The voltage at the terminals of a generator is a function of the excitation and speed. Overvoltage
may result in thermal damage to cores due to excessive high flux in the magnetic circuits. Excess
flux saturates the core steel and flows into the adjacent structures causing high eddy current losses
in the core and adjacent conductor material. Severe overexcitation can cause rapid damage and
equipment failure.
Since flux is directly proportional to voltage and inversely proportional to frequency, the unit
of measure for excitation is defined as per unit voltage divided by per unit frequency (V/Hz).
Overvoltage exists whenever the per unit V/Hz exceeds the design limits. For example, the usual
turbine generator design is for 105 % of rated V/Hz. Overvoltage exists at 105 % of rated voltage and
per unit frequency or per unit voltage and 95 % frequency. Transformers are designed to withstand