In dc distribution systems, a bi-directional inverter can not only link dc bus to ac grid, but can adjust its current commands and operation modes to balance power flow and regulate the dc-bus voltage. This paper presents two approaches, one line-cycle regulation approach (OLCRA) and quarter line-cycle regulation approach (QLCRA), to regulate the dc bus to a certain range of voltages. The OLCRA adjusts the dc-bus voltage every line cycle according to a linear relationship between the dc-bus voltage and the inverter inductor current, and the QLCRA tunes its voltage every quarter line cycle to accommodate fast load variation. The inverter regulates the dc-bus voltage to a higher level when the bi-directional inverter injects more power to the ac grid, while it will regulate the bus voltage to a lower level when it draws higher power from the grid. The two approaches together can reduce the frequency of over/under voltage protection and improve the operational reliability and availability of the dc microgrid without increasing dc-bus capacitance, especially under heavy load requirement. Experimental results measured from a 5 kW bi-directional inverter have verified the feasibility of the proposed two regulation approaches.
