Research On Post Fault Recovery Control Strategy Of PV DC Boosting Integration System

The photovoltaic(PV)DC boosting integration system should have the ability of low-voltage ride through(LVRT).When voltage sags occur on the AC side,the PV array should reduce the output power.However,when the small voltage sag faults on the AC side are cleared,PV array may not be able to effectively detect the system changes and switch back to original the maximum power point tracking(MPPT)control in time.In this case,the stability of the system is compromised and the PV array power generation efficiency is jeopardized.To explore the mechanism of the control strategy switching failure of DC/DC,the power balance characteristics of the system during the disturbance period are analyzed.On this basis,two fault recovery control strategies are proposed.Firstly,the influence of AC voltage sags on DC/DC converter control switching is analyzed.The reason of control switching failure of DC/DC in AC small voltage sag fault clearing is revealed.In addition,the small signal equivalent impedance models of DC/DC and the modular multilevel converter(MMC)are established.According to the Nyquist stability criterion,the stability of the system after the switching failure of DC/DC control strategy is judged.It is concluded that the system stability margin is greatly reduced when DC/DC and MMC both operate in constant voltage control.Secondly,a fault recovery control strategy based on inverter second harmonic injection is proposed.By modifying the negative sequence current reference of converter,the second harmonic is actively injected to the system in a short time when the disturbance disappears,the "communication" between the DC/DC and MMC is realized.Based on the local linearization of PV array’s unique power-output characteristics,the expression of DC voltage second harmonic component and the voltage threshold of DC/DC are obtained.Then DC/DC converters can switch back to the MPPT control quickly and accurately without additional communications between them,which improves the stability margin of the system.Finally,the relationship between the transition resistance and AC voltage,active power deficiency and DC voltage during fault ride through is derived.On this basis,a fault recovery control strategy based on the feedforward compensation for voltage incremental of power grid is proposed,which actively corrects DC voltage reference value of MMC outer control loop.According to the threshold value of DC voltage fluctuation,the correction value of MMC’s control outer loop voltage reference is obtained.In this case,the DC/DC converters can switch from constant DC voltage control to MPPT control in time after the small voltage sag fault is cleared,which effectively improve the utilization of PV generation.The simulation results of different fault types and transition resistance values verify the applicability of the proposed control strategy.