Voltage Stability Real-time Monitoring Based On Wide Area Measurement System

Voltage stability has already drawn common concern of power industry nowadays, and it's very significant to gird's security and stability to monitor voltage stability degree accurately and real-timely. Wide area measurement systems can real-timely and synchronously measure many signals of the nodes where the phasor measurement units are placed, such as voltage phasor, current phasor, internal potential and excitation current of synchronous generator, which presents a new idea for voltage stability real-time monitoring.The placement of phasor measurement units is a basic work for wide area measment systems. In this thesis, after summarizing and studying the existing phasor measurement unit placement methods, a new placement method based on minimum dominating set and modal analysis is proposed. All minimum dominating sets can be calculated, and then considering the modal analysis resultes, the final placement scheme is determined. As the result of this method, key-nodes and key-generators can be directly measured, satisfying grid's minimum complete observability.A good voltage stability index is the key to voltage stability monitoring. Based on summarization and study exsiting voltage stability monitoring approaches in wide area measment systems, several voltage stability indices are derived and analyzed in detail. Then, two new voltage indices are proposed. Through comparing speed, accuracy, linearity of all indices mentioned here, a new index named line flow index is choosed. This index can be calculated and assesses voltage stability quickly just using voltage phasors of the line, power flow through the line, parameters of the line. The results of simulation show that this index has good characteristics of accuracy and linearity.Voltage collapses often start at some lines or nodes, and spread to larger area. If the weakest lines or nodes are monitored and kept in acceptable voltage stable degree, the whole grid will be in voltage stable state. The weakest lines set presented in this thesis keeps a number of lines with weakest voltage stability. In a calculation cycle, just the voltage stability indices of the lines in weakest lines set are calculated, meanwhile, the lines in weakest lines set are renewed. As a result, not only the cycle time of calculation becomes shorter, but also the voltage stability of system is monitored.In order to assessment load margin real-timely, a method called index fitting is proposed, which makes use of the index's good linear characteristics, and fits historical indices to a line, at the same time, keeps historical indices constant renewal. In the fitted line, the distance between current state and critical state is load margin. This method not only reduces the influence of random factors on voltage stability assessment, but also makes use of the historical indices.Finally, taking Chongqing grid in 2007 for example, the result of case study indicates that the phasor measurement units placement method proposed in this thesis is valid, and the voltage stability index proposed in this thesis can reflect voltage stability degree of power system.