Analysis of component current stresses of grid-forming converters for two-phase three-wire isolated AC power grids

Abstract

Isolated ac grids are feasible solutions to energize remote communities usually based on renewables and batteries, interfaced by power electronics converters that require high reliability. The key element of such a system is the grid-forming converter witch provides voltage magnitude and frequency. This paper compares the current stresses of grid-forming converters under an isolated grid mission profile, considering three different grid configurations in terms of line voltage displacement angles: 90° (αβn-system); 120° (abn-system) and 180° (xyn-system). Besides, analytical expressions for the current stress through the semiconductors and the dc-link capacitor are proposed for each system. The lifetime analysis of the grid-forming converter indicates that the xyn-system has the lowest wear-out. Furthermore, this work highlights the influence of high-frequency components on the rms dc-link capacitor current. The xyn-system demonstrates a lower probability of capacitor failure compared to the other system, even presenting higher low-frequency. components. Finally, a sensitivity analysis shows that the grid-forming converter in xyn-system can increase its output power in 12% or reduce its heatsink volume in 25% to achieve the same wear-out B10 lifetime as the abn-system (i.e., benchmark) under nominal conditions. An experiment is conducted using a full-bridge converter to measure the power losses in the semiconductors and the case temperature of the IGBTs. The xyn converter shows 44% lower power losses, and the abn converter shows 18% lower power losses, both compared with the αβn converter.