Research On Control Technology Of Z-source Inverter In Photovoltaic System

With the advantage of solar energy, the photovoltaic(PV) power technology hasbecome a hotspot in new energy field of study. Whether the PV can be promoted woulddepend on the system cost, conversion efficiency, reliability, flexibility, and so on. Thez-source inverter as a new topology, it has its special characteristics. The z-sourcenetwork is mainly consists of two inductors and two capacitors to connect PV array andinverter bridge. The DC-link voltage can be stepped up by increasing shoot-throughtime, therefore the tranditional two-stage control can be simplified as single stagestructure. The z-source inverter used for PV has active significance to reduce systemcost, enhance reliability and improve effficiency. In this dissertation, the z-sourceinverter is used as power converter, based on which, it studies the key controltechnology of efficient grid-connection and related nonlinear behavior, and the maincontents are as following:①The PV array universal model is set up according to the electrical characteristicof solar panel, the multiple maximum power point (MPP) characteristic of PV arrayunder partially shadowing condition is analyzed in detail. To overcome the limitationsof the traditional maximum power point tracking (MPPT) methods, which can easily getinto local optimum during multiple MPP searching process, the MPPT algorithm basedon dual-carrier chaotic search is proposed and the MPP searching under partiallyshadowing condition is realized effectively. In order to further improve the searchingefficiency and precision, based on this algorithm combined with space scale transformhas greatly improved the search effect.②According to the z-source inverter shoot-through state and non shoot-throughstate of the equivalent circuit, the state space model of z-source DC-link circuit isestabilished, and the closed-loop control is adopted to improve system tracking abilityand the anti-interference ability. Based on the analysis of the relation between DC-linkvoltage and z-source capacitor voltage, proceeding from the nonlinearity of system, theDC-link affine model of z-source inverter is established. This model can be linearizedby accurate linearization method based on theory of differential geometry. Thelinearization process is different from tradition, whose accuracy is limited to smalldisturbance around balance of model. With the optimization control, the system canachieve high quality output. In order to further promotion, asymmetric z-source network is futher analysed. The zero dynamic design is used for control, thereby the controlsection is simplified but not degrade the control performance.③The z-source PV grid-connected control technology of AC side is studied. Thehysteresis current control (HCC) is selected by comparing the response speed andprecision of different control methods. In order to overcome the HCC problem ofinconstant frequency which can cause difficult to design filter, constant frequency HCCstrategy is analysed and designed. The fuzzy control techonlogy is adopted to realizeconstant frequency control because it has advantages of easy design, excellentrobustness and lower request to model. Depending on the error change range of actualinverter output, the corresponding membership functions and control rule are designedand the AC side filter inductance can be designed in accordance with the principle of noslack tracking between output current and reference current. Based on the constantfrequency, the shoot-through vector can be inserted into HCC by logical analysis.④For existing nonlinear phenomena of z-source inverter DC side is observed insimulation, the limitation of tranditional small signal modeling which omit higher ordersection, the nonlinear essence of z-source inverter belongs to feedback type inverter hasbeen verified. The DC side and AC side nonlinearity of z-source inverter are discussedaccording to state equation, based on which the stroboscopic mapping discrete model isbuilt. The Jacobian matrix method and Lyapunov index analysis are used to analyse thebifurcation accurately. The input voltageVi nand grid-connected reference currentirefare selected as the variation for analysing the stable region of z-source inductance,z-source capacitance, input voltage and reference current of DC side; three factors, DCoutput voltage, feedback factor and output filter inductance are selected as the variation,take any two in three factors for analysing stable region of AC side. It is helpful toforecast converter working performance in unstable state, especially chaotic state andprovide idea for z-source PV system design, thereby improve the system reliability.