Influence of turbogenerator set parameters on the calculation of shaft transient torsional strains

The paper involves an analysis of the impact of some turbogenerator set parameters which considerably influence its mechanical behaviour on the calculations of torsional strains. The analysis is carried out for a case of a three-phase short circuit at one of two parallel lines through which a generator is supplied followed by a fault clearing. Maximum torsional torques and critical fault clearing times at which they occur are presented. The mechanical parameters which have the largest influence on the torsional strains in the shaft are the stiffness coefficients of the shaft sections and the mechanical time constants (or moments of inertia) of the generator and the turbine rotors. The influence of mechanical damping coefficients is less significant. Maximum torsional torques are also indirectly influenced by electrical parameters of the generator, primarily by the subtransient and the transient synchronous reactance, but their impact is minor. Calculated time responses of torsional torques for the combined fault show that inaccurately defined mechanical parameters can lead to wrong conclusions concerning maximum values of torsional torques which could occur during a fault.