Study On Voltage Stability Of Regional Power Grid

In the second half of the 20th century and at the beginning of this century, in the worldwide scale, there are many blackout accidents of large electric system caused by voltage collapses. This stimulates the researches on voltage stability, which conclude voltage stability is as important as angle stability. In Pearl River Delta region, China, with the cities concentrating and the economy booming, the demand of electric power becomes increasingly huge. However, owning to the economic and environmental impacts, the power sources are always far away from the load centers, and the dependence on external power is extremely high. In the heavy load period, the change of power flow, breakdown of transmission line as well as the shutdown of generators can lead to sharp power change of the local electrical network. In some emergency cases, when the system's reactive power is insufficient, the voltage drops dramatically and becomes uncontrollable, namely voltage collapse. In this paper, a metropolitan grid has been studied, and based on the worldwide research approaches and evaluation methods of voltage stability, the voltage stability problem of urban electrical network is comprehensively analyzed and evaluated via applying diverse methods.Firstly, as the foundation of following analysis, this paper introduces domestic and international concludes about the voltage stability,describes the various research methods and their advantages and disadvantages.Secondly, this paper applies Modal Analysis to study the voltage stability status of region grid. By gradually increasing the load and the output power of reserve units, the power limit of electrical network is found out., considering the nodes which are capable to switch on automatically when voltage is low. This paper also makes full use of jacobian matrix which contains abundant information to analyze every node and every area of region power grid.Then, this paper develops a program to calculate the nearest power critical point of the power grid, based on the open-source software, InterPSS, guaranteeing the result is the most close power limit in all load growth directions and eliminating the optimistic results sought in specific load growth direction. This paper also uses the worst power increasing direction to close down the nodes and areas which are prone to reduce the power limit. Through calculating the power limit, this paper analyzes the reasons why the power flow equation dose not converge, and by substituting shunt admittance for reactive power, improves the converge of the power flow equation,Finally, this paper studies the fast estimation methods of the grid power limit. Considering the network loss sensitivity obtains the characteristic of"quadratic form" function in different load conditions, this paper constructs the corresponding second-order indicator and by using this, estimates electrical network's power margin So it can conveniently find out the lines and the other units that are apt to affect the voltage stability. In order to overcome all kinds of error, this paper applies the multiple node's estimations, and makes up the appropriate approaches to elevate the estimate accuracy.