What is overfluxing relay?
A transformer is designed to operate at or below a maximum
magnetic flux density in the transformer core. Above this design
limit the eddy currents in the core and nearby conductive
components cause overheating which within a very short time may
cause severe damage. The magnetic flux in the core is proportional
to the voltage applied to the winding divided by the impedance of
the winding. The flux in the core increases with either increasing
voltage or decreasing frequency. During startup or shutdown of
generator-connected transformers, or following a load rejection,
the transformer may experience an excessive ratio of volts to
hertz, that is, become overexcited. When a transformer core is
overexcited, the core is operating in a non-linear magnetic region,
and creates harmonic components in the exciting current. A
significant amount of current at the 5th harmonic is characteristic
of overexcitation.