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3.9
Date Code 20171021
Instruction Manual
SEL-421 Relay
Testing
Checking Relay Operation
Brief functional tests and element verification confirm correct internal relay pro-
cessing.
This subsection discusses tests of the following relay elements:
➤
Overcurrent element: negative-sequence instantaneous, 50Q1
➤
Directional element: negative-sequence portion, F32Q/R32Q, of the
phase directional element, F32P/R32P
➤
Distance element: phase-to-phase mho element, MBC2, of Zone 2
mho distance element Z2P
Testing Overcurrent Elements
Overcurrent elements operate by detecting power system sequence quantities and
asserting when these quantities exceed a preset threshold.
Apply current to the analog current inputs and compare relay operation to the ele-
ment pickup settings to test the instantaneous and definite-time overcurrent ele-
ments. Be sure to apply the test current to the proper input set (IW or IX),
according to the Global Current and Voltage Source Selection settings (ESS and
ALINEI, for example) to accept the input. See
Current and Voltage Source Selec-
for more information.
Phase Overcurrent Elements
The SEL-421 phase overcurrent elements compare the phase current applied to
the secondary current inputs with the phase overcurrent element pickup setting.
The relay asserts the phase overcurrent elements when any of the three phase cur-
rents exceeds the corresponding element pickup setting.
Negative-Sequence Overcurrent Elements
The SEL-421 negative-sequence overcurrent elements compare a negative-
sequence calculation of the three-phase secondary inputs with the corresponding
negative-sequence overcurrent element pickup setting. The relay makes this neg-
ative-sequence calculation (assuming ABC rotation):
3I
2
= A-Phase + B-Phase (shifted by –120°) + C-Phase (shifted by 120°)
The relay asserts negative-sequence overcurrent elements when the 3I2 calcula-
tion exceeds the corresponding negative-sequence current pickup setting. If bal-
anced currents are applied to the relay, the relay reads 3I
2
0 (load conditions)
and does not pick up the negative-sequence overcurrent elements.
For testing, apply current to a single phase of the relay, causing the negative-
sequence overcurrent elements to operate. For example, assume 1 A of current on
A-Phase and zero current input on the B-Phase and C-Phase:
3I
2
= 1 A + 0 (shifted –120°) + 0 (shifted 120°) = 1 A (a simulated ground
fault condition)
Ground Overcurrent Elements
The SEL-421 ground overcurrent elements compare a residual ground calcula-
tion of the three-phase inputs with the residual overcurrent setting. The relay
makes this residual current calculation:
3I
0
= A-Phase + B-Phase + C-Phase
Summary of Contents for SEL-421-4
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