Robust PI control of a grid-connected voltage source inverter for virtual inertia response in weak grid conditions

Abstract

Chemical battery energy storage systems (BESS) providing frequency compensation techniques such as virtual inertia response (VIR) can be a solution to tackle the frequency volatility ac-island grids suffer when inverter-based generation such as PV is integrated. However, the inverter connecting the BESS may suffer performance and stability issues when due to a low short circuit ratio (SCR) the inverter phase-locked loop (PLL) and the grid impedance negatively interact. To address this issue this work proposes a robust proportional-integral controller (R-PI) for the inverter inner-loop current control. The R-PI is synthesized by the structured H-infinity method using MATLAB and the performance is compared with a regular proportional-integral controller (PI) designed by the conventional frequency shaping method. A sensitivity analysis that includes the dynamic impact of the PLL is conducted for varying SCR. The analysis is verified by time domain simulations in MATLAB and shows that both the R-PI and the PI maintained the inverter stability under the worst-case SCR of 2. However, the benchmarked transient performance of the regular PI was compromised under the simulated SCR variation of 7.2 to 2, while the R-PI was capable of maintaining this performance throughout the SCR variation.