The Research Of The Low Voltage Ride Through Of DFIG Based On Series D

With the rapid development of wind power industry, wind power is a growing part of electricity production, and the high-volume wind turbine and large scale connected grid have a great increasing impact on the stability of the power grid, so power grid put forward strictrer quirements for low voltage ride through(LVRT) ability of wind turbine. Being the currently mainstream of wind power generation, doubly-fed induction generator(DFIG) has some defects during grid fault, which manifest that it is susceptible to failure effect of grid because of stator directly connected to the grid, and the rotor side converter capacity is relatively too small to fully control DFIG to deal with grid failures, therefore, it is necessary to improve LVRT capability of DFIG. This paper uses a scheme that an active real-time compensation device of dynamic voltage restorer(DVR) assists DFIG to achieve a variety of low voltage ride through. When the grid voltage drops, the DVR completly compensate port voltage of DFIG to maintain terminal voltage at the normal level and effectively restrain the electromagnetic transient impact, which is an ideal scheme to solve grid fault at its source.Under ideal grid condition, the basic working principle and mathematical model of doubly fed wind turbine are introduced, and according to mathematical model of the converter, traditional control strategies of stator flux oriented of rotor side converter and stator voltage oriented of grid side converter based PI controller are established. Characteristic of the quasi-proportional resonant(PR) controller is analyzed and the construction of the vector control of rotor side and grid side converter based on quasi PR controller simplifies the complexity of the current loop control. Secondly, the structure and mathematical model of DVR system is presented. The composite control strategy of inverter based on quasi PR controller ensures accurate track of compensation reference voltage and full compensation of voltage sag. Control of the bidirectional DC/DC converter maintains stable DC voltage. In this paper, voltage detection method of differential αβ-dq ensures the DVR fast starting; while positive and negative sequence separation method of novel resonant regulator can check the voltage sag and the degree of voltage balance.The simulation model of the connected grid 2MW doubly fed wind turbine with three single-phase series DVRs was built by the MATLAB/Simulink software. According to the grid fault type, the this paper respectively discusses impacts on DFIG by instantaneous symmetrical sag, unbalance and serious asymmetry sag of three-phase voltage, and being not off grid operation characteristics of DFIG are simulated and analyzed under the effect of corresponding compensation with DVR. Exciting current is controlled and generater can smooth output active and reactive power in the whole failure process. Whether fault begins or recovers, the electromagnetic transient components are effectively limited and meet national grid code. Therefore, wind turbines periphery adopts the series DVR can enhance the capacity of LVRT. Finally, simulation results show that the coordination of quasi PR controller and DVR can achieve the specified harmonic compensation function, which is of great significance to reduce the harmonic pollution and prolong the life of the wind turbine.