Analysis of Slow Frequency Variations in the Nordic Power System
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- Institutt for elkraftteknikk 
Slow oscillations in system frequency, with a periodic time of 60-90 seconds and amplitude of 50 mHz, has been observed in the Nordic grid. As a result the frequency is outside the limit of 50 +/- 0,1 Hz more often. It is assumed that this cause wear on turbines and generators. It may also cause additional activation of balancing power in the Regulating Power market, and impose an additional cost on the transmission system operators (TSOs). Together, the Nordic TSOs have worked on a report evaluating causes and mitigation of the problem. One of the suggestions is to improve the tuning of the hydro turbine governors. In this Master Thesis a model of the Nordic grid has been used for simulations in PSS/E (Power System Simulation for Engineering), and the effect of governor tuning has been evaluated. In addition, a Simulink model has been developed based on data in the PSS/E model, in order to be able to run longer simulations without having numerical problems. The objective was to find out whether changing the governor parameters, or other parameters, may reduce the problem of low-frequent oscillations. Spectral analysis of the simulated frequency has been used to determine the periodic time of the frequency variations. The modeled frequency responses are similar to the real life frequency response, which confirms that the models are valid for frequency analysis. Simulations run with random load changes, and with different governor settings, indicated that the periodic time and amplitude of the frequency variations are affected by the governor parameters Rp, Rt and Tr. The inertia in the system also has a great impact on the periodic time and variance of the frequency oscillations. Deadbands were implemented on the governors in the Simulink model, and they were found to amplify the amplitude of the oscillations, and to make the periodic time longer.