Research On The Impact Of Variable Speed Wind Turbines Integration On Power System Voltage Stability

With the growing of an installed capacity of wind power, as well as the continual emergence of large scale of wind farm, wind power has more and more influence on the power grid. Wind power has the different characteristic compared with the traditional synchronous generating units. Moreover, the wind speed has the inherent nature of intermittent, random, then, the integration of wind power changes the power flow distribution and the power grid stability. As a result, it's meaningful to investigate wind power integration. This paper focuses on the voltage stability after the large scale of wind farm integration. Two typical wind turbines were chosed to investigated, doubly fed induction generation (DFIG) and direct drive permanent magnet synchronous generator (PMSG).Variable speed and variable pitch wind generating unit based on DFIG, whose rotor was connected to the grid through partial scale frequency converter. The active and reactive power can be decoupled by the frequency converter in this wind generating, which can improve the transformation efficiency, capture the maximum wind energy.Variable speed and variable pitch wind generating unit based on PMSG, which is multi-pole synchronous generator. The wind generating unit was connected to the grid through full scale frequency converter. The wind generating unit was decoupled by the decoupling controller of frequency converter. Therefore, this characteristic of this generating unit was controlled by the control system of converter.The mathematical models of two typical variable wind speed wind turbines were established. Too simplification or too cpmplication model were not suitable to study, which had been proved by the current research. So we used the three-order model after simplifying the complete wind turbine model. We analyzed the model of control system since it is significant for the whole wind generating unit.In the meanwhile, the aggregated wind farm model was established after considering of the distribution of wind turbines, the equivalent wind farm model can represent the whole wind farm through the simulation results.In this paper, static analysis and dynamic time domain simulation were used to power grid voltage stability after wind farm integration. The PV curve was used to analyze static voltage stability, while time domain simulation was used to transient voltage stability under the wind speed disturbance and line trip. The static reactive power compensation (SVC) was added to improve the system stability after windfarm integration.Finally, the mechanism of voltage stability after wind farm integration was analyzed in detail. Based on the differential-Algebraic Equation (DAE) model of power system with wind farm, the chaotic behavior was observed through time domain simulation. The complete dynamic behavior includes period doubling bifurcation (PDB), chaotic motion and Bluesky bifurcation (boundary crisis), which gives rise to the voltage collapse before the saddle node bifurcation. We used the SVC to remove the Hopf. Bifurcation and restrict the chaotic since the oscillation instability due to Hopf. bifurcation.