Obtaining robust performance of a current fed voltage source inverter for virtual inertia response in a low short circuit ratio condition

Low inertia levels are typical in island power systems due to the relatively small rotational generation. Displacing rotational generation units with static inertia-less PV power results in a significant increase in the frequency volatility. Virtual inertia provided by inverter-storage systems can resolve this issue. However, a low short circuit ratio (SCR) at the point of common coupling together with a fast phase locked loop (PLL) will compromise the response performance of the system. To address this issue, a robust PI controller (RPI) for the inner current-loop of a current fed grid-con-nected inverter is proposed. The PLL disturbance and grid impedance are incorporated into a single model and recast to a generalized representation of the system, thereby allowing easy tuning of the RPI by the mixed sensitivity H∞ method. The performance of the RPI is compared with that of a PI controller (PI) tuned by the regular loop-shaping method. The results show that when the SCR is above 10, the performance of both controllers is equivalent. However, lowering of the SCR compro-mises the performance of the system with PI and it becomes underdamped at SCR < 2. On the con-trary, the system with the RPI is capable of maintaining the nominal performance throughout the same SCR decrease.