The magnetic flux inside the transformer is directly proportional to the applied voltage, and inversely proportional to the frequency of the system. Overvoltage and / or underfrequency can cause levels of magnetic flux that can lead to transformer iron saturation.
The use of this relay is justified because, at the moment of over-excitation, the relation V / f will be different from the relation noticed in steady state. When over-excitation occurs, the V / f relay inhibits the differential protection.
Over-excitation of the transformer can cause heating, promoting excitation current, noise and vibration.
During load rejection and other operating conditions, the transformer may be subject to permanent overvoltages. Under these conditions, a high content of odd harmonics is observed, emphasizing the ones of 3a and 5a. This generates the birth of unwanted differential currents, which can generate a misguided performance of differential protection.
To avoid this problem, we can take advantage of a protection against over-excitation: at the moment it acts the differential protection does not act. An alternative is the relay V / f (frequency ratio) in conjunction with the differential relay.