Research Of Leakage Current Restraint On On-grid Transformerless Photovoltaic Inverters

Under the double pressure of the energy crisis and environmental pollution, new energy industry, which is represented by photovoltaic industry, has become one of focuses of attention and research issues of people. As a core device of one photovoltaic power system, research on grid-connected photovoltaic inverter doesn’t only focus on optimization of its output power, but also on optimization of efficiency. Transformerless grid-connected PV inverters don’t need any transformers, and they can make photovoltaic panels linked to power grid through themselves and without transformer isolation, which brings greatly decrease of system volume, reduce costs, improve efficiency. So it has great value of study.Due to the parasitic capacitance to the ground of photovoltaic panels, under the circumstance of no transformers, electrical isolation between power grid and photovoltaic system is lost, and they form a common-mode loop with earth. High frequency oscillating of the common mode voltage brings leakage current, which can not only cause current distortion, but also result in equipment damage, personal injury. Therefore, its amplitude must be strictly detected and controlled. Therefore, restraint of leakage current is one of the key problems of transformerless grid-connected PV inverter. Modulation-based methods need not any additional components and any changing of circuit configuration and controller designing, which are pursued by research staff at home and abroad.This thesis studies on modulation-based methods of leakage current restraint of two-level and three-level transformerless grid-connected PV inverter. In order to analyze the principle of leakage current restraint better, small-signal model of leakage current is built, and the reason why leakage current generates is analyzed from the aspects of its source, impedance of the loop and traditional modulations, and influence of each electric and parasitism parameters to leakage current is declared.With regard to leakage current of two-level inverters, this paper studies on two typical kinds of modulation algorithms, principle of leakage restraint of AZSPWM and RSPWM and their realization. Based on RSPWM, an algorithm of dead-time compensation is proposed, which make it reality that RSPWM can be applied under the drivers with dead-time. In order to measure the performance of AZSPWM and RSPWM, depth analysis upon usage of DC-link voltage and harmonics of output voltage, and comparison with traditional SVPWM is conducted in order to offer academic reference to engineering application of AZSPWM and RSPWM.Regarding leakage current of two-level inverters, this paper studies three medium vector modulation algorithms, 2MV1 Z, 3MV120 and 3MV and their realization. Medium vector modulation algorithms can eliminate leakage current theoretically, but also use DC-link voltage in a large range, so they has great value of research. Modulation ratio, harmonics of output voltage and ripple of input current of medium vector modulation algorithms is conducted, and loss of power devices is simulated, in order to offer theoretical reference to engineering application of 2MV1 Z, 3MV120 and 3MV.This paper provides simulation upon all the theory.