Hybrid Microgrid with Parallel-and Series-Connected Microconverters

In this paper, a hybrid microgrid with series-and parallel-connected microconverters is proposed. A few series-connected low voltage microconverters are used to build a string converter with rated output voltage magnitude. Then, the parallel operation of string converters increases the power rating and redundancy of the microgrid. To realize both grid-tied and islanding microgrid control with proper power sharing, a hierarchical power regulation method is developed with simultaneous point of common coupling (PCC) voltage and microconverter voltage control. First, the well-understood droop control is adopted to obtain PCC voltage reference for power sharing between parallel string converters. Second, a power balancing regulation through microconverter local voltage control is used to redistribute output power demand among series-connected microconverters in a string converter. As the string converter line current measured by all microconverters is the same, it is utilized to develop a novel synchronizer to interconnect the string converter central controller and the microconverter local controllers with only low bandwidth communications. Due to the coordination of string converter central control and microconverter local control, the proposed hybrid microgrid can operate in grid-tied and islanding modes with real and reactive power sharing for each microconverter.