Research On The Key Techniques Of Power Quality Control In Microgrid With Multi-inverter

Microgrid can reconcile the contradiction between distributed generation and the utility grid,and is also an effective way to improve the efficiency of distributed generation.Microgrid has the properties of the diversity of the types of distributed generators,variable operation modes and the volatile and intermittent of the micro-sources,which makes the power quality issues of the microgrid present some new characters other than traditional power system.As the interface unit connecting distributed power supply and the utility grid,microgrid inverter are characterized by the similar structure with power quality compensator,which shall provide convenience for microgrid power quality control.Therefore,researching power quality control strategy in microgrid with multi-inverter is of extremely importance to theoretical research value and practical significance for constructing high power quality and high reliability microgrid.This paper is finished by financial support of the key project of National Natural Science Fund(51237003)"multi-inverter parallel and power quality control method of microgrid" and the basic project of National Natural Science Fund(51477045)"the research of power quality emission characteristics of typical electricity load containing micro supply".The research contents including harmonic control of microgrid inverter in grid-connected mode,power control under unbalance three-phase grid voltage,parallel and average-current control between parallel microgrid inverters under islanded mode and power quality hybrid control of microgrid inverter and active power filter.The major work and innovations in this dissertation are listed below:(1)The proliferation of nonlinear loads and power electronic devices in the power system brings the amount of harmonic current.To this problem,a flexible harmonics control method for three-phase grid-connected inverter without harmonic detection is proposed.It makes current tracking control equipped with the function of harmonic detection and simplifies the procedure of traditional harmonic detection and current tracking control into a single procedure of current tracking control through ingenious design of control structure,so as to realize harmonic compensation and harmonic suppression control without harmonic detection,and to avoid the controlling burden and delay effect from harmonic detection procedure.The synchronous frame universal equivalent model of the controller in stationary frame isdeduced,and the equivalent model not only can reflect the control performance of the input signal at different frequency in stationary frame,but also provide theoretical support for modeling and frequency-domain analysis of the synchronous frame of controller in stationary frame.By the equivalent transformation of controller from synchronous rotating reference frame to stationary reference frame,the frequency-domain model of the entire control system in stationary reference system has been established,and the frequency-domain tracking characteristics and stability of the system have been analyzed.The simulation and experimental results have demonstrated the effectiveness of the proposed method.(2)The imbalanced three-phase grid voltage will cause the distortion of inverter output current and the oscillation of output power,and will bring negative impact on the safe and stable operation of grid-connected inverter.In accordance with the construction features,the power flow of three-phase four-wire grid-connected inverter are analyzed,and then the sufficient condition of constant active power with unity power factor grid-connected in three-phase four-wire system which is maintained together by negative-sequence current component and zero sequence current component under unbalanced voltage environment is deduced.And on this basis,the article respectively deduces the current reference acquisition schemes under three circumstances: maintaining constant active power only through injecting negative-sequence current component,maintaining constant active power only through injecting zero sequence current component and maintaining constant active power and reactive power through injecting zero sequence current and negative-sequence current components together.Based on the problem that the controlling effect of single control objective cannot be taken into consideration at the same time,a negative sequence and zero sequence current coordination control strategy and a negative sequence and reactive power fluctuation coordination control strategy based on weight ideology are proposed.The proposed methods can reasonably realize different control effects while guaranteeing that active power sent to the power grid always keep constant without fluctuation through changing coordination command factors,so as to improve the flexibility of grid-connected control.Through analyzing the influence of parameters in double closed-loop current control strategy on system performance,suitable closed-loop parameters has been selected and the stable and dynamic properties of LCL type grid-connected inverter are improved.(3)Virtual impedance is usually introduced to control system of parallel-connected inverters so as to change the droop characteristics of inverter and improve the power-sharing precision between parallel-connected inverters.In this paper,a novel implementation method of virtual impedance is proposed,and avoids derivation of output current,reduces system calculation and enhances system response speed.The proposed method consists of two parts: crossover-cancellation feedback network,which achieves band-pass effects;and a multilevel TOGI-OSG link,which achieves filtering function.The principle of this method is detailed discussed and its influence on system is analyzed by amplitude-frequency characteristic curve in this paper.In addition,this paper also discusses the filtering characteristic of the proposed method when inverter output current has DC offset and severe distortion.Thus,the method can greatly reduce influence of noises,distortion and DC offset in output current on the system,and finally decrease the THD of output voltage.(4)Targeting at the characteristic that non-linear load in micro-grid is usually dispersive and difficult for concentrated management,a power quality hybrid control strategy combining grid-connected inverter and active power filter based on the concept of frequency domain is proposed.In harmonic domain,grid-connected inverter and active power filter can realize the optimazation of mirco-grid harmonic problem of management,reduce the voltage distortion rate of the point of common coupling and improve grid-connected inverter output current quality through the impedance cordinated control.In fundamental frequency domain,grid-connected inverter and active power filter can improve the accuracy of output voltage at the point of common coupling and enhance the power distribution effect of micro-grid inverters through the compatible operation of reactive power.Through organic cooperation of active power filter and grid-connected inverter,the controlling burden of grid-connected inverters in power quality compensation can be reduced so that grid-connected inverter and power quality compensator can fulfill their respective advantages and their optimized operation.The article proposes a harmonic virtual impedance implementation strategy based on third-order general-integrator crossover cancellation current feedback control method,which can realize the cross decoupling extraction between each harmonic and fundamental frequency signals.It can provide convenience to construct the resistive,inductive and resistance-inductance virtual impedance of each harmonic,and avoid the introduction of differentiation element.Also it has greatly restricted the incremental noise of signals and guaranteed the accuracy of impedance construction.In addition,in order to eliminate the couplingrelationship between the voltage at the point of common coupling and output current of active power filter,a feed-forward control strategy of capacitor voltage is proposed,which can cancel out the effect of the voltage at the point of common coupling and the filter capacitor on the active power filter output current,and improve the tracking accuracy on indirect control.