A Modified Droop Control for Grid-Connected Inverters With Improved Stability in the Fluctuation of Grid Frequency and Voltage Magnitude

The droop controlled grid-connecting inverter (DC-GCI) has been widely used in microgrid (MG). However, the power flow of the droop control is very sensitive to the fluctuation of grid frequency and voltage magnitude, which will result in a very fast inrush current of DC-GCI. Owing to the linear relationship of P-F, current limiters are required to prevent DC-GCI output power from exceeding maximum power. However, the impact of grid frequency fluctuation on DC-GCI cannot be mitigated by the current limiter for the reason that the fluctuation will lead to the current limiter saturation, which will cause the error between the angular frequency of the P-F droop output and the angular frequency of the grid voltage, and the system becomes unstable. It is a challenge to limit the output current, meanwhile ensuring the stable operation of the DC-GCI. To solve these problems, a modified droop control strategy is proposed to mitigate the impacts of the fluctuation of grid frequency and voltage magnitude on the system stability. In addition, the proposed control strategy in this paper realizes the stable operation of the DC-GCI through the cooperation between the modified droop control and the current limiter, and the current limiter saturation is avoided. The current limiter is used to restrain the transient current to compensate for the slow bandwidth of the droop loop, and the modified droop loop is used to stabilize output power and restrain the steady-state output current. The simulation and experimental results from a scaled-down laboratory prototype have been presented to verify the major contribution of this paper.