Improved Droop Control for Suppressing Reactive Circulating Current and Proportionally Distributing Reactive Load between Parallel Converters

Converters are interface of the clean renewable energy integrated into Microgrid. These parallel converters usually adopt droop control for synchronization and load proportion distribution without communication. Due to mismatch between the system impedance (include converter output impedance and line impedance between converter and connecting point) ratio and capacity ratio of converters, a reactive circulating current is caused between converters and the reactive load can't be distributed by converters capacity ratio. The reactive circulating current and load distribution problem will lead to overvoltage and overload of small capacity converter and microgrid unstable. In order to overcome above problems, an improved droop control strategy based on virtual impedance is carried out by analyzing the reactive circulating current and reactive load distribution principle. The virtual impedance used in this strategy is optimal designed to make the system impedance ratio match the capacity ratio between parallel converters. The simulation results show that the goal of suppressing the reactive circulating current and proportionally distributing the reactive load is achieved.