Research On Power Quality Control With Optimized Power Configuration

As energy shortage and environmental pollution become increasingly prominent,new energy,energy storage and other distributed sources have achieved unprecedented development.Concepts of micro-grid and smart grid have been proposed one after another.Traditional power quality controllers are also faced with new demands,that is,to realize dual functions of power quality control and active power generation.The existing multifunctional inverter and unified power quality conditioner(UPQC)with PV grid-connected generation can meet this demand,but they are still faced with many problems,such as low efficiency and high cost.To make the micro-grid operate more efficiently and economically,this dissertation is focused on the system architecture,topology,control and modulation strategy of connecting new energy or energy storage to traditional power quality controllers.To achieve high-efficiency DC-AC conversion,a novel quasi-single-stage three-phase bidirectional DC-AC converter and a family of novel three-port three-phase converter(TPTPC)topologies are proposed.The basic idea is presented based on the traditional three-phase six-switch converter,in which a new low-voltage port is generated by introducing a new power flow branch,i.e.,a bidirectional switch,between the low-voltage port and the midpoint of each switching leg,to realize that partial power can be transferred within single power conversion stage.A modified space vector pulsewidth modulation(SVPWM)is proposed for TPTPC based on the analysis of the influence of modulation strategy on power transfer characteristics.Operation principles and characteristics of the proposed converter are analyzed in detail,and also compared with the traditional two-stage DC-AC converter.Comparison results show that,the power rating,power losses,cost and size of the DC-DC converter are reduced,and much higher conversion efficiency is achieved by the proposed converter.To realize optimized active power configuration,a novel carrier-based pulsewidth modulation(CB-PWM)strategy for TPTPC is proposed.The key parts of the proposed CB-PWM strategy are asymmetrical carriers and modified modulation signals.The influence mechanism of zero-sequence component on power distribution is analyzed firstly based on the CB-PWM strategy of traditional three-level converters.It is found that the active power distribution can be regulated by zero-sequence components.Based on this,the zero-sequence component and modified modulation signals are deduced for TPTPC to realize optimized active power configuration.To realize the asymmetrical carriers,the digital implementation,in which modulation signals are equivalently modified to make carriers invariable,is presented.The equivalent relationship between the proposed CB-PWM strategy and the modified SVPWM strategy is also established.Experimental results show that the proposed CB-PWM strategy applied for TPTPC can realize optimized active power configuration.Meanwhile,it features less computation burden and easier implementation when compared with the modified SVPWM strategy.To meet the demands of dual functions of harmonic compensation and active power generation,a novel quasi-single-stage shunt power quality controller is proposed.Based on the modeling and control,it is revealed that the model of voltage loop of the high voltage port is different from traditional grid-connected converters,and a control strategy that the parameters of voltage loop are adjusted adaptively according to the power transfer ratio is presented.To analyze the influence mechanism of reactive,harmonic and negative-sequence currents on the power transfer characteristics,an analysis model based on rotating coordinates is proposed.This model can approximately decouple active and reactive currents,and transfer harmonic and negative-sequence currents into direct component,which,hence,simplifies the analysis of the influence on power transfer characteristics.Theoretical analysis and experimental results indicate that the influence of reactive,harmonic and negative-sequence currents on single-stage power transfer ratio is insignificant.Hence,the proposed quasi-single-stage shunt power quality controller can not only realize harmonic compensation,but also improve the efficiency of active power generation when compared with traditional solutions.To deal with wide-range and long-term voltage sags,a novel quasi-single-stage dynamic voltage restorer(DVR)is proposed.During shallow voltage sags,the DC-DC converter is idle and the proposed DVR operates as a two-level converter,while during deep voltage sags,only partial active power needs to be processed by the DC-DC converter,and the proposed DVR operates as a multi-level converter.By comparing power transfer characteristics under different compensation strategies,it is found that the in-phase compensation method can make the quasi-single-stage DVR output the minimum compensation voltage,and hence make the DC-DC converter idle within a wider range.The quasi-single-stage DVR can make full use of the advantages of quasi-single-stage architecture,which significantly reduces the power capacity of DC-DC converter and the power losses.Combined with the series and parallel compensators,a novel quasi-single-stage UPQC is proposed,which features comprehensive voltage/current compensation,high-efficiency active power generation and cost-effective.