DC-Link Dynamics Examination of the Parallel-Connected Three-Phase Inverters in Islanded and Grid-Connected Modes

This paper presents a comprehensive analysis of an AC microgrid setup, consisting of parallel three-phase inverters. It introduces a novel controller that integrates the tasks of controlling the DC-link voltage and implementing a current limiting approach. The design is informed by the characteristics of lead compensators and its main objectives are twofold: i) to regulate the DC-link voltage in order to minimize unwanted power exchange among the parallel inverters, and ii) to ensure the current-limiting capabilities of all inverters, regardless of the microgrid's operational state. This includes both standalone and grid-connected modes, voltage sags in the grid side, as well as transitional intervals between the two. In contrast to existing DC-link voltage control methods that do not incorporate a current limiting strategy, the proposed solution effectively restrains the dq axis current values in all operating conditions. This is crucial for preserving system stability, avoiding abrupt changes in DC-link voltage and current during sudden grid modifications and transitions. To validate the efficacy of the proposed controller, a series of simulations were carried out using Matlab/Simulink to evaluate its performance and compare it with an established method. The results explicitly demonstrate that the proposed controller successfully stabilizes the system, minimizes fluctuations in DC-link voltage, and restricts the dq axis current for each inverter without the potential of protection relay tripping.