Deadband Control of Doubly-Fed Induction Generator Around Synchronous Speed

Semiconductor devices in power electronic converters of the doubly-fed induction generator (DFIG) are susceptible to significant junction temperature variations when operating around synchronous speed, thereby reducing the lifetime of the converters. This is due to the fact that the frequency of the rotor current in a DFIG is determined by the stator flux frequency and rotor speed, and hence will lead to low rotor current frequency when operating closer to the synchronous speed, and ultimately result in significant thermal stress on semiconductor devices. In this paper, a multimode operation control strategy is proposed for the DFIG to prevent operating around the synchronous speed (within a predefined deadband); thus, the proposed control strategy can avoid the thermal stress problem. The proposed strategy engages the existing crowbar scheme for DFIG-based wind energy conversion system to intentionally alter the operating mode of the generator between DFIG and induction generator (IG). Smooth transition between the two operating modes can be achieved with the supplementary control strategies. Unity power factor can also be maintained in both operating modes by using the grid side converter as a static synchronous compensator (STATCOM) to fulfill the reactive power requirement of the DFIG in IG mode.