6.92
SEL-421 Relay Instruction Manual Date Code 20171021
Protection Applications Examples
EHV Parallel 230 kV Underground Cables Example
You can select a common time delay or an independent time delay per zone for
phase and ground-distance protection. If you choose independent timing, evolv-
ing faults (such as those changing from single phase to multiphase) cause the
timer to reset and result in additional delay. Select common time delay for this
application.
ECDTD := Y. Distance Element Common Time Delay (Y, N)
The SOTF (switch-onto-fault) protection logic permits tripping by specified pro-
tection elements for a settable time after the circuit breaker closes.
ESOTF := Y. Switch-Onto-Fault (Y, N)
Do not enable the out-of-step logic for this application example.
EOOS := N. Out-of-Step (Y, N)
Do not enable the load-encroachment logic; the minimum apparent load imped-
ance is outside the mho phase-distance characteristics.
ELOAD := N. Load Encroachment (Y, N)
Use Level 1 high-set instantaneous phase overcurrent element for SOTF protec-
tion.
E50P := 1. Phase Instantaneous/Definite-Time Overcurrent Elements (N, 1–4)
This application does not require residual ground overcurrent protection.
E50G := N. Residual Ground Instantaneous/Definite-Time Overcurrent Ele-
ments (N, 1–4)
Enable three levels of negative-sequence overcurrent protection. Use these nega-
tive-sequence current level detectors in conjunction with the communications-
assisted tripping scheme.
E50Q := 3. Negative-Sequence Instantaneous/Definite-Time Overcurrent Ele-
ments (N, 1–4)
Use inverse-time overcurrent protection to provide backup protection for high-
resistance ground faults. The 51S1 element provides backup protection for unbal-
anced faults if both the communications-assisted and step distance protection fail
to operate.
E51S := 1. Selectable Operating Quantity Inverse-Time Overcurrent Element
(N, 1–3)
The relay automatically calculates all of the ground directional elements settings
when you select AUTO or AUTO2.
E32 := AUTO2. Directional Control (Y, AUTO, AUTO2)
Use the POTT trip scheme to quickly clear faults internal to the protected line.
ECOMM := POTT. Communications-Assisted Tripping (N, DCB, POTT,
POTT2, POTT3, DCUB1, DCUB2)
Fuses or molded case circuit breakers often protect potential transformers. Oper-
ation of one or more fuses, or molded case circuit breakers, results in a loss of
polarizing potential inputs to the relay. Loss of one or more phase voltages pre-
vents the relay from properly determining fault distance or direction.
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