Frequency domain-based methods to predict turbogenerator transients with particular emphasis on the field current

Transients occurring in synchronous machines are calculated using Fourier transform methods. Particular attention is paid to the field current oscillograms following sudden short circuits. The field current in the short-circuit test is shown to be a sensitive indicator of the state of the damping circuits present in the rotors of turbogenerators. The paper goes some way to clarifying the degree of effectiveness of the damper cage in three example machines, using a combination of results from testing and from calculations using frequency-response data calculated by finite elements and measured in standstill tests. The Fourier transform methods used are discussed and the exponential Fourier transform shown to be advantageous for transients of long duration. The method is extended to cover the response of the machine to step changes in the field voltage and used to examine the effect of damper-cage design