Research On Low Voltage Ride-Through Control Strategies Of Wind Turbines With Doubly-fed Induction Generator

As the increased penetration of wind power in power grid system year by year, In order to maintain the stability and security of power system operation, the grid department proposed the LVRT (low voltage ride through) requirement of wind turbine net operation.DFIG (Doubly-fed induction generator) as one of mainstream wind turbines domestic and overseas at present, has a lot of advantages, such as a smaller capacity of converter, variable speed operation, active and reactive power decoupling control independently, but because of doubly-fed wind power generator stator connected to the grid directly, making wind turbines is sensitive to the grid fault.In this paper, when minor voltage sags occurs,we can achieve fault ride-through by improved control strategy. First of all,transient excitation accurate model of the DFIG is established under the grid voltage sag, and considering the influence of the wind turbines pitch Angle.Established fourth-order nonlinear model that control the rotor excitation voltage and wind turbines pitch angle of DFIG, based on the feedback exact linearization theory, Nonlinear coordinated control strategy is proposed.The simulation results show that, when grid voltage drop slightly, the proposed control strategy can more effective control of rotor current, protect the excitation frequency converter, suppress speed oscillation of the wind turbines in transient process,promote the voltage recovery, maintain the grid stable,improve operation ability of the wind turbine and adaptability of grid voltage sag.For large voltage values fell,by adding Crowbar protection circuit to achieve low voltage across.This paper analyzes transient short-circuit current of DFIG and resistance selection of Crowbar protection circuit; Plunging Crowbar circuits will cause DC side problem when voltage drop sharply, so nonlinear control strategy of the network side converter is set up,which based on feedback linearization theory. Simulation shows that Crowbar protection circuit can restrain the current well, achieve low voltage ride through; nonlinear controller of the network side can suppress the DC voltage during voltage sag.DFIG will add Crowbar protection circuit when the voltage drop sharply, it run in an asynchronous motor, so DFIG will absorb a large number of reactive power from grid. For reactive power compensation problems caused by the Crowbar circuit, this paper consider coordination STATCOM compensation device, It can provide dynamic reactive current is used to ensure the stability of the power system and to prevent voltage decline further or action again.Through simulation study, the control strategy can solve the problem of the fan reactive power compensation, Coordination of STATCOM can solve the problem of DFIG reactive power compensation, improve the grid power quality, improve the ability of wind turbines low voltage across.