Research On Control Strategy Of Cascaded H Bridge Multilevel DVR

Cascade H-bridge multi-level dynamic voltage restorer(DVR)in series between the grid and the load,can be a short time to eliminate voltage drop,three-phase voltage imbalance and short power and other power quality problems,To solve the problem of voltage quality recognized the most effective measures.However,the cascaded H-bridge multi-level DVR in the implementation of compensation when the DC side of the voltage fluctuations,and when the number of cascades,its digital implementation difficult.In this paper,cascade H-bridge multi-level DVR as the research object,the inverter and the grid power flow,cascade H-bridge inverter modulation strategy and DVR compensation strategy were studied.The details are as follows:In this paper,the topological structure and principle of cascaded H-bridge multi-level DVR are analyzed in detail,and the mathematical model of DVR is established.The relationship between DVR inverter and power flow is analyzed theoretically.When analyzing DVR compensation voltage,Inverter unit output of the active power and voltage amplitude and the voltage fluctuation,load power factor and DVR output voltage,grid voltage,compensation load voltage phase relationship between the three are directly related.Cascade H-bridge multi-level DVR series in the line,through the series transformer to achieve DVR with the power grid coupling,this paper analyzes in detail the series in the transformer transformer current inrush current principle,gives the suppression of transformer inrush current measures.The basic principles of carrier phase shift(CLS-SPWM)and carrier phase shift(CPS-SPWM)are summarized.The CLS-SPWM and CPS-SPWM modulation strategies are compared and analyzed.The difficulties of CPS-SPWM modulation strategy in digital implementation Combined with the advantages of the dual SPWM modulation strategy,a dual CPS-SPWM modulation strategy is proposed,and the concrete modulation method of the CPS-SPWM modulation strategy is given.Based on the phenomenon of level loss under the dual CPS-SPWM modulation strategy,the relationship between the modulation ratio and the number of levels is analyzed in detail,and the relationship between the modulation ratio of the maximum number of cascades of the cascaded H-bridge multi-level DVR output is obtained.Further,The relationship between the modulation ratio and the lower level is obtained.The dq transform method based on the instantaneous reactive power theory,the voltage detection method based on the double dq decoupling transform and the singlephase voltage drop detection method are summarized.The advantages and disadvantages of the in-phase voltage compensation strategy,the complete voltage compensation strategy and the minimum capacity compensation strategy are analyzed in detail,and the application of the compensation strategy is also discussed.Based on the load current vector analysis,the DVR compensation strategy is not consistent with the actual situation.The three-phase imbalance is difficult to analyze and the physical meaning is not clear.Based on the load voltage vector,the relationship between the load voltage and the voltage compensation is carried out.In this paper,a multi-level DVR compensation strategy for cascaded H-bridge based on load voltage vector analysis is presented in detail.System.The PACAD / EMTDC10 k V simulation model applied to 10 k V system is built and the relationship between DVR compensation function,level number and modulation ratio is verified.The simulation results verify the correctness of theoretical analysis.