Modes of Operation and System-Level Control of Single-Phase Bidirectional PWM Converter for Microgrid Systems

Robust system control design and seamless transition between various modes of operation are paramount for multifunctional converters in microgrid systems. This paper proposes a control system for single-phase (1 Phi) bidirectional PWM converters for residential power level microgrid systems which is robust and can tolerate transitions between the different modes of operation. This is achieved by means of a common inner ac current-loop. Each of the operating modes has an individually designed outer loop performing the corresponding regulation tasks, most commonly including the ac voltage and the dc voltage regulation. A modified 1 Phi, phase-locked loop (PLL) system is used for system-level operation with both small steady-state error and fast response; and a novel islanding detection algorithm based on PLL stability is proposed to facilitate the transition between grid-connected mode and stand-alone mode. Finally, a frequency-response based design procedure for the proposed control system is presented in detail for all operating modes, and its performance is verified experimentally using a DSP-controlled 6 kW 120 V rms (ac)/ 300 V (dc) laboratory converter prototype.