Magnetic Flux α Voltage/Frequency
Overfluxing is a dangerous situation in which the magnetic flux density increases to extremely high levels. The high flux density can induce excessive eddy currents in the windings and in other conductive parts inside the transformers. The heat generated by these eddy currents can damage the windings and the insulation. The high flux density also causes magnetostriction inside the transformer core and produces noise. The powerful magnetostrictive forces can also cause damage. The winding temperatures may also increase due to the heat produced.
The magnetic flux density is dependent on the current flowing through the primary windings in a transformer. This current is dependent on the voltage applied across the windings and the winding impedance. The impedance is dependent on the frequency of the applied voltage. If the nominal voltage is applied at a reduced frequency, the low inductive reactance will cause a higher current to flow through the windings.
Overfluxing is usually encountered in Transformers which are directly connected to the generator. It usually occurs when the generator is being started or stopped. As the rpm of the generator and consequently the frequency of the power falls, the same system voltage induces a higher magnetic flux. Modern Automatic Voltage regulators are equipped with V/Hz limiters which limit the voltage in accordance with the frequency.
Overfluxing can be prevented by the use of a Overfluxing relay. An overfluxing is an adaptation of an overvoltage relay. The PT voltage is connected across a resistor and a capacitor in series. The voltage sensing relay is connected across the capacitor. The relay operates in the event of an overfluxing and isolates the transformer