SOLUTION

Over fluxing

The transformers in generating stations need protection against the risk of damage, which may be because when they are operated at flux density levels significantly greater than the designed values. These conditions are most likely to arise when the unit is on an open circuit with the generator field energized, the speed of the machine is considerably below the synchronous speed and the regulator is trying to bring the voltage to the normal rated value. This may result in an unduly large value of V/f and hence a flux.

The flux density in the transformer core is proportional to the ratio of the voltage to frequency i.e. V/f. The power transformers are designed to work with a certain value of flux density in the core. In the generator transformer unit, if full excitation is applied before the generator reaches its synchronous speed then due to high V/f the over fluxing of the core may result. Higher core flux means more core loss and overheating of the core. The saturation of the magnetic circuit is also the probable cause for the over fluxing operation.

Over fluxing is a phenomenon by which the flux linked with the primary and the secondary of the transformer increases to an abnormally high value. This can have a profound effect on the efficiency and life of a transformer mainly because of the heating and magnetostrictive effects.

An increase in magnetic flux densities increases the iron losses and magnetizing current of the transformer. The core and core bolts, as a result, get heated and the insulation of laminations is affected. This can lead to permanent core damage and short circuit faults due to damaged insulation.

Over fluxing Relay

This is used for the protection of power transformers from over fluxing which is a result of overvoltage and causes heating of the transformer and saturation. The winding temperature relay can detect heating but cannot detect the saturation of the core. The transformer will fail to operate if saturated. This protection is one of the functions in the comprehensive transformer protection relay used on power transformers. The setting is based on the ratio of voltage and frequency. The flux is directly proportional to voltage and inversely proportional to the frequency. Therefore the relay setting is a volts/hertz ratio.