型功率方程中状态变量与系数是相互耦合的,不满足维达定理中方程系数与变量无关的前提条件。因此,这些指标在从数学理论推导上是不够严密的,指标结果与实际的电压稳定状态是有出入的。目前,在静态电压稳定的负荷裕度研究方面,一般都是基于状态量解耦的思路。但实际系统状态量之间是耦合的,所以对应稳定临界点的负荷区域的精确求解是非常困难的。
在正常运行状态下,系统一般能够承受偶然事故而不超出约束条件;当系统安全水平降低,进入警戒状态后,在偶然事故下发生电压失稳的可能性才较大。因此,求取对应电压警戒状态的负荷功率区域则更具有实际意义。本文基于戴维南等值模型,在给定的电压水平约束下,提出了对应的视在功率安全域边界的算法,得到了正常运行状态下的负荷功率安全域。采用3机9节点系统的算例仿真表明,若负荷在安全域内,电压就能保持在约束水平以上,且离安全域边界越远,电压水平则越高;若负荷在安全域以外,电压就低于约束水平,且离安全域边界越远,电压水平则越低。该方法简单实用,物理意义清晰,便于运行人员进行负荷控制并保持系统良好的运行状态。
The Voltage stability in power system is very complex. Before then there are many literatures writing about the voltage stability indices, but due to the complexity of the power system and the coupling between the state variables, the application effect of the voltage stability indices are undesirability in the practical operation, for example the voltage stability index based on local information of the line LP、Lq、FV SI、LP N、LQ N、LP P and LQ P, the actual operation shows that the result of those indexes are not match with the P ? Vcharacteristic curve. The derivation of those indexes are depended on the various forms of change of the power equation and the quadratic equation has solution. The analysis of the mathematical derivation of those indexes discover that the state variables of the quadratic power equation is intercoupling with the coefficient, it not meet the prerequisite of the Ouida theorem, that is to say the variable is unrelated with the coefficient of the equation. So the indexes which are derived from the derivation of mathematics theory are not rigorous enough, the result of the index is discriminating with the practical state of voltage stability.
At present, the study about the load margin of the static voltage stability is based on the idea of decoupling of the state variables. But the relation of the state variables in practical system is intercoupling. So it is difficult to exactly solve the load margin of the stability critical point.
Generally, the power system could bear the incidents. But when the system security level is weak and entering alert status, it will possibility cause voltage instability under the incidents. So, it has more practical significance to estimate the load margin of the voltage alert condition. This paper which is based on the Thevenin equivalence theorem proposes the security domain boundary of the load apparent power algorithm and solves load power stability domain of the current operating condition under the same voltage level. This paper adopted 3 machine 9 bus systems’simulation example to illustrate that if the load power is within the range of stable interval, the load node voltage will above the constraint level, the more the distance between the operating point and the boundary of Security domain, the higher the voltage level. If the load power is out of the range of Security domain, the load node voltage will below the constraint level, the more the distance between the operating point and the boundary of Security domain, the lower the voltage level. This method is simple and the physical meaning is clear, and provides a reference for the operating personnel.
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