Torsional subsynchronous oscillations caused by grid-connected wind farms in a complex multi-machine power system under the condition of near strong modal resonance

Abstract

Dynamic interactions introduced by grid-connected wind farms are multivariable. This is why wind farms’ models need to be simplified when the impedance-model-based analysis is applied to examine whether negative resistance is supplied by the wind farms to cause subsynchronous oscillations (SSOs). In this paper, we examine and reveal the mechanism of torsional SSOs caused by grid-connected wind farms from the standpoint of near strong modal resonance (NESMOR). The examination and revelation consider fully the multivariable dynamic interactions introduced by wind farms in a complex multi-machine power system without simplifying the models of wind farms. We carry out the analysis based on the multi-input multi-output (MIMO) closed-loop model with two interconnected subsystems. The analysis indicates that the open-loop modal proximity (OLMP) of interconnected subsystems is a special case of NESMOR. When the NESMOR happens, growing torsional SSOs may be excited by the grid-connected wind farms. Hence, the analysis unveils the connection between the modal condition of OLMP and NESMOR, shedding a deeper insight into the mechanism of torsional SSOs caused by wind farms. In addition, we propose a method to detect the NESMOR. An example multi-machine power system with two grid-connected wind farms is presented to demonstrate the analysis and method.