Underfrequency

The basic underfrequency protection is the most commonly used and is available with multiple independent definite time delayed stages - up to six in some relays. The elements are usually definite time delayed, but can also be used as instantaneous.

In order to minimize the effects of underfrequency on a system, a multi stage load shedding scheme may be used with the substation loads prioritized and grouped. During an underfrequency condition, the load groups are disconnected sequentially depending on the level of underfrequency, with the highest priority group being the last one.

Frequency supervised rate of change of frequency

Considerable load to generation imbalance may result in relatively rapid changes of the system frequency. In such a case maintaining the system stability is an important task, and requires quick corrective action. High speed load shedding cannot be achieved by monitoring the system frequency alone and the rate of change of system frequency becomes an equally critical parameter to use.

Independent rate of change of frequency

This element is a plain rate of change of frequency monitoring element, and is not supervised by a frequency setting. It can be used to provide extra flexibility to a load shedding scheme in cases of severe load to generation imbalances. Since the rate of change monitoring is independent of frequency, the element can identify frequency variations occurring close to nominal frequency and thus provide early warning to the operator on a developing frequency problem. Additionally, the element could also be used as an alarm to warn operators of unusually high frequency variations.

Average rate of change of frequency

Variations in frequency during times of generation - load imbalance do not follow any regular patterns and are highly non-linear. Oscillations will occur as the system seeks to address the imbalance, resulting in frequency oscillations typically in the order of 0.1Hz to 1Hz, in addition to the basic change in frequency.

The rate of change of frequency elements use an "instantaneous" measurement of "df/dt". Due to the oscillatory nature of frequency excursions, this instantaneous value can sometimes be misleading, either causing unexpected operation or excessive stability. For this reason, the relays provide an element for monitoring longer term frequency trend, thus reducing the effects of non-linearity in the system and providing increased security. It is recommended that the average rate of change of frequency element is used in conjunction with the frequency element.