TURBINE UNDER FREQUENCY PROTECTION RELAY SCHEMES AS PER IEEE C37.106-2003

The turbine underfrequency protection scheme may be accomplished by one or more relays. Digital or solid state relays are preferred for their accuracy over a broad frequency range. The required number of frequency setpoints and their associated time delays are dependent upon the characteristics of the turbine.

This relay function may be included in a multifunctional protection package. The first step in designing an underfrequency protection scheme is determining the turbine’s abnormal frequency operating characteristic.

Consultation with the manufacturer should provide the initial design parameters. Modifications to the turbine as well as the known condition from turbine inspections may result in changes to the either the resonant frequency or the allowable abnormal frequency operate time or to both.

From this information, the number of frequency levels that require action can be identified. It should be noted that extreme frequency variations may not require underfrequency relay action as other plant equipment will force the plant to trip.

Once the number of frequency steps is known, the time delay for each step must be determined. Because the allowable underfrequency operation time cannot be identified exactly, some margin should be included in the time delay.

This would allow tripping of the unit prior to damage, with the opportunity to inspect the turbine at the owner’s convenience during a future outage. This allows for application of underfrequency protection, even if the unit has been in operation for many years without having accumulated previous underfrequency operational data.

The time delay margins should consider the importance of the unit, the susceptibility of the system to an underfrequency event and operating agreements with local or regional power authorities. A range of 50–90% of the allowable time per expected event over the blading life is reasonable.

Settings of 50% should be considered if the turbine is in poor condition, there is a high possibility of an underfrequency event or if the unit is not system critical. If the unit is in good condition, an underfrequency event is unlikely, and the unit is critical to the system, a setting near 90% of the allowable underfrequency time should be considered.

It should be recognized that some underfrequency relay timers have an instantaneous reset once the frequency rises above the trip setting, while others accumulate the underfrequency operate time in a memory function (zero reset). The time delay setting should be a smaller percentage of the allowable time if the relay is of the instantaneous reset type, whereas the zero reset relay can be set at a greater percentage of the allowable time.

Figure below will be used to demonstrate an underfrequency relay setting. The example indicates the turbine is capable of continuous operation at frequencies above 58.5 Hz and is limited to a maximum of 10 minutes accumulated over the blading life at 56.0 Hz.

Example of turbine abnormal frequency limitations and settings

These are operating conditions with the turbine at load. The underfrequency relay trip point setting should be set just above 56.0 Hz to allow for relay margin. A setting of 56.2 Hz could be selected. A time delay setting of seven minutes could be used if the unit is in fair condition, not critical to the operation of the system, and it is acceptable to lose 70% of the fatigue life of the blading.

If the unit is in good condition and is critical to the system, a longer time delay of nine minutes could be used to allow the maximum opportunity for system recovery, prior to tripping the unit assuming these events are very rare so that 90% of the fatigue life can be expended on it. An alarm should be provided when the underfrequency relay begins to time out, providing operating personnel a warning of the impending underfrequency trip.

Many operators apply a single multilevel relay having four frequency set points. In many cases, one of the frequency set points will be used to alarm at a relatively high frequency (59.4 Hz for example) to indicate the inception of an abnormal frequency event, with the remaining three set points are at lower frequencies corresponding to the turbine characteristics.


If IEC 60034-3:1996 is applicable, time-frequency protection could be required by a manufacturer for the turbine generator. If such a situation was to develop, the more restrictive of the generator and turbine frequency requirements, over a frequency band, should be used in the determination of appropriate frequency settings for underfrequency relaying. The frequency capability of the generator, when applicable, must be considered in the development of the frequency relay protection scheme.