The Study On Key Techinques Of Three-phase Grid-connected Converter With LCL Filter

Three-phase grid-connected converter with LCL filter has widespreadapplications in active power filter(APF), unified power flow control(UPFC),superconducting magnetic energy storage(SMES), high voltage direct currenttransmission(HVDC), power transmission as well as solar, wind and other renewableenergy power generation. Meanwhile, it is also used as a main connected device in thefuture smart gird technology including energy storage, flexible transmission system,renewable energy power generation and controllable load. In this dissertation, keytechniques in three-phase grid-connected converter with LCL filter are mainlydiscussed.In order to meet the demand of the medium-voltage high-power grid-connectedapplications, firstly a space-vector modulation algorithm based on the decompositionof the reference voltage for the three-level converter and its expansion for multi-levelmodulation is introduced in this dissertation. Comparing to the traditional multi-levelcarrier modulation algorithm, the equivalent relationship between them is proved.Then, as for the neutral-point potential fluctuations of the three-level NPC converter, anew neutral-point potential control algorithm based on zero-sequence voltage which iscalculated by choosing injectable neutral-point current is proposed. The problem, thecoupling between zero-sequence voltage and the reference voltage at the side ofconverter in the traditional neutral-point potential control model, is solved and theneutral-point potential control algorithm is optimized.Since various components’ parameters of LCL filter interact with each other, it iscomplex to design. In this dissertation, by means of the result of pulse-width voltageharmonic analysis of two-level converter based on double Fourier series, the influenceof parameters variation on filtering effect in the independent/grid-connected andsingle grid-connected mode is analyzed, the optimization design of LCL filter basedon harmonic analysis is proposed and the quantization of filtering performance isachieved.The grid-voltage sensorless control of converter is achieved with virtual-fluxoriented control. In this dissertation, a virtual-flux observer based on the stackedprogrammable low-pass filter is adopted and genetic algorithm is used to identify gridparameters before the start of the converter. Accurate observation on virtual grid fluxand its initial value is achieved. To suppress resonance, a new active damping control of feeding back the current at the side of the converter based on a first-order high-passfilter is proposed, which eliminate the need for sensors in the filtering capacitorbranches. By means of discrete root-locus stability analysis of the current loop onthree cases, i.e. no damping control, the active damping control of directly feedingback the current at the side of the converter and the new active damping control, thesuperiority of the new active damping is identified. Combining these two strategies,ultimately, the number of sensors is reduced and system stability is enhanced.Under non-ideal grid, the traditional control strategy is usually hard to obtain theoptimal control effect. The dissertation focuses on the grid voltage unbalance controland how to restrain the fluctuation of DC Bus voltage. Firstly their analysis of controlprinciple is respectively introduced. Towards the control technology under unbalancedgrid voltage conditions, positive and negative sequence current separate controlstrategy is utilized. In addition, this dissertation presents a simple current limitingamplitude method. Meanwhile, considering the complexity of the tradition controlstrategy, a new unbalanced control method basing on the virtual admittance and PRcontroller is presented, in which realized the DC voltage stability and the sinusoidalcurrent control under the static coordinate system, and simplified the unbalancedcontrol ultimately. To the fluctuation of DC Bus voltage, traditional feed-forwardcontrol strategy restricted by current regulator parameters has the relatively slowerresponse and the larger DC Bus voltage fluctuation range. So it is proposed a two-stepfeedforward control strategy. Furthermore, with regard to the three control strategies,which is no feedforward control strategy, traditional feedforward control strategy andthe two-step feedforward control strategy, small signal models and analysis arecomparatively studied in this dissertation. Eventually it proves the two-stepfeedforward control strategy could elevate the response speed of outer voltage loopand reduce the fluctuation of Bus voltage without decreasing the control performanceof the inner current loop.Finally, in order to verify the correctness of the analysis and the effectiveness ofthe control methods, the simulation and experimental results are presented at the endof each chapter.