Analysis On The Low Voltage Ride Through Capability Of Direct-driven Permanent Magnetic Synchronous Wind Generation System

Along with the increasing environmental pollution and the increasing demand for energy by people, wind power generation has caught attention from various countries in the world. As wind power accounts for a great proportion in power network, many countries put forward LVRT (Low Voltage Ride Through) rules about synchronization technology of wind power.Direct-drive wind turbine with permanent magnet synchronous genarator (PMSG) has been rapidly applied for omitted gearbox, no brush and slip ring. This paper mainly discussed the LVRT technology of PMSG. First, the operation of PMSG in three-phase grid voltage symmetrical drops is dynamically analyzed. The current LVRT schemes of PMSG have been researched. Toward their disadvantages (high cost or difficulty to control or slow dynamic response), an improved power conversion based on the DC bus voltage control strategy is proposed. In this strategy, the high dynamical DC bus voltage is inhibited by adjusting the DC current and changing the system input and output unbalanced energy into rotational kinetic energy of wind turbine and generator. And then this control strategy is set up in Matlab/Simulink, when the grid voltage drops to 15%, the capacity of LVRT in PMSG is effectively improved. Second, the fluctuation failure mechanism in the DC bus voltage of PMSG is analyzed when the grid voltage imbalance occurs. A feedback control strategy of voltage fluctuations is proposed to eliminate fluctuations in the amount of energy transfer. And a crowbar circuit in DC bus of PMSG is increased to inhibit the DC bus over voltage further. And the Simulation of PMSG proves that with the proposed strategy the second frequency fluctuation of DC bus voltage is suppressed and PMSG does not run off the grid when a phase drops to 15%. Finally, in connection with requirements of reactive power control and LVRT, a static synchronous compensator (STATCOM) is proposed in the pressure-side of PCC. And a combine control method of grid side inverter and STATCOM is given. And the simulation results show that when voltage drops to 15% caused by the grid failure, the proposed strategy has maked DC bus voltage from the original 1500V down to 1150V and the output reactive power has increased to 3.8Mvar, with which PMSG safely and effectively goes through the power grid failure.Through the above study, the theoretical and experimental basal of LVRT of PMSG is established with our own intellectual property.