Research On Low Voltage Ride Through For Doubly-fed Wind Power Generation System

With the continuous innovation and development of wind power technology,the proportion of wind power generation in China keeps rising,and the automatic disconnection of wind turbines during grid failure will no longer be allowed.For this reason,the relevant departments of our country have formulated the requirements of low voltage ride through(LVRT)grid connection of wind power system.From the core point of view,it can be summarized as the ability of wind power units to keep running on the grid in a certain period of time during grid failure,and the ability of wind turbines to transmit reactive power to the grid to support grid voltage recovery.This thesis is based on further improving the LVRT capability of doubly fed induction generation(DFIG).The main contents of the thesis are as follows:1.The operation characteristics of DFIG unit are analyzed firstly.Based on the understanding of the main components of DFIG wind power system and its mathematical model,the vector control strategy based on the mathematical model of DFIG converter is designed and analyzed.The control strategy includes stator flux oriented control strategy of rotor side converter and voltage oriented control strategy of grid side converter.Secondly,the simulation model of each module of DFIG wind power system is built on the PSCAD/EMTDC simulation platform.2.Aiming at the deficiency of conventional Crowbar,the strategy of LVRT that uses the series dynamic braking resistor and DC-Chopper is proposed.LVRT of conventional Crowbar circuit will lead losing the power control when the grid voltage drops seriously.While using the combined hardware protection scheme can absorb less reactive power from the grid,which is conducive to the recovery of grid voltage.The LVRT capability of the DFIG can be enhanced.3.Research is carried out on the problem that DFIG units need to send a certain amount of reactive power to the grid during faults.Without adding additional static synchronous compensator(STATCOM),taking full advantage of the reactive power characteristics of the grid-side converter,a reactive power support strategy is proposed,that the grid-side converter operates in the STATCOM mode to provide reactive power to the grid during faults.The simulation results verify the economy and effectiveness of the strategy,which is helpful to the recovery of the gridconnection point.4.In the case of the slight drop of the grid voltage,improving the control strategy to enhance the unit’s low voltage ride-through capability can effectively avoid the problem of frequent switching of hardware protection circuits.The nonlinear control method based on state feedback linearization is used to make up for the shortcomings of the linear controller.Firstly,the output function is selected according to the control objective.Then the method of state feedback linearization and coordinate transformation is used to transform the nonlinear system into a linear controllable system.Finally,the converter control strategy is designed by the linear optimal control design method.The control strategy is only related to the parameters of DFIG,which improves the anti-disturbance of the control system.The simulation results show that the nonlinear control strategy has better control performance than the conventional vector control,and improves the LVRT capability of the DFIG.