Pole Slipping in Droop-Based Grid-Forming Inverters

Integration of grid-forming inverter-based resources is considered as a viable solution to address the challenges associated with the dynamics of power systems with high penetration levels of inverter-based resources. Yet, various aspects of dynamic behaviour of grid-forming inverters during disturbances have remained unknown. This paper for the first time introduces the concept of pole slipping in grid forming inverters, referring to the loss of synchronism where the inverter’s internal phase angle deviates beyond 180° with respect to the grid, similar to synchronous generators losing step. It is revealed that this event in grid forming inverters can be stable or unstable. The differences between stable and unstable pole slipping are highlighted. The determining factors for the occurrence of stable and unstable pole slipping in grid forming inverters and equipment-level implications of pole slipping are identified and studied. A solution is proposed to mitigate stable and unstable pole slipping in grid-forming inverters by adjusting the operating reference power during the disturbance. The validity of the proposed concept of pole slipping in grid forming inverters and the proposed solution is tested and verified through extensive time-domain simulations and experimental results. The results verify that pole slipping is more likely to happen in high short circuit ratios (SCRs). It is revealed that unstable pole slipping does not occur at SCRs lower than 3.5. Simulation and experimental results demonstrate that a phase jump as small as 10° may lead to instability in a grid with SCRs equal or higher than 10. This is while in lower SCRs such phase jumps result in long fault recovery without causing pole slipping.