Research On The Control Technique Of High Efficiency Single Phase Grid-tied Inverter

Single phase grid-tied inverter is widely used in renewable energy systems,such as photovoltaic power generation system,photovoltaic enegy storage system and electric vehicle bidirectional charging system,and high efficiency and high power density are two important targets for the inverter.In order to achieve high efficiency,researchers have done a lot of work from four aspects,including soft switching technique,converter topology,control technique and power transistor optimization.This paper has done research on low-and-medium power single phase grid-tied inverters with power rangeing from several hundred watts to several kilowatts,in order to further inprove efficiency from aspects of control technique and converter topology.As for microinverters used in photovoltaic systems,the weighted efficiency is mainly focused,which means the efficiency from light load to heavy load should both be optimized.The conventional control methods for microinverters are simple and fixed,and can't optimize the efficiency under different load conditions.Therefore,this paper proposes a variable frequency control method based on off-time control for a single phase full-bridge grid-tied inverter operating in discontinuous conducton mode,including fixed off-time control method and varied off-time control method.According to the proposed control method,the off-time of the freewheeling switch is adjusted automatically with the instantaneous power,so that the switching frequency and the switching losses are optimized under different loads and the weighted efficiency is improved.This paper gives a complete and detailed parameter design process for the proposed control method,and expetimental results are obtained for verification.Discontinious conduction mode can adjust switching frequency and swithicng losses effectively,but the peak inductor current is large,so it is only suitable for low-power applications.In comparison,the boundary conduction mode can reduce the peak inductor current and is suitable for higher power levels.Besides that,the boundary conduction mode can realize soft switching without adding auxiliary circuits and improve the efficiency,so it receives people's attention and research.However,the single phase full-bridge grid-tied inverter operating in boundary conduction mode has high switching frequency and large switching losses in light load condition,so the light load efficiency is low and the weighted efficiency is impacted.In order to solve this problem,this paper proposes a single phase T-type grid-tied inverter operating in boundary conduction mode with corresponding control method.The circuit combines soft switching technique and multi-level technique,which features zero-voltage turn-on,low equivalent switching frequency,low switching loss and simple topology and control.As a result,the efficiency of the inverter can be improved.This paper emphasizes on some issues of the proposed circuit,including the correction of the reverse inductor current boundary,the configuration of the ePWM module of the T-type leg and the voltage balancing of the DC bus capacitors.The proposed single phase T-type grid-tied inverter can improve light and half load efficiency,while the control method based on boundary conduction mode still has some problems that would impact the output current quality and the whole circuit optimization.In order to further improve the circuit performance,this paper proposes a trapezoid wave control method.It can be used in the transition mode of the T-type inverter to decrease the ultra high switching frequency,narrow the switching frequency range and optimize the circuit design.Furthermore,it can be used in the whole line cycle to cancel the mode transitions,so that the T-type inverter can work in a uniform operation mode.As a result,the waveform quality of the output current is optimized.