Research On Topology And Control Strategy For Single Phase Two-stage Grid-Connected PV Inverter

Due to its full photovoltaic voltage utilization,efficient topology and flexible configuration,the single-phase two-stage photovoltaic(PV)grid-connected inverter has been widely applied in distributed photovoltaic power generation systems.For the purposes of boosting input voltage and keeping dc-link voltage constant,conventional two-stage grid-connected inverter topology uses the boost converter as pre-stage.However,input voltage suffered from an additional power conversion processes during rising first and then falling.As a result,the theoretical efficiency of this topology has not been maximized.This dissertation chooses single-phase two-stage grid-connected inverter system as its research target.The two-stage inverter topology has been improved from structure optimization and efficiency promotion.The current control strategies are respectively researched to the proposed topology based on small-signal linearized model,nonlinear model with direct current controller and nonlinear switching state model.Firstly,through proper device selection,bridge arm structure evolution,shortening current loop path and combining the features of existing two-stage inverter topologys,a high-efficiency two-stage inverter topology with boost and buck mode switching is proposed to achieve maximum efficiency which conventional two-stage inverter topology with constant dc-link voltage can't obtain.The parameter design of the key components for the proposed topology is presented.Furthermore,theoretical analysis and comparison of the losses for the power conductors and energy storage devices are performed between the conventional topology with constant dc-link voltage and the improved topology with boost and buck mode switching.Loesses calculated results show that the proposed topology improves the system efficiency by nearly 1.8%,and moreover,the higher the switching frequency,the higher efficiency improvements of the proposed topology will be observed.Experimental comparison results from engineering test prototypes prove the feasibility and superiority in overall efficiency of the improved topology.Then,a small-signal linearized model of the improved two-stage boost-buck grid-connected inverter topology is built to investigate the applicable current control strategy.According to the amplitude-phase characteristics of the small-signal transfer function between the current of output filter inductor and the duty-ratio of boost converter,the conventional single-target direct current control strategy is difficult to achieve acceptable control performance.Thus,the hybrid current control strategy with dual independent carriers is proposed by introducing a secondary current target using the boost inductor current.Moreover,a dual carrier-overlapping PWM modulation method is proposed to compensate the waveform distortion caused by the energy oscillation during switches conducting of the boost-buck high-frequency with independent carrier modulation.The experimental results verify the feasibility of the proposed hybrid current control strategy.And the designed carrier-overlaping factor effectively eliminates current distortions of the boost-buck with high-frequency switching modes at the expense of smaller switching losses.Furthermore,from the numerical calculation of the traditional direct current control nonlinear simulation solution,the slow-scale and fast-scale bifurcation diagrams with the PV input voltage show that the instability of the boost converter is essentially a non-linear chaos phenomenon.To suppress the irregular waveform in the chaotic current,a resonant sinusoidal disturbance signal with certain amplitude and the optimal phase is added to the reference current.Thus,the weaker periodic components of the chaotic signal compensate the stronger periodic components,and as a result,the system returns to stable condition.On the other hand,two chaotic control strategies for state-associated optimization with coupled coefficient are proposed to force the chaotic region of the original state variable converging to an orderly stable region,thus,the chaos phenomenon of the nonlinear system is suppressed.The effectiveness of the proposed chaotic control scheme is verified through simulation experiment under the specific input parameters.Finally,a nonlinear model of the switches state for the improved topology with finite control set is proposed to optimize dynamic performance and deal with the issues caused by synchronization control of the boost and buck high-frequency switches.A discredited current prediction model is first derived,and then,an evaluation function is established in order to optimize the dc-link voltage and the output filter inductor current.The ultimate simulation analysis and experimental results demonstrate the effectiveness and applicability of the model predictive current control strategy based on the finite control set on the application of the two-stage grid-connected inverter with synchronous boost and buck switching.