A possible four stage load shedding scheme using the average rate of change frequency element is shown in the
following table:
Frequency
"f+t [81U/81O]" Elements
Average Rate of Change of Frequency "f+Df/Dt [81RAV]" Elements
Stage
(f+t) f Frequency
Setting (Hz)
(f+t) t
Time Setting (Sec.)
(f+Df/Dt) f
Frequency Setting
(Hz)
(f+Df/Dt) Df Fr
equency
Diff Setting, (Hz)
(f+Df/Dt) Dt Time
Period, (Sec.)
1 49 20 49 0.5 0.5
2 48.6 20 48.6 0.5 0.5
3 48.2 10 48.2 0.5 0.5
4 47.8 10 47.8 0.5 0.5
In the above scheme, the faster load shed decisions are made by monitoring the frequency change over 500 ms.
Therefor
e 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.
Frequency
"f+t [81U/81O]" Elements
Average Rate of Change of Frequency
"f+Df/Dt [81RAV]" Elements
Stage
(f+t) f Frequency
Setting (Hz)
(f+t) t
Time Setting (Sec)
(f+Df/Dt) f
Frequency Setting
(Hz)
(f+Df/Dt) Df Fr
equency
Diff Setting (Hz)
(f+Df/Dt) Dt Time
Period, (Sec.)
1 49 20 49.2 0.5 0.5 s
2 48.6 20 48.8 0.5 0.5 s
3 48.2 10 48.4 0.5 0.5 s
4 47.8 10 48.0 0.5 0.5 s
Chapter 11 - Frequency Protection Functions P14x
250 P14xEd1-TM-EN-1
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