Chapter 11 - Frequency Protection Functions
FREQUENCY PROTECTION OVERVIEW
Power generation and utilisation needs to be well balanced in any industrial, distribution or transmission network.
These electrical networks are dynamic entities, with continually varying loads and supplies, which are continually
affecting the system frequency. Increased loading reduces the system frequency and generation needs to be
increased to maintain the frequency of the supply. Conversely decreased loading increases the system frequency
and generation needs to be reduced. Sudden fluctuations in load can cause rapid changes in frequency, which
need to be dealt with quickly.
Unless corrective measures are taken at the appropriate time, frequency decay can go beyond the point of no
return and cause widespread network collapse, which has dire consequences.
Protection devices capable of detecting low frequency conditions are generally used to disconnect unimportant
loads in order to re-establish the generation-to-load balance. However, with such devices, the action is initiated
only after the event and this form of corrective action may not be effective enough to cope with sudden load
increases that cause large frequency decays in very short times. In such cases a device that can anticipate the
severity of frequency decay and act to disconnect loads before the frequency reaches dangerously low levels, are
very effective in containing damage. This is called instantaneous rate of change of frequency protection (ROCOF).
During severe disturbances, the frequency of the system oscillates as various generators try to synchronise to a
common frequency. The measurement of instantaneous rate of change of frequency can be misleading during
such a disturbance. The frequency decay needs to be monitored over a longer period of time to make the correct
decision for load shedding. This is called average rate of change of frequency protection.
Normally, generators are rated for a particular band of frequency. Operation outside this band can cause
mechanical damage to the turbine blades. Protection against such contingencies is required when frequency does
not improve even after load shedding steps have been taken. This type of protection can be used for operator
alarms or turbine trips in case of severe frequency decay.
FREQUENCY PROTECTION IMPLEMENTATION
Earlier versions of the product provided just four stages of underfrequency protection and two stages of
overfrequency protection. The present version of the product provides an additional advanced version of
frequency protection consisting of 9 stages each of the following protection functions:
Underfrequency Protection: abbreviated to f+t<
Overfrequency Protection: abbreviated to f+t>
Independent Rate of Change of Frequency Protection: abbreviated to Independent R.O.C.O.F, or df/dt+t
Frequency-supervised Rate of Change of Frequency Protection: abbreviated to Frequency-supervised
R.O.C.O.F, or f+df/dt
Average Rate of Change of Frequency Protection: abbreviated to Average R.O.C.O.F, or f+Df/Dt (note the
Load Shedding and Restoration
The basic version of frequency protection is implemented in the FREQ PROTECTION column of the relevant settings
group. The advanced version of frequency protection is implemented in the ADV. FREQ PROT'N column of the
relevant settings group.
If you require only four stages of underfrequency protection and two stages of overfrequency protection, you can
still use the basic version of frequncy protection, however we recommend using the advanced version in all cases.
If using basic frequency protection you must enable it and disable the advanced frequency protection. If using
advanced frequency protection, you must enable it and disable the basic frequency protection. You do this by
setting the Freq Protection and the Adv. Freq Prot'n settings in the CONFIGURATION column to enabled or
With basic frequency protection you can enable or disable each stage of underfrequency and overfrequency
protection separately with the settings F<1 Status, F<2 Status, F<3 Status, F<4 Status, F>1 Status, F>2 Status.