Testing a transformer by applying to the transformer a test signal, the frequency
of which may be lower than the nominal frequency of the transformer. The voltage
of the test signal may also be lower than the nominal voltage of the transformer.
A number of frequency-dependent parameters are measured, particularly the eddy
current resistance and the hysteresis curve of the transformer, in order to derive
a simulation model which simulates the behavior of the transformer at different
frequencies. Using this simulation model, it is possible to predict operating parameters
of the transformer, such as the terminal voltage on the secondary and the terminal
current in the secondary, during operation with a frequency deviating from the
frequency of the test signal, particularly during operation with the nominal frequency
of the transformer.