Research On STATCOM Compensation Strategy Of H-bridge Under Unbalanced Load

With the development of industry,the growing number of unbalanced loads are connected to the power grid,which causes the imbalance of voltage and current.In order to improve the power quality of the power grid,cascaded H-bridge STATCOM has been applied to voltage power system gradually.For improve the power quality compensation ability of H-bridge STATCOM when the power grid is connected with unbalanced load,this thesis is studied as follows:Firstly,several different topologies of STATCOM device are analyzed,and then its working principle is analyzed.The mathematical models of cascaded H-bridge STATCOM under balanced and unbalanced conditions are established.To improve performance,two modulation methods of bipolar and monopole frequency doubling are studied.It is concluded that the latter has better frequency characteristics.In order to compensate the active power and reactive power separately,a decoupling control strategy is proposed in this thesis.At the same time,considering the unbalanced condition of power grid,we further split the power decoupling control into positive and negative to separate compensation of reactive and unbalanced sequence current on AC grid side.The positive sequence current loop is used to make up for the reactive power on AC grid side,the negative current loop is to make up for the negative current on the AC side.When the power grid is connected to three-phase unbalanced load,it will lead to unbalanced negative sequence in the power grid current.In order to compensate it,cascaded STATCOM needs to output negative current to make up for the AC power grid,which will lead to DC side voltage imbalance.In this thesis,a layer voltage balancing control strategy is proposed,which is divided into overall control,inter phase control and intra phase control.The voltage control adopts the control total active power exchange method,and the effect is remarkable.Zero voltage immition and negative current immition are used in DC side phase to phase voltage control.The consequence indicate the effectiveness of the control strategy,but considering the unbalanced condition of power grid,the simulation results show that the zero sequence voltage injection method can achieve the optimal effect of the whole system,and the zero sequence voltage injection is the final choice for phase to phase voltage control in the law.In the DC side,the phase of the output voltage and current of the H-bridge is adjusted to control,so as to realize the secondary distribution of power and realize the phase voltage balance.Simulation result show that the proposed AC side and DC side control algorithm can settle the current imbalance problem caused by load imbalance on the basis of ensuring direct current side stability.