In order to minimise the effects of underfrequency, a multi-stage load shedding scheme may be used with the
plant loads prioritised and grouped. During an underfrequency condition, the load groups are disconnected
sequentially, with the highest priority group being the last one to be disconnected.
The effectiveness of each load shedding stage depends on the proportion of power deficiency it represents. If the
load shedding stage is too small compared with the prevailing generation deficiency, then there may be no
improvement in the frequency. This should be taken into account when forming the load groups.
Time delays should be sufficient to override any transient dips in frequency, as well as to provide time for the
frequency controls in the system to respond. These should not be excessive as this could jeopardize system
stability. Time delay settings of 5 - 20 s are typical.
An example of a four-stage load shedding scheme for 50 Hz systems is shown below:
The relatively long time delays are intended to provide sufficient time for the system controls to respond. This will
work well in a situation where the decline of system frequency is slow. For situations where rapid decline of
frequency is expected, this load shedding scheme should be supplemented by rate of change of frequency
This section refers to advanced frequency protection. The basic frequency protection works in a similar manner, but the
setting names and DDB signal names are different.
Chapter 11 - Frequency Protection Functions
Frequency Setting (Hz)
Time Setting (Sec)