Dynamic Voltage Restorer Research And Implementation Of The Device Inverter Unit

Dynamic voltage restorer (DVR) is a series-wound equipment used to compensatefor the dynamic power quality problems such as voltage sag and swell, which seems as acontrollable voltage source installed in power system in series and its core is a voltagesource inverter based semiconductor switch. As the executor of compensation voltagecommand, DVR inverter influences the performance and the cost of DVR in great degree.The dissertation mainly researches on the inverter used in low voltage distribution gridDVR based on phase voltage compensation, which includes following parts:1. Based on the analysis and comparison about performance, cost and difficultydegree of realization, the 3-H bridge is chosen from three topologies. To improveinverter's performance, selective harmonic elimination PWM (SHEPWM) is chosen. Thedissertation estimates computing time of inverter's control section and studies on dynamicresponse of filter on the condition of zero initial value as well as of actual initialparameter of filter component, which presents a method to quantificationally computedynamic response time of DVR inverter and gives the suggestion on the selection of thefilter parameters.2. Power exchange between DVR inverter and the power system of the time whengrid voltage fluctuates is analyzed and computed. The dissertation deeply studies on thedetail relation between output voltage and power of the inverter and grid voltage, loadvoltage , command voltage previous and after grid voltage fluctuation under differentcompensation strategies, which presents quantitative and academic base for deciding onDVR's rating voltage, rating energy storage and realization of various compensationstrategy, especially, gives feasible suggestion on the search of reference phase of minimalenergy compensation strategy.3. Based on a great deal computation and comparison, the dissertation summarizesthe sameness and the difference between switch angles of SHEPWM and of CPWM(Centroid PWM) and proposes a method to generate initial condition of SHE equation inWalsh domain according to CPWM technique, which effectively reduces blindness ofsearching initial switch angles in the course of SHE equation solution and difficulty ofequation solution, and improves computation efficiency. So piecewise linear relationbetween the switch angles and the inverter voltage amplitude is simply gained and theinverter can effectively eliminate harmonics and flexibly regulate voltage in the wholerange of output voltage.4. The influence of the filter which is as inner impedance of the inverter and thepower system on the inverter and compensating effect is deeply researched on. Based onthe influence, the suggestion on correcting compensation strategy is given. According toDVR output characteristic, harmonic distribution of the SHEPWM pulse and filter'sinfluence, the dissertation proposes quantitative principle and systematic method ofdesign of the filter, thus, the filter not only can effectively eliminate harmonic but also hasslight influence on the inverter and load.5. At last, the dissertation designs and builds DVR inverter and makes experimentsof inverter solely and in series with grid. The experimental results indicate that theSHEPWM technique based on Walsh transform can effectively eliminate harmonics oflow order in PWM pulse in the whole range of modulation rate, and the filter caneffectively eliminate residual harmonics of high order in SHEPWM pulse and attenuateharmonic of low order generated by dead-time, etc. The inverter has well static anddynamic performance and can meet demand of compensation to dynamic power qualityproblem.