Multimode Control for Power Quality Improvement in an Electronically Coupled Distributed Generation Unit Under Grid Connected and Autonomous Modes

This paper develops a new multimode control strategy for power quality (PQ) improvement in distributed generation (DG) units that employs a voltage source converter (VSC) as the electronic interface medium. The devised control strategy seamlessly operates for both the grid connected (GC) and autonomous/stand-alone (SA) modes of microgrid (MG). A novel computationally efficient comb-filter driven discrete time oscillator is developed for current reference generation (CRG) under GC operation. It constitutes a cluster of zeros to introduce notch peaks at integer multiples of the fundamental frequency thereby ensuring improved harmonic rejection capability. Consequently, it ensures unconditionally balanced and sinusoidal grid currents with improved PQ to comply with grid interconnection codes prescribed in the IEEE std-519 and std-1547 even under electrical anomalies. In the SA mode, the CRG scheme is developed in a stationary reference frame with an improved vector proportional integral (VPI) compensator. Despite the uncertainty of load parameters, the proposed VPI guarantees zero steady-state error and superior transient response. A finite state machine-based synchronization framework is proposed to ensure secure and seamless transfer of MG between GC and SA even under disturbances. The practical viability of the proposed multimode control has been demonstrated by applying various switching rules in both GC and SA modes of MG operation.