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ABB Corporate Research: Grid-forming converter control in microgrids

Background

Microgrids are local small-scale energy grids that can operate independently from a large-scale power grid. While a microgrid may contain a wide range of different renewable sources, today voltage and frequency control is typically provided by dedicated non-renewable generation. In practice, these are diesel generators or gas turbines based on physical synchronous machines (SM). A recent trend is to use power electronics and battery energy storage systems to mimic the behavior of synchronous machines using so-called virtual synchronous generator (VSG) control. The renewable sources use standard power tracking control and therefore do not actively provide voltage or frequency control. Using a single dedicated VSG for voltage and frequency control requires a large centralized battery and, due to its centralized nature, provides little flexibility. In particular, when extending the microgrid, interconnecting it with microgrids, or connecting it to a large scale power grid, stability can no longer be guaranteed.

Thesis Topic

To remedy the drawbacks of the conventional control, the aim of this project is to develop control algorithms for microgrids with decentralized grid forming responsibility: Unlike in conventional microgrid control, where only the SM or VSG are responsible for stabilizing frequency and voltage, all converters share this responsibility, and each is equipped with a small battery storage system. The decentralized control should guarantee “plug-and-play” stability, such that the microgrid can grow arbitrarily without stability issues.

In this project one or more of the following topics will be considered:
- Decentralized grid forming converter control using battery storage
- Control tuning through modal analysis
- Optimal sizing and reducing the number of charge/discharge cycles of the battery storage system
- Validation of the control algorithms and sizing of the battery storage using simulations of low or medium voltage microgrids