Sel 421 - Page 287

U.6.25
Date Code 20090715 User’s Guide SEL-421 Relay
Testing and Troubleshooting
Checking Relay Operation
An ac connection check uses relay metering to verify that the relay current and
voltage inputs are the proper magnitude and phase rotation (see Examining
Metering Quantities on page U.4.33).
Commissioning tests help you verify that you have properly connected the
relay to the power system and all auxiliary equipment. These tests confirm
proper connection of control inputs and control outputs as well (see Operating
the Relay Inputs and Outputs on page U.4.56).
Brief functional tests and element verification confirm correct internal relay
processing.
Selected Element
Tests
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 M2P
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
element pickup settings to test the instantaneous and definite-time overcurrent
elements. 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 Selection on page R.1.2 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 currents 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 negative-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 3I
2
calculation exceeds the corresponding negative-sequence current pickup
setting. If balanced 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)
Courtesy of NationalSwitchgear.com