Transformers need protection against the risk of damage, which may be caused when the transformers are operated at flux density levels significantly greater than the design values. The over fluxing withstand time is generally found to be varying inversely with the working flux density in the core, having higher withstand times during extreme over fluxing conditions.
The over fluxing condition can occur during system over voltage or under frequency conditions.
The basic operating principle is to produce an ac voltage, which depends upon the ratio between AC input voltage and the frequency. The AC input voltage is fed to a step-down transformer, which also provides isolation and the stepped down voltage is fed to a V/F ratio detector circuit. This circuit is a simple operational amplifier integrator with the provision for V/F pickup adjustment. The AC voltage is rectified by true RMS. to dc converter. This circuit gives a frequency output and this frequency increases rapidly with the increase in voltage. The frequency output is given to a curve shaping circuit, which involves counter and comparators. The counter counts the frequency output and the number of counts required for final trip condition is changed by the comparator circuits to get the required timing characteristic.
