不平衡电压条件下VSG-SST控制策略

CONTROL STRATEGY OF VSG-SST UNDER UNBALANCED VOLTAGE CONDITION

当低压配电网电压不平衡时,产生的负序分量将引起传统VSG控制中SST输出级发生电流不平衡和功率振荡现象,为此该文提出基于改进VSG的T型三电平输出级联合控制策略,以提高输出级电能质量。首先,对不平衡电压下VSG电流不平衡及功率振荡进行机理分析;其次,基于VSG控制算法和瞬时功率理论设计新型电流基准发生器,与正负序电流调节器实现级联控制,以此保障SST输出电流平衡及有功/无功功率恒定,实现不同运行工况的可靠切换;最后,对正负序分量分离方法和中点电位平衡算法进行分析研究,并对控制策略进行仿真验证。结果表明,所提联合控制策略有效可行,能保障电压三相不平衡工况下SST输出级并网电流功率质量。

When the low-voltage distribution network voltage is unbalanced,the generated negative sequence component will cause current unbalance and power oscillation in the solid state transformer(SST)output stage in the traditional virtual synchronous generator(VSG)control.Therefore,this paper proposes a T-type three-level output stage joint control strategy based on VSG to improve output stage power quality.Firstly,the mechanism of VSG current unbalance and power oscillation under unbalanced voltage is analyzed.Secondly,based on VSG control algorithm and instantaneous power theory,a new current reference generator is designed,which realizes cascade control with positive and negative sequence current regulator.This ensures the SST output current balance and constant active/reactive power,and realizes reliable switching of different operating conditions.Finally,the separation method of positive and negative sequence components and the midpoint potential balance algorithm are analyzed and researched,and the control strategy is simulated and verified.The results show that the proposed joint control strategy is effective and feasible,and can guarantee the quality of the grid-connected current and power of the SST output stage under three-phase unbalanced voltage conditions.