Plug-and-Play of Grid-Forming Units in DC Microgrids Assisted with Power Buffers

Grid-forming units (GFUs) are fundamental devices in DC microgrids for DC bus voltage regulation. Droop control is widely used for GFU control due to its Plug-and-Play feature. However, it may fail to meet some desired DC microgrid control features and performances, such as zero steady-state DC bus voltage deviation, high system inertia, and fast DC bus voltage regulation. To achieve these desired control performances, some advanced control strategies have been proposed, but the introduction of time-dependent terms or the centralized controller in these control strategies leads to the loss of the key Plug-and-Play feature for GFUs. To maintain the Plug-and-Play feature of GFUs while meeting the desired microgrid control features and performances, this paper proposes a power buffer based control framework. The power buffer is a new device combing a capacitor and a bidirectional DC-DC converter as an interface between GFUs and the DC bus. It enables the development of a novel decoupled control strategy where GFUs are regulated by the droop control while advanced control strategies are applied to the power buffer. Thus, the Plug-and-Play feature of GFUs is maintained, while specific DC microgrid performances are also achieved simultaneously. Supported by this power buffer based control framework, GFUs can be plugged in/out DC microgrids seamlessly at any time without introducing DC bus voltage fluctuations. This allows GFUs, especially mobile GFUs, to achieve high DC bus voltage regulation performance. The hardware-in-loop (HIL) test and real circuit experimental results demonstrate its efficacy.