5.16
SEL-421 Relay Instruction Manual Date Code 20171021
Protection Functions
Frequency Estimation
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 5.13 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 deas-
serts 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 FREQ and FREQP are no longer valid, but they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 5.12 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.
NOTE: These settings are available
only if you have enabled Global
advanced settings, EGADVS := Y.
Although you have the flexibility to select any of the available voltages for the
frequency estimation, the correlation between the selected voltages and the
breaker poles is fixed as shown in Table 5.15.
Table 5.13 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 N N 0 1
Figure 5.12 SEL-421 Alpha Quantity Calculation
FREQP
VF01
VF02
VF03
1
VF11
VF12
VF13
0
EAFSRC
(SEL
OGIC
)
VFn1 – 0.5 (VFn2 + VFn3)
Valpha
Frequency
Estimation
FREQ
DFDTP
FREQOK
FREQFZ
n = 0 or 1
Table 5.14 Frequency Estimation (Sheet 1 of 2)
Label Prompt
Default
Value
EAFSRC Alt. Freq. Source (SELOGIC Equation) NA
V F 0 1 L o c a l F r e q . S o u r c e 1 ( Z E RO , VAY, V B Y, V C Y, VA Z , V B Z , V C Z ) VAY
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