Tuning primary frequency controllers using robust control theory in a power system dominated by hydropower

摘要

A deterioration of the grid frequency quality has been observed in the Nordic synchronous system, and an oscillation with period 40-90 seconds is often clearly visible in the frequency deviation. A working group, consisting of e.g. transmission system operators and power producers, has been initiated to improve and harmonise the technical requirements for primary frequency control in the Nordic countries. In this paper, a method to optimise the tuning of hydropower governors providing primary frequency control to the system is suggested. A model of the Nordic power system is set up, nominal values of the system parameters and their typical ranges of variation are presented, and a controller structure is defined. The objectives of primary frequency control are discussed and interpreted as requirements on the gain of the closed loop transfer functions of the system. These requirements are then used to define tuning goals for an optimisation of the controller parameters. The optimisation is carried out as a weighted minimisation of the closed loop system transfer functions in frequency domain. The result is evaluated in frequency domain and by time domain simulations of a system with added actuator non-linearities. The sensitivity to system parameter variation is analysed in terms of the performance of the optimised controllers in a system with changing parameters, but also in terms of how the optimisation result changes if the nominal system parameters are changed. The results show that compared to the governor tuning currently used in many hydropower plants in the system, retuned governors could reduce the amplitude of the 40-90 second oscillation considerably. A small, floating deadband on the controller output signal is discussed as a means to reduce the number of small movements in the actuators and turbines. The advantage of the presented method is that many different aspects and requirements on primary frequency control are taken into account and the trade-off between different aspects of the performance is visualised and can be controlled directly.