Sharing Transient Loads : Causes of Unequal Transient Load Sharing in Islanded Microgrid Operation

When inverters in voltage control mode are paired with synchronous generators (SGs), they exhibit poor transient load sharing in islanded operation. It is well known that power electronics respond more quickly where 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 relative to the largest load step and could negatively impact the battery life in battery energy-storage inverters. This article provides a detailed examination of the differences between the frequency-regulation characteristics of inverters and generators to explain, in a novel way, the cause of the poor transient power sharing. The tradeoff between an improved transient load sharing and increased voltage and frequency transients is highlighted, and it is demonstrated that equal transient load sharing can be achieved by emulating an SG using a power hardware-in-the-loop (HIL) approach. Validation is provided by simulation and experimental results.