Discontinuous Modulation Technique for Isolated Three-Phase Grid Connected Flyback Inverter With Selective Harmonic Compensation for PV and Fuel Cell Systems

Abstract

Single-stage inverters, such as the proposed differentially structured flyback inverter (DSFI), offer many features, including reduced component count, voltage step-up/step-down capability, minimized cost and footprint, as well as high power density operation. Differential-type inverters comprise three dc-dc converters that are linked in parallel at the input side and differentially from the grid side. These structures are commonly modulated using continuous modulation schemes (CMSs) for simpler operation and low-order harmonic suppression. In the CMS, the three dc-dc modules are activated for the whole operating period, which increases the circulating power between the flyback modules and adversely affects the inverter efficiency. This article, therefore, applies the discontinuous modulation scheme (DMS) scheme for single-stage, three-phase DSFI and provides a comprehensive evaluation of the three-phase DSFI under CMS and DMS schemes. In addition, a simple single-pole selective harmonic compensation (SHC) loop is used for second-order harmonic compensation. The CMS reduces the low-order harmonic components in the grid currents; however, the voltage stress across the inverter’s different components is increased with increased circulating power. On the other hand, the proposed DMS reduces the voltage stress across the different components and the circulating power between the inverter’s modules, resulting in, as a consequence, enhanced system efficiency. The proposed DSFI system is experimentally validated under CMS and DMS based on a 1.6 kW prototype in grid connected mode.