Modeling and Initialization of a Virtual Synchronous Machine for Power System Fundamental Frequency Simulations

The share of renewable energy sources interfaced to the bulk power system through power electronic devices is continuously increasing. As a result, the power systems of the future face challenges of stability and reduced inertia and, to maintain the stability of the low-inertia systems, various concepts of grid-forming converter control schemes have been introduced, which enable converters to behave similarly to traditional synchronous machines. As the research in this area is gaining in importance, the sheer amount of papers with models for different kinds of simulations and the complexity of control algorithms may be daunting for newcomers in this field to understand them and to start modeling such devices. Moreover, different control schemes are often erroneously used interchangeably which adds to the confusion. This papers aims to give a clear understanding of the above aspects by explaining details of modeling and initialization of grid-forming converter control schemes for power system fundamental frequency dynamic simulations (root-mean-square or RMS simulations) by using an example of a virtual synchronous machine (VSM). Starting from comparison of traditional systems and systems with a high share of converters and continuing with a functional overview of virtual inertia based converters in modern power systems, a step-by-step initialization of the static and dynamic model is given, focusing on RMS simulations. Finally, selected simulation examples are shown which illustrate main characteristics of virtual synchronous machines and their comparison to real synchronous machines. The presented dynamic model is made freely available in DIgSILENT PowerFactory format.