Abstract: Power system stabilizers are used in the excitation systems of large shaft driven synchronous dynamoelectric machines for the purpose of enhancing the dynamic stability of power generating systems. Such stabilizers using speed, power or frequency as an input signal when applied with high initial response excitation systems are effective in damping low-frequency local or inter-tie mode oscillations which may occur in the system. The power system stabilizer may adversely affect torsional damping at higher frequencies. The adverse effects of a power system stabilizer at shaft torsional frequencies are obviated by providing a torsional oscillation protection device comprising a composite band-reject filter of the present invention which attenuates the power system stabilizer output at the shaft natural torsional frequencies without introducing an excessive phase lag at the local or inter-tie mode frequencies.