Voltage per Hertz (Relay 24)

Voltage per herzt limiter and protection are used to protect the generator and step-up transformer from damage due to excessive magnetic flux resulting from low frequency and/or overvoltage.

Excessive magnetic flux if sustained can cause serious overheating and may result in damage to the unit transformer and to the generator core.

The ratio of per unit voltage to per unit frequency reffered to as volts per hertz (V/Hz) is a readily measured quantity that is proportional to magnetic flux. Typical V/Hz limitations gor generator (GEN) and step up transformer (XFMR) are shown in following chart.


















This above curve shows the permissible operating time versus (V/Hz) for generator. In addition to the limits of this curve, for periods up to 30seconds, (V/Hz) is limited to 1.2 PU because of excessive leakage flux increase from stator core ends under extreme over-excitation.

The unit step-up transformer low voltage rating is frequently 5% (1.05 PU) below the generator voltage rating, therefore V/Hz limiting and protection requirements are usually determine by the transformer limitation. However the generator and transformer voltage ratings are the same, the generator limitations would be more restrictive.

The V/Hz limiter (or regulator as it is sometomes called) control the field voltage so as to limit the generator voltage when the V/Hz value exceeds a preset value.

The V/Hz protection trip generator, when the V/Hz value exceeds a preset value for a special time. Usually a dual level protection is provided, one with a higher V/Hz setting and a shorter time setting, and the other with a lower V/Hz limiter, it serves as a back up.

For many units, the V/Hz protection becomes overvoltage protection above 60 Hz.