Control And Stability Research For Parallel-connected Distributed Generation In A Microgrid

Microgrid, which is composed of renewable energy sources, energy storage devices and power electronic devices and so on, is gradually becoming an effective way to meet the growing demand of loads. With numberous distributed generation units connecting to the microgrid, mismatched line impedance or inconsistent local loads may cause power sharing and circulating current problems in multiple paralleled inverters, subsequently, affecting the operation stability of the islanded system.In terms of power sharing and circulation problems exsting in paralleled inverters, this paper analyzes the links and differences of the system under the conditions of power sharing based on capacity and zero circulating current, and compares line losses in two conditions, considering the impact of line impedance and local load. Then taking the efficiency of the system into account, an improved droop control scheme based on the averaged power is presented. This strategy injects a voltage compensation to the voltage magnitude to realize power sharing and reducing circulating current in the system with the same capacity or different capacities. It is unnecessary to consider the impact of the line impedance, local load and other factors.In order to design rational droop slopes, the small-signal stability analysis of the droop control strategy has been discussed in an autonomous microgrid with power droop control, generally neglecting harmonic interaction among multiple distributed generation units. However, this paper presents a small signal model including the impact of voltage and current loop and demonstrates the relationship between the point of common coupling voltage, reference voltage and output current. A Thevenin equivalent model of the autonomous network is established to study system stability using impedance-based Nyquist criterion. Then theoretical analysis and simulation results validate that line impedance and the numbers of paralleled inverter system have a significant impact on harmonic stability of the islanded network.Finally, an experiment platform of two paralleled three-phase inverters is built based on TMS320F2812 DSP, and the experimental results prove the feasibility of the strategy.