1原文: A SPECIAL PROTECTION SCHEME FOR VOLTAGE STABILITY PREVENTIONTara Alzahawi Student Member, IEEE Mohindar S. Sachdev Life Fellow, IEEE G. Ramakrishna Member, IEEEPower System Research Group University of Saskatchewan Saskatoon, SK S7N 5A9, Canada Abstract Voltage instability is closely related to the maximum load-ability of a transmission network. The energy flows on the transmission system depend on the network topology, generation and loads, and on the availability of sources that can generate reactive power. One of the methods used for this purpose is the Voltage Instability Predictor (VIP). This relay measures voltages at a substation bus and currents in the circuit connected to the bus. From these measurements, it estimates the Thvenins equivalent of the network feeding the substation and the impedance of the load being supplied from the substation. This paper describes an extension to the VIP technique in which measurements from adjoining system buses and anticipated change of load are taken into consideration as well. Keywords: Maximum load ability; Voltage instability; VIP algorithm. 1. Introduction Deregulation has forced electric utilities to make better use of the available transmission facilities of their power system. This has resulted in increased power transfers, reduced transmission margins and diminished voltage security margins. To operate a power system with an adequate security margin, it is essential to estimate the maximum permissible loading of the system using information about the current operation point. The maximum loading of a system is not a fixed quantity but depends on various factors, such as network topology, availability of reactive power reserves and their location etc. Determining the maximum permissible loading, within the voltage stability limit, has become a very important issue in power system operation and planning studies. The conventional P-V or V- Q curves are usually used as a tool for assessing voltage stability and hence for finding the maximum loading at the verge of voltage collapse 1. These curves are generated by running a large number of load flow cases using, conventional methods. While such procedures can be automated, they are time-consuming and do not readily provide information useful 2in gaining insight into the cause of stability problems 2. To overcome the above disadvantages several techniques have been proposed in the literature, such as bifurication theory 3, energy method 4, eigen value method 5, multiple load flow solutions method 6 etc. Reference 7 proposed a simple method, which does not require off-line simulation and training. The Voltage Indicator Predictor (VIP) method in 7 is based on local measurements (voltage and current) and produces an estimate of the strength / weakness of the transmission system connected to the bus, and compares it with the local demand. The closer the local demand is to the estimated transmission capacity, the more imminent is the voltage instability. The main disadvantage of this method is in the estimation of the Thvenins equivalent, which is obtained from two measurements at different times. For a more exact estimation, one requires two different load measurements. This paper proposes an algorithm to improve the robustness of the VIP algorithm by including additional measurements from surrounding load buses and also taking into consideration local load changes at neighboring buses. 2. Proposed Methodology The VIP algorithm proposed in this paper uses voltage and current measurements on the load buses and assumes that the impedance of interconnecting lines (,) 12Z13Zare known, as shown in (Figure 1). The current flowing from the generator bus to the load bus is used to estimate Thvenins equivalent for the system in that direction. Similarly the current flowing from other load bus (Figure 2) is used to estimate Thvenins equivalent from other direction. This results in following equations (Figure 3). Note that the current coming from the second load bus over the transmission line was kept out of estimation in original (VIP) algorithm. 1)()()(1 111 1221 11 11thththLZEZVZZV2)()()(1 221 1211 21 22thththLZEZVZZV311 111 11)()(EthththIZVZE421 221 22)()(EthththIZVZEWhere and are currents coming from Thvenin buses no.1 and 2. 1EI2EIEquation (1)-(4) can be combined into a matrix form:3*51 21 21 11 11 21 21 121 21 - 121 11 121 11 121 1000000ththththththLththLZZZZZZZZZZZZZZ2121thth EEVV2100EE IIUsing the first 2 rows in the system Equations (1)-(4), the voltage on buses number 1 and 2 can be found as shown in Equation (6) below. From Equation (6) we can see that the voltage is a function of impedances. Note that the method assumes that all Thvenins parameters are constant at the time of estimation. 6 1 2