基于MMC二次谐波注入的光伏直流升压接入系统故障清除协调控制策略

Coordinated Control Strategy of Photovoltaic DC Boosting Integration System During Fault Clearing Based on MMC Second Harmonic Injection

光伏直流升压接入系统在交流电网发生故障时应该具备故障穿越(fault ride through,FRT)的能力,然而交流电网中较大过渡电阻故障被清除后,直流变压器(DC transformer,DCT)控制策略难以自动恢复至最大功率点跟踪(maximum power point tracking,MPPT)控制。提出基于模块化多电平换流器(modular multilevel converter,MMC)二次谐波注入的故障清除协调控制策略。故障清除时,通过修正逆变器控制内环负序分量,主动向系统注入短窗长二次谐波。DCT根据直流母线二次谐波电压幅值切换控制策略,实现光伏发电单元在故障清除后快速切换回MPPT控制。PSCAD仿真结果表明,通过所提方法,在电网故障时,光伏侧均能可靠且快速的切换控制策略,有效提高光伏资源的利用率。

The photovoltaic(PV)DC boosting integration system should have the ability of fault-ride-through(FRT)when the AC grid fails.However,the control strategy of DC transformer(DCT)is difficult to automatically switch back to its original maximum power point tracking(MPPT)control when the fault by large transition resistance is cleared.A fault clearing coordination control strategy based on modular multilevel converter(MMC)second harmonic injection was proposed.By correcting the negative sequence current reference of converter,the second harmonic is actively injected to the system when the fault is cleared,and DCT can switch back to the MPPT control in time by monitoring the amplitude of the DC second harmonic voltage.The PSCAD simulation results show that the control strategy of DCT can reliably and quickly switch by using the proposed method under various fault types of the host grid,which effectively improves the utilization rate of the photovoltaic.