On the Relationship between Inter-Area Modes and Power System Inertia

Increased use of wind and solar power is leading to reduction of inertia in several power systems. Lower inertia (or kinetic energy) has a significant effect on various dynamic phenomena in power systems. One of the affected phenomena is electromechanical inter-area oscillations. The characteristics of inter-area modes change as synchronous generators participating the modes are replaced by converter connected wind and solar farms. This paper analyses the characteristics (frequency and damping) of the main inter-area mode in the Nordic power system (i.e., the 0.3-0.4 Hz mode between Finland and Sweden) over a period of 5 years. The estimated frequency and damping of the mode are then compared with the system inertia from the same period. The results show that the frequency of the inter-area mode has a strong correlation with system inertia. The damping of the mode has practically no correlation with inertia. Understanding these relationships is important and may be used in the future in evaluating system stability. Furthermore, the relationship between inter-area mode frequency and inertia may be utilized in estimating the total kinetic energy of the system in the future.