Impact of System Strength and Control Parameters on the Small-Signal Stability of Grid Following and Grid Forming Inverters

Inverter-driven small signal stability issue has been identified as a new stability class by global power industries, which are the bottleneck for high solar/wind power penetration. This new stability problem is mainly caused by the interactions between controllers of the converters and other power system elements. To eliminate the oscillation problems, it is crucial to develop fundamental theories to fully understand the IBR dynamic behaviours and identify the cause of oscillations. In this paper, the small-signal mathematic models are established for grid-following invertor (GFLI) and grid-forming invertor (GFMI), based on which the small signal analysis is performed on both GFLI and GFMI to study the impacts of system strength and control parameters on their small-signal stability. Small-signal analysis reveals that low system strength can lead to instability in GFLI, but the instability of GFMI happens when it connects to a grid with high system strength. It is also found that those instability issues can be resolved by tuning their control parameters. However, parameter tuning brings side effects to those systems. Therefore, a modified inner current controller is novelly developed for GFMI to maintain its stability without bringing other negative effects. The developed controller and theoretical analysis are verified in the EMT testbed built in PSCAD.