A Grid-Connected Single-Phase PV Microinverter With Very Low DC Bus Capacitance, Low THD, and Improved Transient Response

Nowadays microinverter with power levels ranging from 150 to 300 W, has become the trend for some grid-connected PV systems specially for residential PV systems. Also, in marine industries, photovoltaic microinverters can be used to produce a part of the electrical energy required on ships. In single phase microinverters there is a double-frequency ripple on the DC bus voltage that causes harmonics and reduces power factor. In order to reduce this ripple, large electrolytic capacitors are often used at the DC bus that the drawback is short lifetime of these capacitors. To overcome this problem, it is necessary to replace electrolytic capacitors with thin film capacitors that have long life and low capacitance. By decreasing the capacitance the amplitude of double frequency ripple on the bus voltage increases so the common bus voltage controller (PI) can’t solve the tradeoff between the output current THD and bus voltage fluctuations (overshoot and undershoot). In this paper, by using the proposed control methods a microinverter is designed and simulated so that this tradeoff has been solved without using additional hardware. This microinverter is connected to the grid (220V, 50Hz). It has a very small 20µF bus capacitor. The bus voltage has negligible overshoot and undershoot in response to a 200W input power step. THD of the injected current to the grid is 0.44% at 250W output power. The effectiveness of the proposed method is validated by simulation results in Matlab-Simulink software.