Research Of Power System Voltage Stability Based On Load Models

In recent years, as the power system is developing into the direction of large units, large power grid, ultrahigh voltage and long distance power transmission at home and abroad, the voltage problem is highlighted increasingly. However, the static load models which are usually used as load models hampers the research of power system voltage stability.The induction motor load is the main kind of dynamic load models, this paper introduces the simulation model of a three-phase induction motor and its realization in detail based on MATLAB/Simulink, according to the mathematical model of the induction motor in which the electromagnetic transient process is taken into account based d-q synchronous coordinate system. The paper also analyzes the static characteristic and dynamic characteristic of the simulation model by setting disturbances in a single-load infinite-bus system, which is usually used to analyze the voltage stability problem, the results of simulation shows that the dynamic characteristic of the simulation model plays an important role in voltage stability of power system. When disturbances were set in system, with the terminal voltage of the load decreasing, the active power and reactive power absorbed from the system decreases, but it will increase after fault clearing, and then the terminal voltage recoveries. However, the reactive of the system will be shortage if the reactive power absorbed from the system increases all the time, and the terminal voltage will further decline, the system voltage will be instable.In the research of transient voltage stability, the critical fault clearing time which keeps the system stable is important. In the paper, the influence of different parameters in induction motor load on transient voltage stability was compared, the results shows that the resistance of stator has little influence on the critical fault clearing time, the resistance of rotor, the mutual inductance and the inertia time constant has influence, the self inductance of the stator and the self inductance of the rotor has greater effect on the critical fault clearing time. The paper also analyze the influence of induction-motor ration on dynamic stability of power system, the results shows that stabilities of voltage is easily instable when induction-motor ration is increased.At present, Static Var Compensator (SVC) and Static Synchronous Compensator (STATCOM) are used in power system to restrain the voltage instability. The paper compares the impact of SVC and STATCOM on the transient voltage stability when a three-phase fault happens based on the IEEE 2 machine and 5 bus system, the results shows that both the SVC and STATCOM can improve the transient voltage stability. However, the STATCOM can supply more reactive power, and it has fast response, thus the STATCOM can improve the voltage stability of power system more significantly.