GE MiCOM P40 Agile Technical Manual page 276

Chapter 11 - Frequency Protection Functions
A possible four stage load shedding scheme using the average rate of change frequency element is shown in the
following table:
"f+t [81U/81O]" Elements
(f+t) f Frequency
Stage
Setting (Hz)
1 49
2 48.6
3 48.2
4 47.8
In the above scheme, the faster load shed decisions are made by monitoring the frequency change over 500 ms.
Therefore tripping takes place more slowly than in schemes employing frequency-supervised df/dt, but the
difference is not very much at this setting. If the delay jeopardises system stability, then the scheme can be
improved by increasing the independent "f" setting. Depending on how much this value is increased, the frequency
at which the "f+Df/Dt" element will trip also increases and so reduces the time delay under more severe frequency
fluctuations. For example, with the settings shown below, the first stage of load shedding would be tripped
approximately 300 msecs after 49.0 Hz is reached and at a frequency of approximately 48.7 Hz.
"f+t [81U/81O]" Elements
(f+t) f Frequency
Stage
Setting (Hz)
1 49
2 48.6
3 48.2
4 47.8
250
Frequency
(f+t) t
Time Setting (Sec.)
20
20
10
10
Frequency
(f+t) t
Time Setting (Sec)
20
20
10
10
Average Rate of Change of Frequency "f+Df/Dt [81RAV]" Elements
(f+Df/Dt) f
(f+Df/Dt) Df Frequency
Frequency Setting
(Hz)
49 0.5
48.6 0.5
48.2 0.5
47.8 0.5
Average Rate of Change of Frequency
"f+Df/Dt [81RAV]" Elements
(f+Df/Dt) f
(f+Df/Dt) Df Frequency
Frequency Setting
(Hz)
49.2 0.5
48.8 0.5
48.4 0.5
48.0 0.5
(f+Df/Dt) Dt Time
Diff Setting, (Hz)
0.5
0.5
0.5
0.5
(f+Df/Dt) Dt Time
Diff Setting (Hz)
0.5 s
0.5 s
0.5 s
0.5 s
P14x
Period, (Sec.)
Period, (Sec.)
P14xEd1-TM-EN-1