Virtual Synchronous Machine Control for Grid Transmission Compliance Studies

The increasing penetration levels of renewable energy systems (RES) such as wind and solar could affect the stability threshold of electricity transmission and distribution systems. The power electronic converters used to interface RES to the grid tend to be operated in "grid following" mode (typically acting as controlled current sources) and depend on high quality synchronization to the grid through a phase locked loop (PLL). The behavior of the PLL during grid transients can deteriorate, and this combined with the deterioration voltage reference signal and reduced system inertia can lead to system collapse during abnormal events and under post fault conditions. The virtual synchronous machine (VSM) control strategy has been proposed as a "grid forming" control mode to improve converter behavior from a grid perspective. However, the inverter is controlled as a voltage source and can be susceptible to fast changes in grid state. This paper presents an investigation into the VSM algorithm under abnormal grid conditions including single and three phase faults, frequency changes and grid phase/vector shifts. The paper describes current and power limiting strategies which protect the power converter whilst allowing it to support the grid.