Research On The Highly Reliable Doubly Grounded Transformerless Single-Phase PV Grid-Connected Inverters

In recent years,human consumption of fossil energy has increased dramatically,and air pollution and energy crisis are imminent.New energy sources such as photovoltaics and wind power generation are highly valued due to their advantages of renewable and pollution-free.The distributed generation system of photovoltaic array is a key part of the new energy generation system,and the grid-connected inverter is an important part of the distributed power generation system.Therefore,the study of photovoltaic grid-connected inverters is of great significance to promote the development of new energy power generation and new energy business.There are advantages in the PV grid-connected inverters to use transformerless topologies in order to acquire smaller size and weight.However,the common mode leakage current exists in the transformerless PV inverters,which may cause the problems of personal safety,severe electromagnetic interference,distorted grid current,and extra loss.In order to reduce the common mode leakage current,extra power devices should be added into the grid-connected inverters and the control method should be changed.In addition,the existence of the shoot-through problem of the bridge arm restricts its application in occasions with high reliability requirements.In order to solve the above mentioned problems,highly reliable doubly grounded transformerless single-phase PV grid-connected inverter is proposed.The inverters are transformerless topologies without shoot-through problem.Dead-time in gate signals is removed,which improves waveform quality of the grid current.The application of the doubly grounded structure can eliminate the leakage current without adding additional devices.Operating principle of the proposed inverter is analyzed in detail.The operating modes of the proposed inverter are analyzed.Hysteresis current control is employed.Hysteresis current control circuit,current sampling circuit,voltage sampling circuit,digital logic circuit and conditioning circuit are designed and the filter.According to the theoretical analysis,the power device is selected.The calculation method of current reference is givenThe proposed inverter is a two-stage structure.There are two switchs operates at high frequency in each half-line period in the proposed inverter.In order to reduce the number of high-frequency switchs,an improved highly reliable doublely grounded transformerless single-phase PV grid-connected inverter is proposed.Only one switch operates at high frequency in each half-line period in the proposed inverter.Besides,the proposed inverter is a single-stage structure.The characteristics of highly reliable and doubly grounded are remained.Operating principle of the proposed inverter is analyzed in detail.The operating modes of the proposed inverter are analyzed.Hysteresis current control is employed.Hysteresis control circuit,current sampling circuit,voltage sampling circuit,digital logic circuit and conditioning circuit are designed and the LCL filter.The power devices are selected.The perturbation and observation algorithm is used for tracking the maximum power point(MPPT)of the PV array.The calculation of reference currents is given.The theoretical analyses of the proposed inverters are verified by Matlab/Simulink simulation software.Finally,350W prototypes are built and verify the theoretical analysis.