E00858
Frequency
Rapid decay
Slow decay
T
ime
fn
f
f
1
2
Figure 117: Frequency supervised rate of change of frequency protection
6.3.2 SETTING GUIDELINES
We recommend that the frequency supervised rate of change of frequency protection (f+df/dt) element be used in
conjunction with the time delay
ed frequency protection (f+t) elements.
A four stage high speed load shedding scheme may be configured as indicated below, noting that in each stage,
both the "f+t" and the "f+df/dt" elements are enabled.
Frequency "f+t [81U/81O]" Elements
Frequency Supervised Rate of Change of Frequency "f+df/dt
[81RF]" Elements
Stage
Frequency Setting
(Hz)
Time Setting (Sec.) Frequency Setting (Hz)
Rate of Change of Frequency Setting (Hz/
sec.)
1 49 20 49 1.0
2 48.6 20 48.6 1.0
3 48.2 10 48.2 1.0
4 47.8 10 47.8 1.0
It may be possible to further improve the speed of load shedding by changing the frequency setting on the f+df/dt
element
. In the settings outlined below, the frequency settings for this element have been set slightly higher than
the frequency settings for the f+t element. This difference will allow for the measuring time, and will result in the
tripping of the two elements at approximately the same frequency value. Therefore, the slow frequency decline
and fast frequency decline scenarios are independently monitored and optimised without sacrificing system
security.
Frequency "f+t [81U/81O]" Elements
Frequency Supervised Rate of Change of Frequency "f+df/dt
[81RF]" Elements
Stage
Frequency Setting
(Hz)
Time Setting (Sec.)
Frequency Setting
(Hz)
Rate of Change of Frequency Setting (Hz/
sec.)
1 49 20 49.2 1.0
2 48.6 20 48.8 1.0
3 48.2 10 48.4 1.0
4 47.8 10 48.0 1.0
P24xM Chapter 11 - Frequency Protection Functions
P24xM-TM-EN-2.1 233
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