Grid Services with Grid-Following and Grid-Forming Inverters in Hybrid Photovoltaic-Battery Microgrids

This paper focuses on essential grid services in a hybrid microgrid comprising a photovoltaic (PV) system with a grid-following(GFL) inverter and a battery energy storage system (BESS). In such configurations, the PV inverter typically operates in maximum power point tracking (MPPT) mode, necessitating the BESS inverter to assume a grid-forming (GFM) role during islanded operation. Additionally, the BESS inverter emulates synchronous generator functionality under weak grid-connected conditions. However, relying solely on GFM inverters significantly increases the microgrid's operational and capital costs. This dependence also accelerates the charge-discharge cycles of the battery, adversely affecting its lifespan. To mitigate these challenges, this study proposes a coordinated grid service control strategy that leverages the BESS inverter as a GFM unit and the PV inverter as a GFL unit. The proposed approach ensures improved voltage and frequency regulation in the hybrid microgrid. The results demonstrate notable improvements in frequency and voltage stability under step load changes and variations in solar irradiation, both in islanded mode and in weak grid-connected conditions at different short-circuit ratio (SCR) values.