On the Relationship between Ultra Low Frequency Oscillation and Power System Inertia

In recent years, ultra low frequency oscillation (ULFO) has been observed in various power systems. In the ULFO phenomenon, the frequency of the system continuously slowly oscillates around the nominal value. Such oscillation exists also in the Nordic power system, where the frequency of the system oscillates around the nominal 50 Hz value with a period of around 40 to 90 s. Such oscillation deteriorates the frequency quality of the system, might cause unnecessary control actions and wear and tear of turbines, and potentially even risk the frequency stability of the system. This paper analyses the characteristics of the ULFO during the latest seven years in the Nordic power system. The paper shows, based on measurements, that the ULFO has a relationship with the inertia of the system. More specifically, system inertia is clearly correlated with the frequency of the ULFO. As the inertia of the system increases, the frequency of the ULFO decreases, and vice versa. Furthermore, the amplitude of the ULFO is also correlated with inertia and smaller inertia leads to higher amplitude of the ULFO. These findings give important insight into the characteristics of ULFOs and similar analysis is also recommended for other power systems experiencing an ULFO phenomenon.