Dual-Port Grid-Forming Control of Double-Stage Interlinking Converters in Islanded Hybrid AC/DC Microgrids

Hybrid AC/DC microgrids (MGs) are comprised of AC and DC MGs interconnected through an interlinking converter (ILC). This paper presents a novel control strategy for double-stage ILCs (consisting of bidirectional DC-DC and DC-AC converters) in islanded hybrid AC/DC microgrids. To provide global power sharing among AC and DC DERs, a PI controller obtains a single relative power reference for the AC and DC DERs based on the energy content of the DC link capacitor. This relative power is then translated into references for the output voltage angular frequency at the AC side of ILC and the output voltage at its DC side, in accordance with the AC and DC DERs' droop characteristics. Then the DC-AC and DC-DC converters realize these references through a voltage-controlled method (VCM) in their inner control loops. Accordingly, the proposed method features grid-forming capability at both AC and DC terminals of ILC, which provides it with some unique advantages. First is the improved resiliency; the system remains stable even if all DC DERs are disconnected or all AC DERs are disconnected. Secondly, stability margin and voltage transients are improved thanks to the PLL-less design and adaptation of VCM. Small-signal stability analysis and hardware-in-the-loop test results demonstrate the stability and dynamic performance of the proposed method not only in normal operation but also under DER disconnection. The results show that the steady-state AC and DC voltage deviations are less than 5% and the AC voltage undershoot is less than 1V.