电气工程及其自动化毕业设计外文翻译--一种特殊的预防电压波动的保护方案(编辑修改稿)

电气工程及其自动化毕业设计外文翻译--一种特殊的预防电压波动的保护方案(编辑修改稿)内容摘要：

the next data window es in. • The monitoring 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. Coordinated control can 5 also be obtained if munication is available. Once we have the time sequence of voltage and current we can estimate unknowns by using parameter estimation algorithms, such as Ka lm an Filtering approach described [6]. stability margin (VSM) due to impedances can be expressed as ( ZVSM )。 where subscript z denotes the we have: L o a dth evL o a dZ Z ZZV S M  [12] The above equation assumes that both load impedances ( 1Z , 2Z ) 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 starts to decrease again. Now assume that both loads are functions of time. The maximum critical loading point is then given by Equation(13): 011  dtdsS Critical [13] Expressing voltage stability margin due to load apparent power as ( SVSM ), we have: C rit ic a l L o a dC rit ic a lS S SSV S M  [14] Note that both ZVSM and SVSM are normalized quantities and their values decrease as the load increases. At the voltage collapse point, both the margins reduce to zero and the corresponding load is considered as the maximum permissible loading. Fig. 5. VIP algorithm . Voltage Stability Margins and the Maximum Permissible Loading System reaches the maximum load point when the condition: thevload ZZ  is satisfied (Figure5).Therefore the voltage stability boundary can be defined by a circle 6 with a radius of the Th233。 venin‟s impedance. For normal operation the thevZ is smaller than loadZ (. it is outside the circle) and the system operates on the upper part (or the stable region) of a conventional PV curve [2]. However, when thevZ exceeds loadZ the system operates on the lower part (or unstable region) of the PV curve, indicating that voltage collapse has already occurred. At the maximum power point, the load impedance bees same as the Th233。 venin‟s ( thevL ZZ  ). Therefore, for a given load impedance ( loadZ ), the difference between thevZ and loadZ can 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 . 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 loadZ . The online tracking of thevZ 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 pensation by means of SVC‟s and synchrouns condensers, starting of gas turbines, low priority load disconnection, and shedding of lowpriority load [8]. Figure 6 shows the most monly used remedial actions . 3. Conclusions 7 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 online tracking Th233。 venin‟s equivalent for tracking the system trajectory. The algorithm is simple and easy to implement。