Subsynchronous Oscillations in Inverter-Dominated Power Systems

The number of electric power conversion devices connected to AC grids of different voltages has markedly increased worldwide in recent years due to the steady growth in renewable power capacity. Unique features of power inverters (high performance, the dominant role of the automatic control system, etc.) modify dynamic properties of power systems, resulting in new operating states and processes. These are, most notably, broadband oscillations and subsynchronous oscillations (SSO) in particular. The latter are the subject of the present study. One of the causes of such oscillations is interaction between automatic control systems running inverters and the external grid. A comprehensive SSO analysis within the study involved frequency analysis of simplified models of the grid-tie inverter under different patterns of active and reactive power control, as well as the detailed modeling in the time domain. The findings of the above analysis served as a basis for a taxonomy of SSO causation mechanisms, which are related to the dynamics of inverter's automatic control system operation under various network topologies and operating conditions. Furthermore, the factors increasing the probability of SSO events in power grids with inverter-based generators are highlighted. Testing different designs of closed-loop automatic control systems provided evidence for the validity of the proposed taxonomy.