The Effect of Hysteretic Damping on Turbogenerator Shaft Torsional Oscillations

Electromechanical interaction between an electrical system and turbogenerator units may, in certain circumstances, lead to shaft oscillatory behaviour which has an onerous effect on the life expenditure of the shaft. The rate of decay of the oscillations produced as a result of a system disturbance is determined by both the electrical and mechanical components of damping. Due to the advances made in the modelling of electrical machines over the past fifty years the effect of the electrical component of damping is well understood and easily incorporated into a system model. The mechanical components of damping are known to be variable, but nevertheless are usually represented as constant viscous coefficients in the shaft system equations of motion. The present paper proposes a method for the evaluation of a variable damping coefficient which depends on the property of the shaft material when undergoing high stress cyclic variations. The effect of this term is to more rapidly reduce the high torque oscillations and reduce low torque oscillations less rapidly. This effect has been observed in practice and hitherto not predicted from simulation studies. The investigation relies on previous work when taking account of the effects of various steam conditions. Emphasis is placed on the derivation of a shaft model which includes equivalent variable viscous damping and representative results are collected to show the effect of this important parameter on the pattern of derived shaft torques based on a representative 600 MW turbo-generator.