Modeling of a Virtual Synchronous Machine-based Grid-interface Converter for Renewable Energy Systems Integration
To date, the common, and with some exceptions the only, method to interconnect high power renewable energy sources and energy storage systems to the grid has been the use of power electronics converters that operate as current sources to the grid for the purpose of achieving maximum primary-source power tracking. When the grid is not available, such sources would not be allowed to continue operating and would be shut down after anti-islanded algorithms recognized the loss of the grid. Although existing standards and requirements still limit grid-interface converters to regulating voltage in the grid, the functionality discussed in this paper will inevitably be part of the power system's operation in the future. This paper addresses physical and mathematical equivalency between power electronics converters and synchronous generators, and emphasizes how an inherent synchronization feature of the synchronous generators can be used to improve the performance of the grid-interface converters. It has also been shown that if operated as a voltage source, the grid-interface converter could have significant stabilizing effects on the system dynamics due to non-delayed power delivery.
Fast, digitally controlled converters offer endless possibilities for the most optimal utilization of renewable resources. Moreover, power electronics-based DG can enhance power system controllability due to their fast dynamic response to the power system disturbances and deviations of the voltage and frequency. However, the grid-interface converter's phase-locked loops can cause frequency instability in cases where grid output impedance becomes very high, or when there are multiple grid-interface converters connected to the same or an adjacent bus with a "weak" link to the strong grid. On the other hand, thousands of synchronous generators can work in parallel, and share the power with much "milder" interactions in the sense that they synchronize with the grid and to each other. This paper addresses that issue by showing what features of the synchronous generators' operation offer dynamic advantages that could be useful in the future for control of the next generation of grid-interface converters. This paper additionally shows how grid-interface converter with an adaptive virtual inertia has an ability to stabilize the system after large disturbance.
One of the main contributions of this paper is the way how frequency-locked loop of the grid interface converters is realized (duty-cycle angle obtained by integrating dc-link voltage). Although obvious choice for the grid-interface converters that behave as synchronous machines, this synchronization concept could open up a research space for reinvestigating the need for the conventional phase-locked loops (PLL) in the grid-interface converters in general.