SHORT CIRCUIT FLOW ON A BUS
SHORT CIRCUIT FLOW ON A BUS
(OP)
Hi gentlemen, I am presently making some evaluation of an existing protection scheme. I have here an 11kV switchgear with three bus bar sections (bus A, B and C) connected by normally closed tie breakers. There are three incomers (one incomer in each bus section).
Neglecting load contribtions and considering only the incomer contributions. If I have a three phase short circuit at an outgoing feeder at Bus A, upstream incoming contribution of equal value (say 3000A)will flow through the three incomers. Will CT of the bus tie connecting bus B-C sense 3000A and the CT of the bus tie connecting bus A-B see 2 x 3000A = 6000A considering current flow direction to the faulted outgoing feeder at bus A? Or will both bus ties see total current of 3 x 3000A=9000A regardless of any outgoing feeder fault?
Thanks and best regards...
Neglecting load contribtions and considering only the incomer contributions. If I have a three phase short circuit at an outgoing feeder at Bus A, upstream incoming contribution of equal value (say 3000A)will flow through the three incomers. Will CT of the bus tie connecting bus B-C sense 3000A and the CT of the bus tie connecting bus A-B see 2 x 3000A = 6000A considering current flow direction to the faulted outgoing feeder at bus A? Or will both bus ties see total current of 3 x 3000A=9000A regardless of any outgoing feeder fault?
Thanks and best regards...
GO PLACIDLY, AMIDST THE NOISE AND HASTE-Desiderata






RE: SHORT CIRCUIT FLOW ON A BUS
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RE: SHORT CIRCUIT FLOW ON A BUS
GO PLACIDLY, AMIDST THE NOISE AND HASTE-Desiderata
RE: SHORT CIRCUIT FLOW ON A BUS
RE: SHORT CIRCUIT FLOW ON A BUS
A much better solution would be to protect the bus sections with bus differential relays and let the feeder relays protect the feeders. That way you will get fast selective clearing of faults on the bus without the delay inherent in an overcurrent scheme.
RE: SHORT CIRCUIT FLOW ON A BUS
Going back to my first subject above. Will the bus section CTs (connected to bus sections: AB and BC) see the total short circuit(all incomers + load side contributions) for a 3 phase short circuit at an outgoing feeder in Bus 'A'?
Is the assumption I made on the first figure indicated by RalphChristie above correct and be sense by the bus section CT and relay. By the way, the bus section protection only employs a non-directional overcurrent relay. Yes a bus differential protection is the best solution but for a stone-aged existing switchgear, I guess we will end up for a switchgear replacement as a strong recommendation.
GO PLACIDLY, AMIDST THE NOISE AND HASTE-Desiderata
RE: SHORT CIRCUIT FLOW ON A BUS
RE: SHORT CIRCUIT FLOW ON A BUS
Fault contributions from incomer's and other feeders connected to Bus A will not flow through the bus sectionalizers if the fault is on a feeder connected to Bus A.
RE: SHORT CIRCUIT FLOW ON A BUS
Actually, if it was possible and out of a wild imagination, what current I will measure if I were to put a current measuring device at the bus sections for a fault on a bus A feeder. Am i going to see a value as Ralph's first diagram or I will see total (9000A) on any of the buses?
GO PLACIDLY, AMIDST THE NOISE AND HASTE-Desiderata