Reserch On Novel Y-Source Photovoltaic Inverter With High Step-Up Voltage

The proportion of the solar energy in the comtemporay energy structure is higher and higher,while the photovoltaic(PV)system,as the most important way in the solar energy application area,develops fast in recent years.In PV system,the inveter is a critical device,which is gaining credibility of more and more scholars.Y-source impedence network inverter studied in this paper is a single-stage system,which can realize boost and inversion at the same time.It has some advantages such as high voltage gain,simple structure,little output waveform distortion and high reliability.This paper firstly studied the drawbacks of traditional Y-source inverter,such as discontinuous input current,high startup current,effects introduced by the leakage inductances of coupling inductor on voltage gain and dc bus voltage spike.In terms of above issues,the improved Y-source inverter was proposed.This topogoly built a new input current branch,which overcame the drawback of the traditional Y-source inverter that the input current is discontinuous during the shoot-throug state.Meanwhile,the added input inductor could significantly reduce the inrush current at startup.Moreover,this paper anylzed the voltage and current stresses in details as well as the suppression of leakage effects in the improved Y-source inverter.The dc bus voltage gain and voltage spike of two topologies were compared in depth,which further illustrated the superiority of the new topology.To analyze the influences of parameters in the improved Y-source inverter on the system stability,the small-signal model based on state-space averaging method was established,and the placements of zeros and poles of the transfer functions were analyzed in details.In the traditional control strategy of improved Y-source inverter,three PI controllers are adopted to realize the control of MPPT,outer voltage loop and inner current loop,respectively.However,the parameter setting of PI controller is complex,the robustness is poor and the adaptive adjustment canont be achieved.To overcome above issues,this paper proposed adopting port-controlled hamiltonian(PCH)model and energy-shaping method to realize the grid-connected control of the system.This method established the accurate PCH model of the PV system,then an energy-shaping controller with multi-input and multi-output characteristic was designed.The boost and inversion was integrated in one controller,and the control parameters of the inverter could be calculated directively.Meanwhile,to compensate the loss of the shoot-through duty ratio,a self-adaptive method method based on energy-shaping control method was proposed,which could compensate the lost dc bus voltage.Commonly,electrolytic capacitors were connected to the dc bus of the inverter,which using the voltage fluctuation of the capacitors to absorb the double frequency power ripple fron the ac side.To ensure the quality of the output waveforms,the fluctuation of the dc bus voltage was limited,thus a high capacitance of electrolytic capacitors was needed,which further increased the volume and cost of the inverter.Meanwhile,the lifetime of electrolytic capacitors was short,which inevitablely influenced the whole lifetime and reliability of the system.Thus,this paper proposed a new power decoupling topology based on the improved Y-source inverter.This topology added no addtionanl switches but only two capacitors on the ac side.The input current of the inverter was superimposed on the original voltage and current double loop control algorithm after passing through the decoupling controller,then the two capacitors were controlled to absorb the double frequency power ripple from the ac side.Meanwhile,in terms of off-grid mode and grid-connected mode,the responding operation methods as well as the control methods were analzed,which can make sure the constan load voltage during the off-grid mode and the unity power facter during the grid-connected mode.Due to light weight,small size,low cost and high efficiency,the transformerless inverter had received more and more attention.However,the common mode current problem of transformerless inverter must be solved.Thus,this paper analyzed the generation mechanism of the common mode current in improved Y-source inverter,and proposed a novel tranformerless Y-source inverter topology,combined with PWM modulation stragtegy and the existing common mode current suppressiong methods.This topology connected the negative terminal of PV array to the neutral line of the grid directly,and bypassed the loop of the common mode current,while the virtual dc bus was adopted to ensure the quality of the grid current.In terms of the unbanlance of modulation stragegy during the positive and negative half cycle,the voltage stress and current stress during the negative half cycle were analyzed in depth.Meanwhile,in order to reduce the charging current of the virtual dc bus capacitance,the charge and discharge expression of virtual dc bus capacitance during the negative half-period was derived,and the capacitance value in the new topology was optimized based on the deriation.The prototype based on TMS320F2812 was built after the above research,and the proposed improved Y-source topology as well as the control stretegy,power decouling method and common mode current suppression method was realized on this platform.The experimental results verified the validity and feasibility of the proposed topologies and methods.