Improvement In Low Voltage Ride Through Capability Of Doubly Fed Induction Generator

With the gradual upgrading of installed capacity of wind farms, the proportion ofwind power in the grid is also growing. If the wind generators in the wind farms arecut off, it will have a big impact on the safe and stable operation of the power system.Wind generators are required to ride through the fault, remain connected during gridfault and provide reactive power to the grid to make for the grid voltage restoration.This capability is called low voltage ride through (LVRT).In this paper, the topology of doubly fed induction generator (DFIG) is analyzed.The back to back PWM converter is used in DFIG. It not only has a goodperformance of the output, but also greatly improves the performance of the input.Sinusoidal input current can be obtained at any power factor, and the energy flow isbidirectional. And it studies the transient characteristics of DFIG during a fault andthe cause of rotor over-current and DC bus over-voltage.Traditional DFIG crowbar uses a fixed crowbar resistance, and it’s difficult tosuppress both the rotor current and DC bus voltage and shorten the working time ofcrowbar at the same time. Aimed at this shortage, a low voltage ride-through schemeof doubly fed induction generator based on the crowbar with a parallel dynamicresistor is proposed, the adaptive control strategy and the resistance setting method isgiven. The LVRT characteristics of the crowbar with a parallel dynamic resistor aresimulated during different dips, which are compared with the crowbar of variableresistance under the IGBT pulse control. The results show that the LVRT effect of thecrowbar with a parallel dynamic resistor is better. It can not only suppress both therotor current and DC bus voltage, but also shorten the working time of Crowbar,which is conducive to the recovery of the fault point voltage.When the system contains several wind farms, the crowbar will be applied duringone fault next to one of the wind farms. At the same time, the rotor-side converter willbe locked and the DFIG will become a reactive power load like asynchronousgenerators. It will absorb reactive power from the system and make the furtherdeterioration of the voltage. Meanwhile, it will adversely affect the adjacent windfarm. To solve the above problems, a comprehensive protection scheme is proposed tocoordinate the LVRT between the neighboring wind farms. Simulation results show the effectiveness of the scheme. It can avoid cascading crowbar action, and maximizethe compensation of reactive deficiency and raise the export voltage of neighboringwind farm.