R.1.15
Date Code 20111215 Reference Manual SEL-421 Relay
Protection Functions
Frequency Estimation
example, to switch to an alternate voltage, set ALINEV to VZ (enables setting
ALTV) and then set ALTV to PLT31. To perform the switch use the protection
latch control inputs PLT31S and PLT31R (Set and Reset, respectively).
Frequency Estimation
The SEL-421 measures power system frequency based on fixed rate sampled
voltages. The relay measures the time between zero crossings, filters the time
measurement, and processes the measurement with an algorithm to remove
line anomalies.
The output of the frequency estimation routine is used to alter the protection
algorithm sample rate which is in samples per cycle. The protection
algorithms are processed at 8 samples per power system cycle.
In most applications, no settings changes are required for frequency
estimation. Advanced Global settings (see Table 1.12) are provided for special
cases, and these allow selection of the voltage input terminal(s) to be used as
frequency estimation inputs. An alternate frequency source may also be
enabled and selected by control from a SEL
OGIC control equation.
The SEL-421 will not measure frequency using the voltage from any phase
that has an open-pole condition. For example, with the factory default settings
listed in Tabl e 1.1 2, if Relay Word bit SPOB is asserted, the signal present on
the VBY input is treated as zero volts and will not be a contributing factor to
the frequency estimate. If all three poles are open, or all selected voltage
Table 1.12 Frequency Estimation
a
a
These settings are available only if you have enabled Global advanced settings, EGADVS := Y.
Label Prompt Default Value
EAFSRC Alternate Frequency Source (SELOGIC Equation) NA
VF01 Local Frequency Source 1 (ZERO, VAY, VBY, VCY,
VAZ, VBZ, VCZ)
VAY
VF02 Local Frequency Source 2 (ZERO, VAY, VBY, VCY,
VAZ, VBZ, VCZ)
VBY
VF03 Local Frequency Source 3 (ZERO, VAY, VBY, VCY,
VAZ, VBZ, VCZ)
VCY
VF11 Alternate Frequency Source 1 (ZERO, VAY, VBY,
VCY, VAZ, VBZ, VCZ)
ZERO
VF12 Alternate Frequency Source 2 (ZERO, VAY, VBY,
VCY, VAZ, VBZ, VCZ)
ZERO
VF13 Alternate Frequency Source 3 (ZERO, VAY, VBY,
VCY, VAZ, VBZ, VCZ)
ZERO
Table 1.13 Frequency Estimation Outputs
Name Description Type
FREQ Measured system frequency (Hz) Analog Quantity
FREQOK Measured frequency valid
a
a
Deasserts approximately two seconds after a pole-open condition or after all frequency
estimation voltages drop out.
Relay Word bit
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