Sudden 3-phase short-circuit characteristics of turbine generators from design data using electromagnetic field calculations

Abstract

The transient performance of turbine alternators following a sudden 3-phase short circuit is obtained from design data using electromagnetic field calculations. The 3-phase stator windings are replaced by equivalent current sheets lumped into the stator slots, and the 2-axis theory is used to calculate the short-circuit currents in the stator windings. A 2-dimensional analysis of the field over a half pole pitch is made and the electromagnetic problem is solved numerically. The nodal method is used to give a relationship between current and magnetic field at each node. Currents in the stator windings are taken to depend on rotational voltages, but the currents in the field winding, damper wedges and the solid rotor depend on the rate of change of the magnetic field in addition to the constant source current in the field winding. This leads to a formulation of governing electromagnetic equations coupled with circuit conditions. In the numerical formulation several conductivity matrices are defined to allow for the evaluation of the various currents and the electromagnetic field distribution, at each instant, simultaneously. Induced eddy currents, currents in the machine windings and the flux distributions are obtained step-by-step using the predictor-corrector method. The problem is nonlinear, the permeabilities of the iron in the various regions being dependent on the resultant field pattern; the problem is solved iteratively using Newton-Raphson and sparsity techniques. The machine performance is obtained during the subtransient and a part of the transient periods for armature and field currents. They are compared with the experimental results on a 333 MVA turbine generator.