3 or 4 pole bus section switches

[Freeportpower]

Gents,

Imagine 3 off 11kv/400V transformers feeding an LV switchboard with bus section switches between each section. All interlocked to prevent transformers operating in parallel. (2 from 3 scheme).
I say the bus section switches should be 4 pole to prevent earth fault circulating current and ensuring disconnection of the faulty circuit.
A fellow engineer insists on 3 pole and a solid neutral throughout.
Any thoughts ?

 

[davidbeach]

Solid Neutral; grounded in exactly one location. Will save you vast amounts of headache down the road.

 

[rbulsara]

Either is doable.

Your fear of adverse effect on earth fault detection is valid. (It is not exactly the circulating currents but the multiple paths available for earth fault current, resulting in improper sensing).

4 pole breakers are less common and more expensive, hence seldom seen, at least in the USA.

There are so called 'modified differential schemes' available which only requires a few CT wires between CTs at each source that will allow proper sensing of earth (ground) fault current for multiple sources. Most switchgear manufacturer will provide it upon request.

See attached, for example:

http://www.geindustrial.com/Newsletter/spring05multiple.pdf

davidbeach's recommendation is also valid, except that for larger transformers many owners/engineers do not feel comfortable not seeing the neutral at the transformer firmly bonded to earth.






Rafiq Bulsara

http://www.srengineersct.com

 

[Freeportpower]

Thank you both.

The type of power system will be TNS according to the UK standard BS7671 so I think the 4 pole bus section scheme will the most appropriate in this case as the system will normally run with all bus sections open.(Do you call these bus couplers over the pond?) Not too sure about the protection relays described being available here. Earth fault protection could comprise a restricted earth fault scheme backed up with a standby earth fault relay.

 

[rbulsara]

-Nothing wrong with 4 -pole breakers, in fact they offer a simpler solution just expensive.
-The bus couplers are generally called Bus Tie Circuit breakers or just Tie circuit breakers on west side of the pond.

-The relays are not special nor are the CTs, only how the CTs are wired is important.

I am not intimately familiar with nomenclature of UK standards, but the scheme described above is based purely on technical principles, so it would work any where in the world, as long as it fits the type of earth fault sensing applied. Someone from UK would perhaps chime in.

Rafiq Bulsara

http://www.srengineersct.com

 

[ScottyUK]

I'd say you are on the right track with a direct and solid connection from the neutral point of each transformer to the substation earthing mat and to use four-pole breakers. You are absolutely correct in recognising the problems which occur when multiple paths exist through the earthing system in parallel with the neutral conductor. The alternative method is also valid but brings operational difficulties when testing as the neutral can not be considered a 'dead' conductor unless the board is disconnected from the system where it can be tested section by section when a four-pole scheme is used. The same applies to testing neutral-earth connections.

Regarding terminology, a switch which splits a single busbar substation into two half bars is a bus section switch or bus sectioniser, while a bus coupler is a switch which links the main and reserve busbars of a double busbar substation. Two separate functions and two different names. In the UK 'bus coupler' is widely misused to describe a bus sectioniser by people unfamiliar with double busbar arrangements.


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If we learn from our mistakes I'm getting a great education!

 

[sibeen]

I would say it depends upon the downstream application. I've had some bugger of problems with 4 pole switching on a TNS system where the downstreeam usage was a computer centre.