Research On Current Control And Conducted EMI Suppression Of High Efficiency Photovoltaic Micro-Inverter

With the energy crisis and environmental pollution becoming more and more serious,clean and pollution-free renewable energy generation has been paid more and more attention.As an important part of the new energy photovoltaic power generation applications,the micro-inverter not only needs to improve the work efficiency by using soft switching technology,but also needs to improve the quality of grid-connected current and electromagnetic compatibility.This paper focuses on the grid-connected current control and conducted EMI suppression of two-stage micro inverter based on soft switching control.In this paper,the operation principle of active-clamp forward-flyback converter in the critical continuous mode and the discontinuous mode is analyzed in detail.Based on the principle,the implementation of forward-flyback converter operating in critical continuous mode is described.To overcome the drawbacks of the zero crossing detection circuit and hardware delay circuit,a new improved zero crossing detection and a driving delay method are proposed respectively.For the second stage – full-bridge inverter,the unipolar modulation method is used and the principle of the inverter operating in boundary current mode is analyzed.The output currents are distorted near the zero crossing of the grid voltage since the varied switching frequency goes through the resonant frequency of the LCL output filter.To improve the current performance,a gradient of the switched inductor current envelope method is proposed in this paper.To implement the boundary current control of the full-bridge inverter,a new hybrid control method combining the hardware reset control and software predictive control is proposed.In this paper,the influence of the delay caused by the non-ideal factors such as the control circuit and the driving circuit,and the dead time of the soft switching process on the inductor current and the output current waveform are introduced.To improve the quality of the grid current waveform,an accurate delay time and dead time compensation method is proposed by analyzing each part of the delay composition,dead time effect of hardware reset control and software predictive control in the switching process.The drain-to-source voltage of the power switch and the cathode-to-anode voltage of the power diode in two-stage micro inverter are regarded as the interference voltage sources which introduce conducted EMI.The conducted EMI propagation paths of the forward-flyback and full-bridge are analyzed respectively.Then the equivalent circuits of the common mode and differential mode noises are derived,and the transfer voltage gain of the common mode and differential mode noises are given.Finally,the EMI conduction model of micro inverter is established.According to the modeling method of the EMI conduction,the suppression effect of various conduction EMI suppression methods is analyzed.In this paper,a simulation model is built in PSIM.The simulation results validate the improved achievement of critical conduction mode forward-flyback converter,and the feasibility of unipolar full bridge inverter delay time and dead time compensation.To experimentally verify the proposed control method,a photovoltaic inverter rated at 300 W is built.The experimental results prove the feasibility of the delay time and dead time compensation in the hybrid control and the output current shows excellent performance with compensation.The multiple conduction EMI suppression methods are verified,and the conducted EMI of the micro inverter can meet the requirements of the standard test.