Research On Improved Droop Control Strategy Of Inverters In Microgrid

Generally, the microsources were connected to the microgrid through inverters, thus the microsource control was indirectly controlled by inverter. While the microgrid operated in islanding mode, the droop control scheme was commonly used. It was associated with the microgrid safety and reliability to consider about how to essure the power quality and improve the accuracy of load power distribution, which had also become a core issue of microgrid control. To the photovoltaic(PV) microgrid, the inverter control technology was studied around the improved droop control.Affected by light intensity, temperature and other factors, the output of PV power was random and intermittent. For each PV system, to achieve its output power schedulability, and maintain the inverter DC voltage constant, energy storage system was required. Based on the deep analysis of the PV output characteristics, vanadium redox battery-super capacitor hybrid energy storage system was configured to smooth the PV power fluctuations. Taking both the technical and the economic performance into account, a capacity allocation mathematical model was established based on an overall dispatch strategy of energy storage system. Then, the particle swarm optimization(PSO) was programmed under the Matlab environment, and used to determine the optimal capacities of vanadium redox battery and super capacitor.The three-phase voltage source inverter(VSI) played an important role in the microgrid, so it was analysed detailedly and modeled. To improve its control performance, the multiple feedback loop control, containing an outer power loop, and an inner voltage and current loop, was adopted in the control circuit. Moreover, the related parameters design principles were given, which layed foundation for the presentation of an improved droop control scheme.Due to the dispersion of microsources, line impedances were different, resulting in unequal inverter voltages. So, it was difficult for traditional droop control to achieve reasonable distribution of load power. To solve this problem, an improved droop control strategy based on inverter output voltage adjustment was presented. Once there was large load fluctuation, the reference voltage amplitude of each inverter was adjusted roughly using the relationship between it and the output power. Since it leaded to much electrical fluctuation, an output voltage minute adjustment by drooping coefficient was added. Moreover, two parallel inverters using the improved droop control were carried out in Matlab/Simulink platform. The simulation results show that the proposed scheme achieves rational load power distribution and good power quality, and the circulating current between parallel inverters is small, the system steady performance and dynamic performance have been both improved.