Dynamics of Droop-Controlled Microgrids with Unequal Droop Response Times

One increasingly popular approach to integrate distributed generation (DG) energy sources into an electrical grid system, is to assemble them into microgrids which then connect to the main utility grid at a single or multiple points of connection. A microgrid can also operate as a standalone islanded network which is physically disconnected from the grid. During island mode the generated power of each DG unit must be carefully controlled to ensure reliable power distribution and modular operation. Droop control is commonly used for this purpose, commanding individual voltage magnitude and frequency for each DG unit to achieve balanced active and reactive power sharing. A droop controller involves several stages of processing, particularly output power calculation and updating the voltage command setpoint. These stages contribute to a time delay defined in this paper as the droop response time (DR time), which can vary between multiple DG units because of different designs and controller implementations. This paper explores the effect of differences in droop controller response times on the ability of a microgrid to balance its commanded DG generations with its required load profile. It is shown that unequal droop response times can substantially degrade the power sharing dynamics and the modularity of the microgrid.