Angle Droop Design for Grid-Forming Inverters Considering Impacts of Virtual Impedance Control

Grid-forming inverters (GFMs) based on active power-frequency droop control and reactive power-voltage droop control have been developed in recent years, which can generate voltage and frequency references for islanded power systems or microgrids to improve system-wide synchronization and power-sharing capability. It is known that the control delays in GFMs may cause system harmonic instability. To eliminate this issue, virtual impedance techniques are normally adopted. This paper gives a comprehensive examination of the impacts of virtual impedance control block Z
v
in GFMs on system stability. It is revealed that although Z
v
is effective in eliminating system harmonic stability, it may ruin system synchronization stability as a side effect. To compensate for the negative impacts caused by Z
v
, an active power-angle control block is added into the conventional droop control. Design criteria are also proposed accordingly to ensure system stability over the entire frequency range. Simulations and experimental testing are conducted to validate the analysis and the proposed design guidelines. It can be concluded that the Z
v
control-caused grid-synchronization instability issues can be eliminated with an appropriate active power-angle droop design.