VSM-based Inverters within Automated Hybrid Energy Systems

Abstract

The evolving isolated energy system technologies are based on grid inverters, which play an important role in the parallel operation of renewable and conventional generation facilities. Characterized by zero inertia, these inverters lack the capacity to participate in voltage and frequency regulation. Traditionally, grid inverters operate as current sources to ensure maximum possible power generation. This function requires a reference voltage source and cannot be performed if the source is disconnected. It should be noted that modern microgrids often utilize specially designed inverters that operate as a current source when an external voltage source is available, while when the reference voltage is lost, they start forming a network, which supplies the microgrid loads with energy from renewable energy sources and existing energy storage systems. Under these conditions, the network is typically formed when such an inverter switches to the voltage-source inverter (VSI) operation. It is important to note that a conventional VSI does not possess an inherent load regulating effect, unlike motor loads or other consumers, which exhibit load sensitivity to changes in voltage or frequency. This circumstance complicates the parallel operation of VSI in the network, since the VSI inverter outputs a strictly defined frequency disregarding its possible variations when the electrical loads in the electrical network change. For this reason, VSI cannot provide a regulating response to load changes. It is also crucial to note that when VSI operates in parallel with other inverters, the “primary-secondary” operation is also impractical. Instead, distributed regulation and proportional load sharing are required. One of the most promising solutions is the implementation of virtual synchronous machine (VSM) algorithms without a phase-locked loop in parallel-operating inverters connected to renewable energy sources. The paper focuses on the development of a VSM-based inverter and considers algorithms for its implementation for off-grid and grid-parallel operation. The study also analyzes network disturbances and provides a comparative analysis of the results.