The mode-based damping torque analysis (M-DTA) method for studying the effect of an external controller on power system low-frequency oscillations is proposed in this paper. First

based on the interconnection model between the system and the controller in the frequency domain

the oscillation loop corresponding to the electromechanical oscillation mode is built

and then the mode-based damping torque of the controller can be calculated. Then

the application of the M-DTA method in the power system is illustrated. The derivation shows that in the single-machine infinite-bus power system

the M-DTA method is completely equivalent to the classical damping torque analysis (C-DTA) method. In the multi-machine power system

the mode-based damping torque directily reflects the effect of the controller on the oscillation mode

overcoming the shortcomings of the C-DTA method in which there is no direct correspondence between the damping torque and the oscillation mode. By deriving the relationship with the residue index

the M-DTA method shows higher accuracy than the residue method in applications

such as controller parameter adjustment. Finally

two example power systems are presented to demonstrate the application of the proposed M-DTA method.