Sel 551 - Apply the Feeder Relay Negative-Sequence Overcurrent Element (Guidelines 1 to 3)

F.4
SEL-551 Relay Instruction Manual Date Code 20110408
Setting Negative-Sequence Overcurrent Elements
Coordinating Negative-Sequence Overcurrent Elements
I
F
= Maximum load current through feeder relay = 450 A
I
R
= Maximum load current through line recloser = 150 A
51F = Feeder relay phase time-overcurrent element
51QF = Feeder relay negative-sequence time-overcurrent element
51R = Line recloser phase time-overcurrent element (phase “slow curve”)
Traditional Phase Coordination
Figure F.3 Traditional Phase Coordination
51F: pickup = 600 A (above max. feeder load, I
F
)
51R: pickup = 200 A (above max. line recloser load, I
R
)
Figure F.3 shows traditional phase overcurrent element coordination between
the feeder relay and line recloser phase overcurrent elements. Phase
overcurrent elements must accommodate load and cold load pickup current.
The 450 A maximum feeder load current limits the sensitivity of the feeder
phase overcurrent element, 51F, to a pickup of 600 A. The feeder relay cannot
back up the line recloser for phase faults below 600 A.
Apply the Feeder Relay Negative-Sequence Overcurrent Element
(Guidelines 1 to 3)
Applying negative-sequence overcurrent element coordination Guideline 1 to
Guideline 3 results in the feeder relay “equivalent” phase overcurrent element
(51EP) in Figure F.4. Curve for 51F is shown for comparison only.
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