P.3.120
SEL-411L Relay Protection Manual Date Code 20151029
Protection Functions
Frequency Estimation
Note that frequency measurement is not the same as frequency tracking. The
relay first measures the frequency and then tracks the frequency by changing
the processing rate.
The relay measures the frequency over the 20–80 Hz range (protection
frequency, see FREQP in Table 3.57), but only tracks the frequency over the
40–65 Hz range (see FREQ in Table 3.57). If the system frequency is outside
the 40–65 Hz range, the relay does not track the frequency. Instead, it clamps
the frequency to either limit. For frequencies below 40 Hz, the relay clamps
the frequency at 40 Hz. For frequencies above 65 Hz, the relay clamps the
frequency at 65 Hz.
To measure the frequency, the relay calculates the alpha component quantity
and then estimates the frequency based on the zero-crossings of the alpha
component. Relay Word bit FREQOK asserts when the relay measures the
frequency over the range 20–80 Hz. If the frequency is below 40 Hz or above
65 Hz, FREQ reports the clamped values of either 40 Hz or 65 Hz. In this
case, the relay no longer tracks the frequency. Instead, it uses either 40 Hz or
65 Hz to calculate the internal quantities.
If the frequency is in the 20–80 Hz range, but outside the 40–65 Hz range (for
example, 70 Hz), FREQP shows the frequency the relay measures and FREQ
shows the clamped frequency. In this case, FREQP = 70 Hz and FREQ = 65 Hz.
Table 3.54 summarizes the frequency measurement and frequency tracking
ranges.
If the frequency is below 20 Hz or above 80 Hz, the relay no longer measures
the frequency. Relay Word bit FREQFZ asserts and Relay Word bit FREQOK
deasserts to indicate this condition. FREQ and FREQP are no longer valid, but
they display the frequency at the time that the relay stopped measuring the
frequency.
NOTE: The relay measures/tracks
the frequency to a rate of 15 Hz/s.
The relay has six voltage inputs (VAY, VBY, VCY, VAZ, VBZ, and VCZ) that
can be used as sources for estimating the frequency. Assign any of the six
voltage inputs to VF01, VF02, and VF03. Note that assigning ZERO will set
that input to zero. The relay also provides an alternate frequency source
selection where you can assign any of the six voltage inputs to VF11, VF12,
and VF13. The relay uses VF01, VF02, and VF03 as sources if the SEL
OGIC
evaluation of EAFSRC is 0. The relay uses VF11, VF12, and VF13 as sources
if EAFSRC is 1. The relay calculates the alpha quantity, Valpha, as shown in
Figure 3.71 using the mapped sources. Note that the alpha quantity is based on
the instantaneous secondary voltage samples from the mapped resources and
is an instantaneous quantity.
Table 3.54 Frequency Measurement and Frequency Tracking Ranges
Frequency
Range (Hz)
Measures
Frequency
Tracks
Frequency
FREQOK FREQFZ
40–65 Y Y 1 0
20–39.99 Y N 1 0
65.01–80 Y N 1 0
Below 20 or
above 80
NN01
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