Research On Common DC Bus Transformerless Dual-Frequency Grid-Connected Inverter

With the global demand for energy,the photovoltaic energy has become one of the most popular renewable energy due to its advantages of environmental friendliness and low maintenance cost.The grid connected inverters have received extensive attention as the interface between PV systems and the grid.The grid connected inverter without isolation transformer can saves cost,improves efficiency,reduces volume and weight compared with the grid connected with isolation transformer.However,it is necessary to pay attention to the leakage current of the transformerless grid connected inverter.The excessive leakage current can cause electromagnetic interference,additional power losses and personal safety issues.Many topologies and control methods have been proposed to suppress the leakage current.To suppress the leakage current of the transformerless grid connected inverter and improve the efficiency of the inverter,a novel transformerless common DC bus dual frequency grid connected inverter and control scheme are proposed in this paper.The topology consists with a main unit and an auxiliary unit,and the two units are connected in parallel with the grid through a common DC bus.The proposed scheme eliminates the circulating current between the two units,suppresses the leakage current more effectively and achieves a higher efficiency.Combined with the corresponding parameter design methods,a simulation model of the dual-frequency grid connected inverter with common DC bus is established in this paper.The main unit adopts the HERIC topology with the hybrid modulation strategy and the auxiliary unit adopts the H-bridge topology with the bipolar pulse width modulation.The main unit operates at a low switching frequency to reduce switching losses and the auxiliary unit eliminates the current ripple of the main unit with the feedforward compensation control strategy.The simulation results shows that the dual-frequency grid connected inverter with common DC bus eliminates the circulating current between the two units and ensures the power quality.Compared with the traditional HERIC topology and the dual-frequency inverter with independent DC bus,the proposed inverter can suppress the leakage current more effectively.Based on the parameters obtained in the parameter design section,a experiment platform of the dual-frequency grid connected inverter with common DC bus is established.Compared with the traditional HERIC topology and the dual-frequency inverter with independent DC bus,the THD value of the common mode voltage of the proposed inverter is only 5.42%,and the proposed inverter achieves better common mode performance and leakage current suppression effect.The THD value of the grid connected current of the proposed inverter is only 1.73%.Comparing the filter inductor currents,it can be seen that the proposed inverter is not affected by the circulating current,and the circulating current is effectively eliminated.The simulation and experimental results verify the effectiveness of the proposed inverter.