Reactive Power Control For Doubly Fed Induction Generator

With the growing shortage of fossil energy, the development and utilization of wind energy has achieved an unprecedented development in many countries. Variable speed constant frequency doubly fed induction generator has been the mainstream model.The DFIGs based wind farm connects to the local grid, which produces a series of impacts on the grid because the wind farm connected to gird changes the power flow distribution of the original gird. In all impacts, the stability of voltage, the reactive power optimization of network and wind turbine low voltage ride through have a very close relationship with the reactive power control of wind farm. Using vector control technology, the reactive power and active power of DFIG can be easily decoupled. Both the grid-side converters and stator of DFIG can provide or absorb reactive power. Using the reactive power of DFIGs based wind farm to stabilize the voltage or power factor at the point-of-common coupling is proposed in most related studies in recent years.In normal working conditions, only one of LVC mode and PFC mode can not utilize the reactive power of the wind farm reasonably because of the difference of the local grid structural strength, local load and etc. A switching control scheme is proposed in this paper. The switching control scheme can select one control strategy from three strategies:voltage control, power factor control and zero reactive power automatically according to the actual voltage and power factor at the PCC, and under the premise of a stable voltage, the control object is to improve power factor. In the period of low voltage default, the rotor side converter immediately switches from active power prioritization to reactive power prioritization based on the existing voltage scheme and injects a certain value of reactive current to the rotor according to the depth of voltage sag. By the improved voltage control scheme, the DFIG can inject more reactive power than the active current prioritization voltage control scheme into the grid under the tolerable conditions of device.The simulation results proved that in normal conditions, the switching control scheme has the obvious superiority over simple voltage control scheme or power factor control scheme and in the period of low voltage default, more reactive power can mjected to the grid compared with the simple active power prioritization voltage control scheme. Meanwhile, mechanical impact and electrical parameters oscillations caused by reactive power prioritization are relieved.