Design of Control System for Smooth Mode-Transfer of Grid-Tied Mode and Islanding Mode in Microgrid

A complete control system of microgrid with hierarchical control structure based on voltage-source control is proposed in this article, in order to realize the smooth mode-transfer of grid-tied mode (GTM) and islanding mode (ILM), and satisfy the individual design requirements of GTM and ILM simultaneously. This system consists of GTM control, ILM control and mode-transfer control, and high similarity of GTM and ILM control is the most important feature. ILM control adopts an improved parallel-operation droop control, to realize accurate power sharing and eliminate the steady-state voltage bias; GTM control adopts an improved grid-tied droop control, to realize accurate power-flow control and restrain the harmonic current injected to grid. These two methods adopt similar control equations to generate their respective reference voltages (V_ref), and uses the same output-voltage control to track V_ref. Moreover, the mode-transfer control adopts a grid-tracking control loop to synchronize the ac bus of microgrid with the grid. Based on this design, GTM and ILM can switch directly and smoothly between each other, because the mode transfer is transformed to the switching of V_ref generated by GTM and ILM control. Finally, experiment results are presented, which validate the overall performance of the proposed control system.