Electrolytic capacitors as a decoupling reservoir restrict the lifetime of photovoltaic (PV) microinverters. This has led to the development of several improved decoupling circuits that can reduce the capacitor value to allow the use of nonelectrolytic types. In this paper, the minimum decoupling capacitor value for the proper operation of discontinuous conduction mode flyback PV microinverters is analyzed by taking into account the total harmonic distortion (THD) and PV power utilization ratio. The results presented show that the decoupling capacitor value influences the THD more than PV power utilization. A decoupling capacitor selection method for single-stage and two-stage flyback inverters is proposed. Experimental results obtained on an 80-W test bench are presented.Index Terms-DC-AC power conversion, harmonic analysis, solar power generation.

Flyback-type micro-inverter is an attractive solution for photovoltaic ac-module applications because of several advantages; such as simple current control, working without ac current sensor and potentially low cost. However, it suffers from the dc-link voltage ripple that causes it to merge with the third and fifth harmonics on the output current. For this reason, a very smooth dc-link voltage is essential in order to keep total harmonic distortion (THD) at a low level. However, the voltage ripple is inevitable in practise for single-phase inverters because of pulsating instantaneous power. This study presents a new volt-second-based current control method for flyback micro-inverters which improves THD caused by the voltage ripple. The proposed method works effectively up to approximately 20% of the dc-link voltage ripple. In addition, the volt-second-based control method is able to allow reduction of the power decoupling capacitor value. Experimental results validating the proposed method are also presented.

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The solid state transformer (SST) was announced as the one of the most emerging technology by the Massachusetts institute of technology (MIT). An SST generally includes a DC or AC buses in high and/or low voltage side in order to connect the external input sources to the SST. We proposed discontinues conduction mode (DCM) modified dual active bridge inverter topology for integrating photovoltaic (PV) panels to SST. The proposed inverter achieves soft switching, hence it has high efficiency. Moreover, it can be operated by open loop control with eliminating high frequency current sensor. In this paper, the analysis of the proposed inverter is presented, and simulation results are given for validating the proposed inverter.Index Terms-DCM dual active bridge converter, grid connected photovoltaic inverter, solid state transformer