Research On The Dual-buck Photovoltaic Grid-connected Inverter And Its Nonlinear For Sliding Mode Controller

The existing dual-buck photovoltaic grid-connected inverter(DPGCI)topologies do not isolate the DC voltage from the grid during the freewheeling time,and the high-frequency variation of common-mode voltage results in large leakage current,which is difficult to meet the standard of VDE-0126-1-1.To connect with power grid,existing DPGCIs need to install the isolation transformer or high-voltage AC capacitor with high price and large volume.Aiming at this problem,the transformerless DPGCIs with low cost and small size are proposed based on the analyses of the topologies and common-mode leakage current of existing DPGCIs.Meanwhile,the common-mode leakage current of the proposed transformerless DPGCIs are analyzed.Sliding mode controller(SMC)is recognized as a robust controller with fast dynamic response,therefore it is widely used in the control of inverter.The sliding mode controlled inverter is in nature a nonlinear controlled time-varying nonlinear system,and it has complicated nonlinear behaviors in practice.To study the non-linear behavior and non-linear analysis method of SMC inverter,the sliding mode controlled transformerless DPGCI with single-filter inductor installing is taken as an example in this paper.The main research contents include:(1)Because the large high-frequency common-mode leakage current exists in the conventional DPGCI with symmetric-inductor installing which is modulated by unipolar SPWM strategy,a transformerless DPGCI with symmetric-inductor installing topology is presented.Then,the unipolar SPWM strategy,operating modes and common-mode leakage current of the transformerless DPGCI with symmetric-inductor installing are analyzed.(2)Double frequency-based DPGCI is well known as its equivalent higher frequency and lower current ripples,but the large leakage current exists in the line-frequency cycle.To overcome this imperfection,a new transformerless double frequency-based DPGCI topology is proposed.Through the comparative research of conventional double frequency-based DPGCI and the transformerless double frequency-based DPGCI,it is verified that the high-frequency common-mode leakage current of the transformerless double frequency-based DPGCI can be suppressed effectively.(3)DPGCI with single-filter inductor installing has the advantages of fewer filter inductors and smaller volume,but the large leakage current exists in the positive half cycle.To overcome this imperfection,a new transformerless DPGCI with single-filter inductor installing topology is proposed.Researches show that the high-frequency common-mode leakage current of the transformerless DPGCI with single-filter inductor installing can be suppressed effectively.Further,the effect of switch's collector-emitter capacitance on the common-mode leakage current is explored in detail when the transformerless DPGCI with single-filter inductor installing converts from non-decoupling states to decoupling states.The experimental results show that the influence of collector-emitter capacitance can be eliminated by the way of matching switch's collector-emitter capacitance parameters,and this scheme is valid for other topologies,such as,the transformerless DPGCI with symmetric-inductor installing and transformerless double frequency-based DPGCI non-isolated grid-connected inverter.(4)To suppress the leakage current of DPGCI,and increase power level,the output waveform quality for transformerless PV systems,a cascaded transformerless DPGCI topology is firstly presented,along with a new unipolar Dual-carrier Sinusoidal Pulse Width Modulation(DSPWM)method,which can ensure that the high-frequency common-mode voltage is constant.Then,the high-frequency common-mode loop models of the cascaded transformerless DPGCI are established.Based on the models and unipolar DSPWM method,operating modes and the characteristics of common-mode leakage currents of the cascaded transformerless DPGCI are discussed,respectively.Meanwhile,the influence of grid voltage on the leakage current is analyzed,and the proposed cascaded transformerless DPGCI can be extended to n-stage inverter.(5)To analyse the nonlinear behaviour of the sliding mode controlled transformerless DPGCI with single-filter inductor installing(SMCTDPGCI-SII)system,the discrete model of the SMCTDPGCI-SII system is firstly established based on the stroboscopic map method.With the case of folded diagram,a novel nonlinear behavior(state-mutation phenomenon)is observed that the state variable changes from the period-1 state to the period-2 state at a certain switching cycle.Then the root cause of state-mutation phenomenon is explored.Meanwhile,regarding the variation of state-mutation switching cycle with the parameters of SMC,a novel criterion is proposed to determine state-mutation switching cycle of the state variable.And then a new bifurcation-diagram-drawing method of variable sampling point is proposed.The bifurcation diagram drawn by this method can not only accurately determine the stability domain of sliding mode control parameters,but also be used to analyze the bifurcation instability of the SMCTDPGCI-SII system.