The excitation regulator of generators in current power systems are widely equipped with superimposed type of underexcitation limiters(UELs)

which have important impacts on the dynamic stability of power systems. But the mechanism has not been fully recognized. Based on an extended Heffron-Phillips model

the variation law of the new coefficient of the model with the change of UEL parameters and operation point is analyzed. The damping torque of generator is divided into four components by using the electromagnetic torque analysis method. They are damping torque provided by UEL

damping torque provided by power system stabilizer(PSS)

the inherent damping torque of system and damping torque related to coefficient D. The impacts of UEL parameters on the various parts of the damping torques are deduced and analyzed in detail. The results indicate that UEL boundary line with a large slope may cause damping torque provided by UEL to become negative

and UEL will worsen the positive damping torque provided by PSS in the usual case of UEL parameters. A typical example of the simulation system further validates the conclusions proposed.This work is supported by State Grid Corporation of China(No. XT71-15-059).