In the analysis of the sub-synchronous oscillation (SSO) based on the complex torque coefficient (CTC) method, it is required to establish a detailed small-signal electromagnetic transient equation for the entire power system. This paper demonstrates a method of replacing the small-signal electromagnetic transient equation of grid to the d-q frame impedance model, reflecting the change of the grid in the change of the impedance model. Using this method, re-establishing small-signal electromagnetic transient equations for each component is not required when the grid side changes. Hence, the analysis and modeling of the grid is simplified, and the electrical damping coefficient is used to visually predict whether the system has the risk of SSO, which greatly simplifies the modeling of the SSO analysis in pure AC power system and saves time.

sub-synchronous oscillation,complex torque coefficient,impedance model,electrical damping