Efficiency Enhancement For Two-stage Transformerless Photovoltaic Grid-tied Inverters

In recent years,with the issues of environmental pollution and fossil fuel shortage,the development and utilization of renewable clean energy has been paid more and more attention.Among them,the installed capacity of photovoltaic(PV)grid-tied generation systems is growing rapidly.Compared with isolated grid-tied inverters,non-isolated grid-tied inverters feature smaller size,lower weight,and higher efficiency.Therefore,the non-isolated inverters are widely used in distrbutied PV grid-tied generation systems.As the output voltage of PV panels is usually lower than the peak value of grid voltage,two-stage topology structure is usually used.Thus,the output power of PV panels should be fed into the utility grid with two-stage power conversion.However,the PV output power can be single-stage tranferred,when the PV panel output voltage is higher than the instantaneous value of grid voltage.As a result,the efficiency of the grid-tied inverter can be enhanced.Therefore,the main work of this paper is to study two semi-two-stage non-isolated grid-tied inverters with low leakage current and high efficiency.Firtstly,a semi-two-stage H5-based non-isolated PV grid-tied inverter is researched.Based on the conventional two-stage H5 non-isolated PV grid-tied inverter,a new power processing path that consist of a switch and a diode connected in series is introduced.As a result,the PV output power can be single-stage tranferred,when the PV panel output voltage is higher than the instantaneous value of grid voltage.However,the PV output power is still two-stage transferred,when the PV panel output voltage is lower than the instantaneous value of grid voltage.Since the H5 topology is used,the freewheeling path is disconnected with the PV panel in the freewheeling modes.Therefore,the leakage current is suppressed,and the efficiency is enhanced.A semi-two-stage dual-buck type non-isolated PV grid-tied inverter is studied as well.Based on the conventional two-stage dual-buck non-isolated PV grid-tied inverter,two new power processing paths that consist of a switch and a diode connected in series are introduced.As a result,the PV output power can be single-stage tranferred,when the PV panel output voltage is higher than the instantaneous value of grid voltage.Further,the PV output power can be still single-stage transferred in the freewheeling modes,when the PV panel output voltage is lower than the instantaneous value of grid voltage.On the other hand,both the power switches and diodes can be optimal designed in the semi-two-stage dual-buck inverter.Therefore,compared with the semi-two-stage H5-based non-isolated PV grid-tied inverter,the efficiency of the the semi-two-stage dual-buck non-isolated PV grid-tied inverter is higher.Finally,the experimental prototypes of these two semi-two-stage non-isolated inverters are designed and built.The parameters of power-stage circuits,the design guidelines of control parameters,as well as the system structure are presented.Further,both the leakage currents characteristics and the efficiency are verified and compared with convetional two-stage inverters.Experimental results are consistent with the simulation and theoretical analysis.Therefore,the presented semi-two-stage inverters can be applied in the PV grid-tied systems.