Research On Control Method Of The Unified Power Quality Conditioner In Microgrid

With the continuous development of the microgrid technology, thestructure of the microgrid and the power load will be increasinglycomplicated, and the probability of various power quality problemsappeared in microgrid system gets higher and higher. In order to solve thepower quality problems in microgrid about the voltage fluctuation,three-phase unbalance, harmonic pollution and so on, this paper used theUPQC to control the power quality. The UPQC control policy directlyinfluences the comprehensive compensation ability for power quality.Therefore, this paper mainly studied the UPQC control method.Deeply analyzed the basic structure, the working principle and theinstruction signal generating principle of the UPQC, and according to thestate space theory established a unified math model of the UPQC, whichlaid a good foundation for the study on the UPQC control method.According to the delay problem in the harmonics detection methodof the UPQC, a harmonic current forecasting method for the UPQC basedon the forward linear prediction theory was proposed in this paper. Theoptimal forecasting coefficient canonical equation, minimum forecastingerrors and rank update equation were deduced. The forecasting coefficientand minimum forecasting errors of one-three order filter were given. Thismethod could predict the next time harmonic current, to realize minimumerrors compensation by adaptive forecasting control. The simulationresults show that the method has high forecasting precision and goodcompensation effect.According to the poor comprehensive compensation effect，whichthe usual control method separately controlled the series and shunt part ofthe UPQC, a optimizing control method of base on robust H₂/H_∞controllers was proposed. Uncertain parameters of structures areintroduced into the state equation, meanwhile, both the H∞disturbancesuppression and the optimal H2performances of the closed-loop controlsystem are satisfied simultaneously for all allowable uncertainties. Therobust H₂/H_∞controller design was transformed into a optimal problem with constraints of linear matrix inequalities, the robust H₂/H_∞outputfeedback controllers can be applied，then realized the optimal control forthe UPQC. The theoretically analysis and simulation results showed thismethod can get preferable control effect, fast dynamic response and goodrobustness.According to the poor control effect for the constraint conditions ofthe input-output and status with the usual control methods. A UPQCcontrol method based on variable constraint predictive theory wasproposed. The convex combination coefficients for the state feedbackmatrix were obtained through on-line optimization, and obtained thecurrent control, the corresponding control constraints were determinedaccording to the elliptical stability domain, this method could increase thefreedom of online solving effectively and reduce computing time. Thesimulation and experiment results showed that the method was feasibilityand effectiveness.In order to analysis the control effect about these control methods inthis paper, a simulation model about the mocrogrid with the UPQC wasestablished, and simulated and compared the control effect about thepower quality problems as the voltage fluctuation and harmonic pollution.The simulation results showed that the control method based on variableconstraint predictive theory has better control effect.