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Research On Key Technique Of Multi-Inverter Parallel System And Grid-on Inverter In Photovoltaic Power Generation System

Posted on:2010-08-14Degree:DoctorType:Dissertation
Country:ChinaCandidate:M YuFull Text:PDF
GTID:1102360275986956Subject:Electrical engineering
Abstract/Summary:PDF Full Text Request
While the world's power demand is increasing,photovoltaic power distributedgenerating system is gaining more and more visibility.This dissertation is focused on theresearch of grid-on current control and current-sharing control,which are the keytechniques in power conversion ofphotovoltaic energy.The grid-on inverter is connected with power system via L filter or LCL filter.Thecommom problem of current control is researched with L filter first,then the specialproblem in LCL grid-on current control is researched.The technical requirements forgrid-on PV system are the total harmonic distortion,power factor and current trackingerror of grid current.The relationship between these requirements and the naturalfrequency of the close-loop characteristic roots(FCCR)is analyzed in the paper.Then atheory in which FCCR is used to quantize the technical requirements is proposed in thepaper.Taking PI control algorithm for example,the quantitative relationship is presented.As the system stability may fall down in grid-on inverter with a L filter while thecontroller is discreted,the quantitative relationship between critical FCCR and samplingfrequency is analyzed.As the system steady performance may fall down while thecontroller is discreted,the steady performance of discrete control system is compared tothat of continues control system.Considering the technical requirements,the stability andthe steady performance,the quantitative requirement for sampling frequency is present indiscrete control system.Considering that there is a step delay in discrete control in grid on inverter with a Lfilter,the quantitative relationship between critical FCCR and sampling frequency isanalyzed,and the steady performance is analyzed.Considering the technical requirements,the stability and the steady performance,the quantitative requirement for samplingfrequency is present in discrete control system with a step delay.As LCL filter may not satisfy the engineering requirements for some switchingfrequency in grid-on inverter,the range of switching frequency is analyzed.As thestability may fall down because of resonance of LCL filter,the stability of current controlsystem in continues time is analyzed,and the conclusion is that the stability is not affected by the resonance.As the performance may not satisfy the technical requirements in grid-on inverterwith inner inductor current feedback control,the current tracking performance andanti-interference performance are analyzed,and requirements for FCCR and parameter ofLCL filter are obtained.As the stability may fall down in grid-on inverter with a LCL filter while thecontroller is discreted,the quantitative relationship between critical FCCR and samplingfrequency is analyzed and obtained.When discrete control is with a step delay,thequantitative relationship between critical FCCR and sampling frequency is analyzed andobtained.As the relationship between regulated reference voltage quantity and circulatingcurrent need to definite clearly in distributed logic current-sharing control,the concept ofvirtual circulating impedance is proposed,and its mathematical model is obtained.As thevoltage feedback control may affect the current-sharing control performance,themathematical model is analyzed.The conclusion is obtained,that the magnitude ofcirculating impedance is decreased sharp and the characteristic is changed by voltagefeedback control.As the magnitude control and phase control are coupled in roots mean square(RMS)current-sharing control,the respond speed of current-sharing control becomesslower.To solve the problem,a novel decoupled strategy based on virtual circulatingimpedance is proposed in the paper.To further accelerate respond speed,a novel instantaneous current-sharing controlstrategy based on virtual circulating impedance controller is proposed in the paper.As thedifference in inverter modules' parameters may decrease the current-sharing performance,the mathematical model of circulating current in parallel system with the instantaneouscurrent-sharing control strategy is analyzed,and the conclusion is obtained,that thestrategy is robust.The results of experiment and simulation verify that the current-sharingcontrol strategy has very good performance.
Keywords/Search Tags:Grid-on, Current Control, Parallel Operation, FCCR, Discrete Control, a Step Delay Instantaneous, Current-Sharing Control, Virtual Circulating Impedance, LCL filter
PDF Full Text Request
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