Sel 551 - Convert “Equivalent” Phase Overcurrent Element Settings to Negative-Sequence Overcurrent Element Settings (Guideline 4)

F.5
Date Code 20110408 Instruction Manual SEL-551 Relay
Setting Negative-Sequence Overcurrent Elements
Coordinating Negative-Sequence Overcurrent Elements
Figure F.4 Phase-to-Phase Fault Coordination
51EP: pickup = 300 A (below max. feeder load, I
F
)
Considerable improvement in sensitivity and speed of operation for phase-to-
phase faults is achieved with the 51EP element. The 51EP element pickup of
300 A has twice the sensitivity of the 51F element pickup of 600 A. The 51EP
element speed of operation for phase-to-phase faults below about 2000 A is
faster than that for the 51F element.
Convert “Equivalent” Phase Overcurrent Element Settings to Negative-
Sequence Overcurrent Element Settings (Guideline 4)
The “equivalent” phase overcurrent element (51EP element in Figure F.4)
converts to true negative-sequence overcurrent element settings (51QF in
Figure F.5) by applying the equation given in guideline 4. The time dial (lever)
and curve type of the element remain the same (if the element is a definite-
time element, the time delay remains the same).
Time (Seconds)
10.0
5.0
2.0
1.0
0.5
0.2
0.1
Phase Current (Amps)
125 250 500 1000 2000 4000
51R
51F
I
F
I
R
51EP