Research On Control Strategy Of Three-phase Unified Power Quality Conditioner

At present,a variety of devices based on power electronic technology are widely used in the market.These devices often have non-linear characteristics,making the power quality problems of power system become more and more serious.At the same time,with the gradual improvement of power quality requirements of power users and a growing number of sensitive equipments,all kinds of electric energy control devices begin to appear.As the equipment that can deal with many kinds of power quality problems comprehensively,Unified Power Quality Conditioner,(UPQC)can solve both voltage quality problems and current quality problems at the same time,so it is widely used.In this paper,the control strategy of three-phase UPQC is studied.Based on the investigation of the current research status of UPQC,a new three-phase UPQC topology structure without power frequency transformer in series part is proposed to realize the electrical isolation between the series part and the parallel part of UPQC,greatly reducing the volume of the whole device and improving the safety of the device.Combined with the engineering practice,the modular design of the device is carried out under the new topological structure,which is convenient for the extension and maintenance of the device.The whole three-phase UPQC is divided into series part,parallel part and high frequency isolation DC/DC part.In this paper,the three parts of UPQC are described according to the chapters,from the main circuit topology,converter working principle,detection method,compensation strategy and other aspects of research,by comparing the current common detection method and compensation strategy,combined with the control objectives of each part,the appropriate control strategy is selected for the series part,parallel part and high-frequency isolation DC/DC part.In terms of detection methods,in order to achieve the same frequency and phase of compensation voltage and grid voltage,the phase-locked loop method based on sogi is used to track the grid voltage phase.For the detection of grid voltage drop,the series part selects the defect voltage method with better rapidity as the detection method.For the detection of positive sequence,negative sequence and zero sequence components of specific subharmonics of three-phase current,the parallel part selects improved d-q method is selected as the detection method of harmonic current.In the aspect of compensation strategy,because the series and parallel parts control the AC quantity,PR controller which can track the AC signal without static difference is selected.In order to reduce the influence of the error caused by the frequency fluctuation of power grid on the control link,band width adjustment coefficient is added on the basis of PR controller,by selecting appropriate resonance coefficient,the controller can also guarantee large gain in a wide bandwidth range.The control target of the high frequency isolated DC/DC part is the DC voltage at the low voltage side of the device,so the PI control for DC signal,which is more mature at present,is selected as the control method of this part.Under the appropriate simulation and experimental parameters,the voltage detection method and compensation strategy,current detection method and compensation strategy,high/low voltage side DC voltage stability strategy of three-phase UPQC are verified by Matlab/Simulink simulation platform and experimental platform built in the laboratory.The simulation and experimental results prove the feasibility and effectiveness of the proposed control strategy.