Virtual Transformer Control for DC-DC Interlinking Converters in DC Microgrids

In a DC microgrid, a converter connecting two DC buses is a DC-DC interlinking converter (ILC). In the ILCs, this paper proposes a simple control solution to handle two control issues simultaneously: a) high-voltage-side-series low-voltage-side-parallel (HSLP) modular ILC, b) control adaptability in different grid-working modes. Existing closed-loop control methods are complex and usually do not consider the two issues together. Open-loop controls feature very low cost for realization. Some open-loop controlled ILCs show good performances in the two issues: automatic voltage sharing in HSLP modules, good adaptability in different grid-working modes. The secret is that in these open-loop controlled ILCs: when the steady converter power changes, steady voltages of the two ports are nearly proportional. However, open-loop controls are restricted to certain converter topologies and they cannot modify the converter performances based on sampling feedback. For DC-DC ILCs, this paper proposes a very simple but comprehensive closed-loop control named ‘virtual transformer’. This control uses closed-loop feedback to learn the proportional-port-voltage feature from the open-loop controlled ILCs. Thus these good performances and the low cost are inherited, while the main shortcomings of open-loop controls are overcome.