Bifurcation Research On Voltage Stability Of Power System Containing Wind Farms

The wind power is the most developed ways of generating electric power withnew energy resources. The development of the wind power in China can not onlymake improvement in the optimization of the national energy mix and the protectionof the deteriorating environment, but also help promote the stable and economicaloperation of the power system.The remarkable achievement in the construction oflarge-scale wind farms promotes the rapid development of wind power industry. Thewind power integration has prominent influence on the whole power system,particularly the voltage stability of the power system.The wind power system is adynamic system characterized by its strong nonlinearity and can be analyzed using thebifurcation theory, a powerful tool of studying the structural stability of nonlineardynamic systems. In this paper, the voltage stability of the power system containingwind farms is investigated and some kinds of bifurcations closely related to thevoltage instability are analyzed thoroughly. The research is as follows:(1) The dynamic and static equations for the general induction generators arederived and an applicable mathematical model is established for the wind generatorset composed of the general induction generators. The influence of the wind velocityfluctuation in wind farms on the active-reactive operating characteristic of theinduction generators and the impact of the variation of wind turbine’s output power onthe system voltage are analyzed. Furthermore, the effect mechanism of wind farms onthe voltage stability of local power grid is achieved. The study shows a large numberof reactive power absorbed by wind farms results increasing reactive powertransporting from grid to wind farms, and then larger line voltage drop reduces windfarms voltage stability.(2) A multi-parameter continuation tracking algorithm is proposed to calculatethe bifurcation boundaries or hyper-surface, the influence of the active power outputfluctuation of wind farms and the static var compensator on the voltage stability of thewind power system composed of general induction generators is analyzed, theinfluence of the injection of active power of wind farms and the static varcompensator control parameters and the excitation system parameters on thesaddle-node bifurcation is discussed. The quadratic polynomial nonlinear analytical expression method proposed is capable of describing the saddle-node bifurcationboundary and hyper-surface of the parameter space analytically. The simulationverifies the accuracy and effectiveness of the method.(3) The influence of the power parameters of wind farms and the static varcompensator parameters on the Hopf bifurcation point is analyzed and the relationshipbetween wind farm active power output and wind farms power system voltagestability is obtained. From the view of the structural stability, the static varcompensator control effect on system characteristic polynomial is analyzed and themechanism of the static active power compensation improving dynamic voltagestability is obtained.The study shows the static var compensator improves the busvoltage immunity and strengthen the system equilibrium point stability by adjustingthe local system structural parameters, which can delay the Hopf bifurcation andimprove the dynamic stability of the nonlinear power system voltage.(4) The control strategy for the Hopf bifurcation which combines the static varcompensator and the linear feedback control method together is proposed，the linearfeedback controller of Hopf bifurcation for the conventional power system and thepower system including wind farms is designed. The simulation result verifies theeffectiveness of the Hopf bifurcation controller on the Hopf bifurcation and improvesthe dynamic voltage stability of the system.Multi-parameter bifurcation analysis is better to reveal stability bifurcationmechanism caused by paramete variation of wind farms and the static varcompensator, which contributes to the theory and application value of the bifurcationcontrol to the nonlinear powers system voltage stability containing wind farms.