1.National Electric Power Conversion and Control Engineering Technology Research Center (Hunan University), Changsha 410082, China;2.Shenzhen Power Supply Co., Ltd., Shenzhen 518000, China

The voltage drop on the source side in the distribution network will lead to the transient instability of droop-controlled grid-connected inverters. Subsequent traditional unchecked reclosing will bring overcurrent problems, posing significant challenges to the safe operation of the inverter. To address the problems of transient power angle instability and overcurrent in the droop-controlled inverters due to the source-side voltage drops in the distribution network, as well as the impulse current generated by traditional unchecked automatic reclosing, this paper proposes an optimal control method for droop-controlled inverters during the full cycle of the fault transient. This method effectively improves the grid-connected operation stability of inverters and reduces the impulse current during reclosing to achieve a smooth grid connection. Firstly, the paper analyzes the transient characteristics of the inverter during the fault, and the relationship between the instantaneous overvoltage of the reclosing and the voltage amplitude, phase angle and frequency deviation on both sides of the high-voltage-side circuit breaker. Secondly, during the voltage drop phase, the reference value of the inverter power is dynamically adjusted based on the relationship between the active power of the droop-controlled inverter and the voltage output characteristics of the point of common coupling, as well as the influence of reactive power on the line current amplitude. This adjustment is made according to the actual working conditions, and the transient power angle stability before and after the fault is controlled to limit the output current of the inverter below the safety threshold. During the reclosing transition phase, the comprehensive adjustment mechanism of the frequency-phase angle is introduced to reduce the voltage frequency and phase angle difference. This mechanism reduces the number of proportional-integral controllers in the command calculation link and greatly shortens the time required for quasi-synchronization compared to the existing grid-connected control mechanism. Finally, the correctness of the theoretical analysis and proposed method is validated by combining simulation and experimental results.