The concept of damping and synchronizing torques for a single-machine-infinite-bus system was first introduced in (F. P. Demello and C. Concordia, “Concepts of synchronous machine stability as affected by excitation control,” IEEE Trans. Power Apparatus and Systems, vol. PAS-88, no. 4, pp. 316 – 329, April 1969). Thus, electromechanical torque Te deviations of a machine can be expressed in terms of its speed ω and angle δ deviations, known as damping and synchronizing torque, respectively. The damping and synchronizing torques are the in time phase components proportional to the speed and angle deviations , and are defined as:
where Kd (p.u./rad/sec) and Ks (p.u.) are damping and synchronizing coefficients, respectively; and ω0 (rad/sec) is the system angular frequency.