A High-Efficiency Transformerless Inverter For Grid-Connected Photovoltaic Systems

As a resource-rich and widely distributed clean energy, solar energy becomes one of the maindirections of new energy developments. Presently, PV power generation is mainly used ingrid-connected application. The transformerless single-stage grid-connected inverter with unipolarSPWM modulation strategy has the highest efficiency in grid-connected PV inverters. However, theleakage current caused by non-isolation can not be ignored. Therefore, research on transformerlessinverters with high efficiency and low leakage current has a great significance and practical value.This paper proposes a new topology which is based on the foreign patent H5, named oH5. Thenew oH5topology changes the suppression means of leakage current and then the high-frequencyleakage current is eliminated in theory. Meanwhile the oH5changes the devices’ stress so that reducesthe losses of devices and improves the efficiency of the converter further. A novel modulation strategyis also presented to fulfill the requirements that the inverter could provide reactive power and at thesame time the efficiency of the converter is not affected when the converter is operated at unity powerfactor.In order to reduce the size of the inverter and improve the filter effects, the LCL filter isintroduced. According to power electrical requirements and engineering experience, the parametersdesigning criteria and detailed designing process of LCL filter are given. In order to eliminateresonance problems caused by the LCL filter while not affect the efficiency of the converter, activedamping solution has been used. Then, a four-pole configuration control strategy of the grid currenthas been proposed. The resonance form of conjugate poles of resonant frequency has a larger dampingfactor to achieve active damping, while the conjugate poles of fundamental frequency are configuredin the form of quasi-resonance in order to achieve a high open-loop gain for fundamental frequency.This configuration also achieves the no error control for fundamental frequency.By adding the high-precision PLL (phase lock loop) technique, the non-blind islanding detectionfunction as well as the efficient MPPT (Maximal power point tracking) technology, this prototype hasachieved all important functions that the grid-connected PV inverters required. The simulations andexperimental results verify that the theory analysis of the inverter topology is correct and the controltechnology is effective.