| With the continuous advancement of China’s social modernization,various sensitive loads have been widely used in people’s production and life,it is especially important to ensure that the power quality meets the power requirements of these devices.Among the various power quality problems such as voltage sags,voltage swells,and voltage harmonics,voltage sags occur the most and cause the most losses to business users.Dynamic voltage restore(DVR)is a kind of power quality management device developed for voltage sag and other issues.It can quickly compensate load voltage when voltage sag occurs,ensure that the load is not affected,and have good dynamic characteristics,is currently the most cost-effective solution to voltage sags.This paper designs a three-phase dynamic voltage restorer for medium voltage distribution networks,and studies its topology,voltage sag detection algorithm,compensation and control strategy.This paper first introduces the basic constituent units and working p rinciples of DVR devices,compares and analyzes the advantages and disadvantages of the different structures of each unit.Aiming at the problem that the traditional DVR device is mainly used in low voltage levels,a new DVR topology combining a parallel converter and a multi-winding transformer is proposed.This structure uses multi-winding transformer shunting to reduce the requirements for the current tolerance of the converter,and an equivalent circuit model of the DVR structure is established.Next,the modulation strategy and output harmonics of the parallel converter are analyzed,and a carrier phase shifting strategy is designed to modulate the converter.Secondly,the voltage sag detection method and compensation control strategy of DVR are studied.Different voltage sag detection methods are compared.The software phase-locked loop and dq transform based on instantaneous reactive power are used to detect the voltage sag.Then the DVR mathematical model in the dq coordinate system is deduced.Based on the model,a control strategy for grid voltage feedforward combined with DVR output voltage feedback is designed to compensate the dq axis components respectively.At the same time,the output voltage and active power of the DVR in three commonly used c ompensation strategies are analyzed,and in-phase compensation is selected as the compens ation strategy for the DVR.The circulating current between the secondary windings of the transformer,which appears by using carrier phase shift modulation,is analyz ed,and a circulating current suppression strategy is proposed.The construction of DVR simulation model was completed using Matlab / Simulink software,and the simulation experiment of voltage compensation and circulating curre nt suppression with system voltage of10 kV was performed.The simulation results show that the designed DVR structure and its control strategy can quickly output the compensation voltage,and the harmonic content and windings circulation is small.Finally,a 380 V principle prototype with DSP and FPGA as the control core was built,the hardware circuit and software flow of the prototype were designed,and the control code was written using the CCS development platform.Low voltage compensation experiments were performed according to the existing experimental conditions,and the experiment verified the feasibility of the designed DVR structure and related control strategies. |
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