Improvement Of The Convergence Of Calculating Voltage Stability Using Circulating Power Flow Method

Power system stability can be divided into three categories as its own characteristics, namely, the power angle stability, voltage stability and frequency stability. As a long time, the power angle stability arouses the universal concern, and also very complete analysis theory and methods have been set up. But at the same time, the study of voltage stability has progressed very slowly. Until the 20th century, some voltage instability accidents happened in some large power grid in the world since the 70's, which cause huge economic losses and large-scale long power cut, voltage stability problem has gradually been concerned. In recent years, as the evolution of power system from the integration from power generation to transmission to an open and competitive environment, the uncertainty and insecurity of power system planning and operation increased, the voltage insecurity has become one of the main factors which limit the power transmission. Therefore, the study of voltage stability analysis and control has become a matter of great urgency.Voltage stability is an ability of system voltage keeping or resuming the extent permitted so as to avoid voltage collapse when a little or large disturbance occurs to a power system. Its analysis methods mainly include static methods, dynamic methods and other methods. The static methods essentially take the voltage stability as a static problem, which take the power flow limitation as for the static stability limit point, thus use different methods to calculate this limit point. This paper is just based on this theory which uses circulating power flow methods to get the voltage stability limit point.Circulating power flow is base on the common power flow. First it increases the load of one or several node in fixed step, and then power flow is recalculated. It is circulated until power flow doesn't converge. It can get the voltage stability limit load and its voltage so as to draw the P-V curve. Thus we can explain the performance of system's voltage stability according to the ability of nodes to support the load change.First this paper develops and completes the common power flow program which can use Newton-Raphson method and P-Q decomposition method. And then taking the IEEE-39 buses system as the example, it tests the correctness and exactness of the program through the comparison with the result of SFD program. Meanwhile it verifies the convergence characteristics of two methods through the comparison between the iteration information of Newton-Raphson method and P-Q decomposition method. Based on this program, taking 12 buses system and IEEE-39 system as examples, using Circulating power flow algorithm, it calculates the voltage stability to get the voltage limitation and draw the P-V. curve.