SHORT CIRCUIT FLOW ON A BUS

[nightfox1925]

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...

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[RalphChristie]

Hope I understand your question correctly.

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[nightfox1925]

Yeah you got it Ralph. The issue that bothers me is that I have an overcurrent relay at both bus section circuit breakers and I am presently evaluating and checking whether the operating times would coordinate with the incomers and further upstream in the event of both bus bar A fault and an outgoing feeder fault. Making sure I ma taking the right value of short circuit current flow in the bus sections.


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[jghrist]

Coordination should be easier between feeder relays and bus tie relays because more current will flow in the feeder than in the bus tie. For an inverse relay, that means the feeder relay will operate faster. You still have to coordinate for equal lower currents to cover the case where only one source is supplying current and it flows through both the bus tie and the feeder.

 

[davidbeach]

Directional overcurrent on the bus ties will allow setting differing currents in each direction to allow better coordination with the out going lines.

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.

 

[nightfox1925]

Thank you gentlemen for the suggestions.

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.

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[davidbeach]

The CTs will see the current that flows at that point. If the entire fault current flows at the point the CTs are installed, the entire fault current will be seen. If only a portion of the fault current flows at the point of the CTs, only that portion of the fault current will be seen by the CTs. You need to diagram out the buswork of your particular installation and locate the CTs on the diagram. Then you can trace the flow of fault current (from a fault study) and see what currents are seen at what locations.

 

[jghrist]

Assuming that by

bus section CTs (connected to bus sections: AB and BC)

you mean CTs in the sectionalizing point between Bus A and Bus B, and between Bus B and Bus C in Ralph's diagram, the answer to your question is no.

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.

 

[nightfox1925]

jghrist, the tie breakers are normally closed as I have indicated in my query. The contributions from the other incomers and feeders will flow through the point of fault through the bus sections which are connected normally.

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?

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