Improved control strategies for primary frequency control in Swedish combined heat and power plants

Abstract: A model of the Swedish power plant Mälardalen Energi, developed by Solvina AB was simulated in Dymola with different types of control strategies in order to evaluate if they could improve the operational flexibility of the plant and, thus, its ability to deliver frequency control to the Swedish power grid. Several different control strategies and process modifications were tested and compared to previous simulations done by Solvina. Overall, the results show that improvements could be done to the systems deviation in steam net pressure and district heating temperature by implementing a slightly more complex control structure or by modifying the system slightly. The simulations showed that implementing a feed forward on the already existing control structure would improve the deviations of the outlet temperature of the district heating network. Furthermore, implementing a split range control structure that first throttled the steam exhaust drains and then opened the turbine inlet valve, decreased the steam net pressure drop when increasing the production demand. Lastly, modifying the turbine with inlets at different pressures and letting a split-range controller manipulate the flows between them gave the overall best performance. Finally, several factors such as economy, performance, difficulty to implement and potential confusion for the operators need to be considered in order to determine which of the control structures that are the best.