Long Term Characteristics of Ultra Low Frequency Oscillations in the Nordic Power System

In the Nordic power system, the frequency of the system oscillates constantly around the nominal 50 Hz value with a period of around 40 to 90 s, i.e., in the frequency range around 0.011-0.025 Hz. This ultra low frequency oscillation (ULFO) deteriorates the frequency quality in the entire system, causes unnecessary control actions and may cause wear and tear of turbines. In the worst case, the oscillation may endanger frequency stability of the system. This paper analyzes the properties of this oscillations during a period of seven years using an ambient modal identification method (mode meter). The paper shows that the ULFOs can be analyzed using ambient modal identification methods and a similar approach is recommended to be used for analyzing ULFOs in other power systems as well. Furthermore, the paper gives insight into the long term characteristics of the ULFOs in the Nordic power system and the results indicate that for example the amplitude of the ULFOs has an increasing trend during the analysis period (and their damping has a decreasing trend). These findings indicate a need for monitoring the ULFOs and develop actions for mitigating them.