DC Bus Regulation and Suppression of Circulating Current in an Isolated DC Microgrid

This paper addresses load current sharing, DC bus regulation, and circulating current issues of parallel-connected DC-DC converters in an isolated DC microgrid environment. Droop control is a popular technique for load current sharing in a DC microgrid. The main drawbacks of the conventional droop method are poor current sharing and a drop in DC grid voltage due to the droop action. Circulating current issues will also arise due to mismatches in the converter's output voltages. This work introduces a hierarchical control strategy to improve the performance of the DC microgrid. Integral sliding mode control and droop control are used at the primary layer, while an average voltage controller and an outer voltage regulator are used at the secondary layer. Sliding mode control, distinguished by its robustness and fast dynamic response, is utilised to manipulate the output voltage and the input current of each converter simultaneously, and hence it can also provide protection against over-current. Furthermore, at the secondary layer, an average voltage controller is added with a dedicated outer voltage control loop. The average voltage controller reduces the circulating current by equalising the converter output voltage, while the outer voltage control loop regulates the DC bus voltage of the microgrid by shifting the droop lines. In the proposed scheme, only the converter output voltage and DC bus voltage of the microgrid need to be transmitted through low-bandwidth communication channels to individual units. The dynamic performance of the proposed control scheme is evaluated using the MATLAB simulation environment.