Inertia Emulation in Droop-Based DC Microgrids With Equivalent Converter Impedance Reshaping

With the high penetration of renewable energy sources, the inertia of microgrids has declined. In dc microgrids, the lack of inertia leads to bus voltage fluctuations, which may threaten system stability. To mitigate this issue, virtual inertia can be provided by energy storage units via emulating the inertial properties akin to virtual dc motors. Although the fast voltage change can be restrained, the principle of virtual inertia remains elusive and there is no suitable implementation for V-I droop. With these considerations, in this article, the voltage regulation issue is analysed from an impedance perspective, and it is revealed as transient current sharing between the dc-link capacitor and equivalent converter impedance. On this basis, an inertia emulation strategy is proposed based on capacitor current regulation and droop gain modification. With the proposed strategy, the equivalent converter impedance is reshaped in the medium-frequency range to absorb the transient current, and the capacitor current is suppressed. Consequently, the virtual inertia is provided to improve voltage regulation. Remarkably, in the proposed strategy, no additional control loops or measurement is required, and it seamlessly integrates with existing V-I droop control methods. Then, the proposed inertia emulation is validated with hardware-in-loop and experimental test results.