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Topic Title: Switchboard with three transformer incomers
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Created On: 04 June 2013 10:56 AM
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 04 June 2013 10:56 AM
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makisman

Posts: 9
Joined: 05 October 2007

Hi,

I am looking for guidelines for rating the busbars of a switchboard with three busbars and three transformer incomers. Any information will be appreciated.

Regards,

Efthymios
 04 June 2013 12:07 PM
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MAXMIRA

Posts: 79
Joined: 25 January 2011

What are the transformerFLC and fault current ratings, are they all on load at the same time
 04 June 2013 12:09 PM
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MAXMIRA

Posts: 79
Joined: 25 January 2011

What are the transformerFLC and fault current ratings, are they all on load at the same time
 04 June 2013 01:27 PM
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makisman

Posts: 9
Joined: 05 October 2007

Hi,

Thanks for your replies.

My questions is about a switchboard with three bus sections, each one having a 2.5MVA transformer incomer (440V secondary). The transformers are sized as 3 x 50% of the total load of the switchboard.

Does the current rating of the busbar have to be based on 2.5MVA or 5MVA load?
What is the difference if the busbar section in the middle has loads connected or not?

Regards,

Efthymios
 04 June 2013 02:12 PM
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jimmyngcheong

Posts: 4
Joined: 23 December 2002

My understanding is that the rating of the busbar must cater for the expected total load of the installation over its lifetime and there are normally some provision to allow connection of future circuits. This is the first data which you must determine. You mention 5 MVA.
You must also consider whether all 3 transformers could be operated in parallel, this would have a strong bearing on the fault levels. From your description, it seems that each transformer can carry 50% of the total load, so that one transformer is sized for the total load. Is this correct?
For your last question, any section of the busbar must be able to handle the total full load

-------------------------
Jimmy Ng
 04 June 2013 02:40 PM
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makisman

Posts: 9
Joined: 05 October 2007

Hi,

- Total load: 3.2MVA (4198A)
- Transformer size: 2.5MVA (3280A)
- Switchboard with three bus sections
- Busbar rating: 4000A
- Bus couplers: Normally Open
- Transformers in parallel only during changeover

The transformers are 3 x 50% so the total load that can be supplied from the switchboard should be 5MVA.

Any comments on this configuration in terms of the busbar rating?

Regards,

Efthymios
 05 June 2013 02:31 PM
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jimmyngcheong

Posts: 4
Joined: 23 December 2002

From your information, it seems that at least two transformers would be needed to supply the total load. This would be the case if one transformer is out of service(OOS), for instance being under maintenance. This would mean that the bus couplers might need to be closed to continue supplying the section for which the transformer incomer is OOS or other reason. Would this be a reasonable mode of operation?In which case the busbar should be able to handle the total load current and with some spare capacity to cater for any future machinery/equipment which the building users would most likely add/replace/upgrade over the years. I am inclined to follow on the sizing of the transformers (2x2.5 MVA) as preliminary without knowing all the factors ( installation use, essential loads, etc...)

The busbar fault levels would be determined by whether a scenario could be envisaged that all three transformers are connected with all three bus couplers closed due to manual intervention? Hope this helps

-------------------------
Jimmy Ng
 05 June 2013 02:36 PM
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jimmyngcheong

Posts: 4
Joined: 23 December 2002

I mean if the two bus couplers are closed, effectively the 3 transformers would be operating in parallel. The equivalent transformer impedance would then be much lower.

-------------------------
Jimmy Ng
 05 June 2013 02:43 PM
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makisman

Posts: 9
Joined: 05 October 2007

Hi Jimmy,

Thanks for your reply.

I would imagine that the two bus couplers are normally open, with only one of them closing when one transformer trips.

My question is related to the busbar current rating. With 3 x 50% transformers (2.5MVA), the total load that can be supplied by the switchboard is 5MVA (6560A). The busbar rating is 4000A.
If I assume that more or less the load is equally split between the two busbars, each busbar will be connected to maximum 3280A. So, the busbar current rating has some margin for unequal loading.

Regards,

Efthymios
 06 June 2013 08:43 PM
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Zuiko

Posts: 521
Joined: 14 September 2010

If you have two 5% impedence transformers in parallel this is an effective impedence of 2.5%; your short circuit rating is going to be 40x your total maximum load; which is an enormous value, so if you are going to run your transformers in parallel for any period of time, you need to design for this.

Edited: 10 June 2013 at 10:04 PM by Zuiko
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