Design And Realization Of Large-Capacity Multi-Level Dynamic Voltage Restorer

Requirements of power quality is getting higher and higher by the power users with a largenumber of high-voltage large-capacity sensitive loads are widely used. The supply voltagesags in a variety of power quality problems has the most serious impact on power users.Currently most of the researchers believe that dynamic voltage restorer (DVR) is the mosteffective compensation device to solve voltage sags. DVR can generate compensation voltagein the millisecond time and put the compensation voltage into the grid, so the load voltage willnot be affected.This paper describes DVR working principle, system architecture, and makes detailedanalyses to DVR system structure. In this paper super capacitors are selected as DC energystorage unit, cascaded multi-level inverter as DVR core power unit. The output filters andtransformers are determined to couple with the grid. The SPWM modulation method based oncarrier phase-shift technology is selected as the cascade multi-level inverter modulation methodafter comparing and analyzing several multi-level inverter modulation methods. At the sametime the mathematical model of multi-level DVR is determined.This paper discusses the traditional voltage sag detection method and three-phase voltagedetection method based on homeopathic reactive power theory. In order to solve thenon-applicability that traditional methods are used in the single-phase voltage sag detection, thesingle-phase abc-dq derivation voltage detection method is determined to use in DVR aftercomparing and analyzing the single-phase abc-dq transform voltage detection method and thesingle-phase abc-dq derivation voltage detection method.Three compensation strategies of DVR are analyzed and compared in this paper. They arein-phase voltage compensation strategy, complete voltage compensation strategy and minimumpower compensation strategy. The minimum power compensation strategy is selected as thetheoretical basis of the DVR compensation voltage considering the load characteristics. Thecontrol strategy of cascade21-level DVR is discussed amply and voltage single closed-loopcontrol strategy with feed-forward PD is determined as the final control strategy. On these bases,hardware selection, software design, the main circuit structure and total system structure diagram of cascade21-level DVR core controller is completed.Finally, applying cascade21-level DVR to solve the problem of supply voltage sags issimulated in the PSCAD/EMTDC. Simulation results verify the correctness of the theory, andalso prove that cascade21-level DVR can solve the problem of supply voltage sags effectively,so cascade21-level DVR could ensure the quality of power supply of high-voltagelarge-capacity sensitive loads.