Current Sharing Approach for DC Microgird Based on Superimposed Frequency Droop Control

Abstract

Accurate current sharing between micro-sources is an important control objective in DC microgrid. The traditional droop control based on virtual resistance is difficult to meet the requirements of sharing accuracy and stability at the same time because of the differential line impedances and low-speed communication. A new current sharing approach for paralleled micro-sources based on superimposed frequency droop control is proposed in this paper. A small ac voltage with low frequency and low amplitude is superimposed onto the output dc voltage of each converter. Therefore, dc converters can be coordinated together with the frequency of the small ac voltage, without any communication. Utilizing the droop characteristic between superimposed frequency and output DC current, reactive circulating power would be generated between converters and then be used to regulate the output DC voltage of each converter. Then, the accurate current sharing can be achieved. On this basis, a reactive power limiter is proposed to compensate the voltage drop, so as to make the bus voltage operating within the specified range. Small signal analysis is used to verify the system stability and select proper parameters. Finally, the feasibility and effectiveness of the proposed control strategy are validated by the results obtained from simulations.