電氣工程及其自動(dòng)化畢業(yè)設(shè)計(jì)外文翻譯--一種特殊的預(yù)防電壓波動(dòng)的保護(hù)方案（英文）

1、A SPECIAL PROTECTION SCHEME FOR VOLTAGE STABILITY PREVENTION Tara Alzahawi Student Member, IEEE Mohindar S. Sachdev Life Fellow, IEEE G. Ramakrishna Member, IEEE Power System Research Gr

2、oup 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 o

3、n 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 measu

4、res voltages at a substation bus and currents in the circuit connected to the bus. From these measurements, it estimates the Thévenin’s equivalent of the network feeding the substation and the impedance of the l

5、oad 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.Key

6、words: Maximum loadability; Voltage instability; VIP algorithm.1. IntroductionDeregulation has forced electric utilities to make better use of the available transmission facilities of their power system. This has

7、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 permi

8、ssible 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 react

9、ive 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 conven

10、tional 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

11、 of load flow cases using, conventional methods. While such procedures can be automated, they are time-consuming and do not readily provide information useful in gaining insight into the cause of stability problems

12、[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] e

13、tc. 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 produ

14、ces 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 imm

15、inent is the voltage instability. The main disadvantage of this method is in the estimation of the Thévenin’s equivalent, which is obtained from two measurements at different times. For a more exact estimation,

16、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 cons

17、ideration 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 interco

18、nnecting lines (Z12,Z13) are known, as shown in (Figure 1). The current flowing from the generator bus to the load bus is used to estimate Thévenin’s equivalent for the system in that direction. Similarly the cu

19、rrent flowing from other load bus (Figure 2) is used to estimate Thévenin’s equivalent from other direction. This results in following equations (Figure 3). Note that the current coming from the second load bus o

20、ver the transmission line was kept out of estimation in original (VIP) algorithm. ) 1 1 ( 1 ) 1 12 ( 2 ) 1 12 1 1 1 1 ( 1 ? = ? ? ? + ? + ? th Z th E Z V Z th Z L Z V [1]) 1 2 ( 2 ) 1 12 ( 1 ) 1 12 1 2 1 2 ( 2 ? = ? ?

21、? + ? + ? th Z th E Z V Z th Z L Z V [2]1 ) 1 1 ( 1 ) 1 1 ( 1 E I th Z V th Z th E = ? ? ?[3] 2 ) 1 2 ( 2 ) 1 2 ( 2 E I th Z V th Z th E = ? ? ?[4]0-7803-8886-0/05/$20.00 ©2005 IEEE CCECE/CCGEI,

22、Saskatoon, May 2005 545th Z load V th E + = I [8] V and I are directly available from measurements at the local bus. Equation (8) can be expressed in the matrix form as shown below. ? ? ???

23、? ? ???0.0i Vr V=? ? ? ???? ? ? ???th X th Ri th Er th E) () (? ? ???? ? ?????0 0 0 0. 1 00 0 0 0. 0 1r I i Ii I r I[9] B = A X [10] The unknown parameters

24、can be estimated from the following equation: B T A AX T A =[11] Note that all of the above quantities are functions of time and are calculated on a sliding window of discrete data samples of finite, preferably short

25、length. There are additional requirements to make the estimation feasible: ? There must be a significant change in load impedance in the data window of at least two set of measurements. ? For small changes in Th

26、3;venin’s parameters within a particular data window, the algorithm can estimate properly but if a sudden large change occurs then the process of estimation is postponed until the next data window comes in. ? The mo

27、nitoring device based on the above principle can be used to impose a limit on the loading at each bus, and sheds load when the limit is exceeded. It can also be used to enhance existing voltage controllers. Coordinat

28、ed control can also be obtained if communication is available.Once we have the time sequence of voltage and current we can estimate unknowns by using parameter estimation algorithms, such as Kalman Filtering approach

29、 described [6]. 2.2. Voltage Stability Margins and the Maximum Permissible Loading System reaches the maximum load point when the condition: | Zload | = | Zthev | is satisfied (Figure 5). Therefore the voltage st

30、ability boundary can be defined by a circle with a radius of the Thévenin’s impedance. For normal operation the | Zthev | is smaller than | Zload | (i.e. it is outside the circle) and the system operates on the

31、 upper part (or the stable region) of a conventional P-V curve [2]. However, when | Zthev | exceeds | Zload | the system operates on the lower part (or unstable region) of the P-V curve, indicating that voltage colla

32、pse 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 Zthev and Zload can be

33、considered as a safety margin. Hence the voltage stability margin (VSM) due to impedances can be expressed as (VSMZ); where subscript z denotes the impedance. Therefore we have: Load Zthev Z Load ZZ VSM ? =[12] The abo

34、ve equation assumes that both load impedances (Z1, Z2) are decreasing at a steady rate, so the power delivered to bus 1 will increase according to Equation (7). However once it reaches the point of collapse power sta

35、rts to decrease again. Now assume that both loads are functions of time. The maximum critical loading point is then given by Equation (13): 0 dt1 ds Critical 1 S = =[13] Expressing voltage stability margin due to load

36、apparent power as ( S VSM ), we have: Critical SLoad S Critical SS VSM ? =[14] Note that both VSMZ and VSMS are normalized quantities and their values decrease as the load increases. At the voltage collapse point, both

37、 the margins reduce to zero and the corresponding load is considered as the maximum permissible loading. Fig. 5. VIP algorithm 2.3. Advantages of the proposed VIP algorithm By incorporating the measurements from

38、other load buses (Figure 3), the proposed VIP algorithm achieves a more accurate value of Zthev. The on-line tracking of Zthev is used to track system changes. The proposed improvements in the VIP algorithm will resu

39、lt 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 sync