Frequency Stability in Grids with a High Share of Variable Generation: The Role of Gas Turbines

Sammanfattning

The electric power system is undergoing a major change through the implementation of a large number of variable renewable energy sources (VRE), driven by the goal to reduce carbon-dioxide emissions from electricity production. This transition increases the need for a more flexible and dynamic power system capable of maintaining voltage and frequency stability under changing operating conditions. Under normal operating conditions, the grid is supported by numerous generating units across the synchronous area, such as the Nordic power system. However, during disturbances or planned maintenance, transmission lines may be disconnected, potentially forming an islanded network that must rely entirely on local resources to maintain balance and stability -- otherwise a power outage is imminent. This thesis investigates how gas turbines can support a subtransmission power grid with a high share of wind power, particularly during critical scenarios such as island operation and three-phase faults. Due to their fast start-up times and strong ramping capabilities, gas turbines remain well-suited for providing frequency support and reserves in a flexible power system. A simplified model of the Stenungsund region in western Sweden has been developed in DIgSILENT PowerFactory, including wind power, gas turbines, and battery storage. Transient RMS simulations were carried out to evaluate the grid's stability during fault conditions and disconnection from the main grid, where the number of gas turbines is varied from one to six units. In general, the results demonstrate that gas turbines can play a key role in maintaining system stability during transient events in grids dominated by inverter-based generation. Under island operation, gas turbines effectively handled active power imbalances and maintained frequency stability, with increasing numbers contributing to a more resilient grid. The results also show that by incorporating future capabilities for the gas turbines, such as increased inertia and fast power injection, frequency stability can be greatly improved. However, during short-circuit events, the results indicate that increasing the number of gas turbines without reinforcing the grid can reduce transient stability and increase the risk of loss of synchronism following a short-circuit. This highlights the need to ensure sufficient grid strength when expanding local generation capacity to maintain synchronism during fault events.