Transient load sharing between inverters and synchronous generators in islanded microgrids

When inverters in voltage control mode are paired with synchronous generators they exhibit poor transient load sharing in islanded operation. The inverter initially picks up the majority of every load step, and the generator's output slowly increases until they reach their steady state load sharing given by droop settings. This excessive transient power output from the inverter constrains its rating compared to the largest anticipated load step, and could negatively impact battery life in battery energy storage inverters. This paper examines the transient load sharing characteristics between synchronous generators and inverters in voltage control, i.e. grid-supporting-grid-forming control, and investigates methods to improve it. An equivalent circuit is proposed to describe the initial power sharing ratio, and the effects of increased inverter droop slope and increased governor integral gain are investigated. An inverter control strategy of emulating a synchronous generator is shown to provide good transient load sharing. Validation is provided by simulation and experimental results for real and reactive loads.