Study On The Control Strategy Of The Grid-connected Converter In The Non-ideal Grid

In distributed generation systems, the grid impedance generally can not beignored. In this case, the traditional synchronous reference frame phase-locked loop(SRF-PLL) and vector control of three-phase grid-connected converters will result insevere power fluctuations and harmonic currents. It will also lead to fluctuations ofthe DC bus voltage, contaminate power grid further and undermine the stability of thegrid.In the weak power grid, the impedance can not be ignored. So in this paper, themethod of grid synchronization based on inherently frequency-adaptive virtualflux(VF) estimation is applied. If the grid impedance is known, the synchronizationmethod can directly estimate the virtual flux of the virtual PCC grid point. And themethod overcomes the disadvantage of traditional virtual flux estimation, lacking offrequency-adaptive characteristics. Even when the grid voltage is unbalanced anddistorted, the method can also achieve grid synchronization quickly and accurately.In this paper, the mathematical models of the three-phase grid-connectedconverter have been analyzed. Based on virtual flux Instantaneous Power Theory,control strategies based on virtual flux under the unbalanced grid has been proposed.The grid synchronization is achieved by a method for virtual flux estimation withinherent sequence separation in the stationary reference frame. The estimated positiveand negative sequence(PNS) virtual flux components are used as basis for calculatingcurrent references corresponding to the following objectives under unbalancedconditions:1) symmetrical currents at grid side,2) elimination of double-frequencyactive power oscillations, and3) elimination of double-frequency reactive poweroscillations.Because traditional PI controller cannot track sinusoidal command currentwithout steady-error, so in this paper two PR regulators are used to control thepositive and negative sequence command current in two-phase stationary reference frame. It not only greatly simplifies the control of three-phasegrid-connected converter, but also enhances the robustness of the system. Furthermore,the resonance frequency of the PR regulator is adjusted by the grid frequency trackedby the frequency-locked loop(FLL). Therefore, the control system has thefrequency-adaptive characteristics.Finally, the simulation models of grid synchronization based VF estimation and control strategies of grid-connected converter under the unbalanced grid have beenestablished. And the simulation results of static and dynamic performance aredescribed in detail. The simulation results and experimental results both agree withthe theoretical analysis.They all verify the correctness of the proposed controlstrategies.