Mechanisms Causing the Occurrence of Subsynchronous Oscillations in Power Systems Containing Power Inverters: Part 1

Modern power systems are undergoing changes as a result of wide-scale introduction of power inverters. The unique properties of such devices alter the dynamics of power systems and give rise to new processes. One of them is a phenomenon accompanied by the occurrence of a wide range of subsynchronous oscillations associated with the response of the applied grid inverter control system in rotating dq coordinates to various grid operation and circuit conditions. Given the existing gaps in understanding the mechanisms causing the occurrence of subsynchronous oscillations due to various combinations of the controllers used in the grid-tied inverter control system and the grid parameters, the first part of the article addresses the development of simplified mathematical models of the grid-tied inverter with various degrees of detail. The models reflecting the influence on subsynchronous oscillations of various external controllers, phase-locked loop, and their response to variations in the grid density, the level of grid inverter loading by active power, and voltage at the connection point are considered in a sequence. A frequency analysis is performed for the obtained models, the results of which made it possible to reveal different mechanisms causing the occurrence of subsynchronous oscillations, as well as influencing factors. For each mechanism causing the occurrence of subsynchronous oscillations, the necessary degree of detail in the used mathematical model is justified.