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Topic Title: Transformer to sub connection
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Created On: 12 April 2018 06:10 AM
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 12 April 2018 06:10 AM
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callumcox

Posts: 10
Joined: 10 April 2018

Hi in a few weeks we are going to swap a outdate sub over to a new one. The incoming phases on the new Air circuit breaker have 3 connection points per phase,it is rated at 2500amps. So I am thinking using 3 cables per phase back to the transformer. What types of cables you using?,what sizes would you suggest,thanks
 12 April 2018 09:05 AM
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mapj1

Posts: 10249
Joined: 22 July 2004

This is not my area, so wait for others, but I fear without a bit more info, the answer will be like asking 'how long is a piece of string?'

It is very common with high current connections to use parallel cables. This driven by considerations of easier installation (tighter corners,lighter sections) cooling (air can get to more surface), ease of termination.
Then comes the question of how the cables are grouped, and while thermal effects want them apart, magnetic considerations really want the currents that cancel to be close by one another, so multiple trefoils or parallel runs may be the solution, but please not all of one phase together, then a gap and all of the next phase..
The ACB is 2500A - is this a good match to the transformer without adjustment? Often the breaker is programmed to some fraction of its total rating, so the transformer rating is also something you need to know.
Cable size will depend on current per core, and how long the run is, and the route, and in many ways this is the same as any cable, just the numbers are bigger. Multiple cables in the 300-500 mmsq region are common, but only because fatter than that is hard to get and very hard to use, and at about the same price point it may become economic to think about a rigid bus.

If you have a bit more info, containment (concrete channels, up in the air, buried) 5m or 25m, location - lots of bends ?
others may be able to fill in a bit more about 'typical' arrangements
I dont think anyone will be able to tell you a definite design without seeing it though.

-------------------------
regards Mike
 12 April 2018 09:18 AM
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OMS

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Joined: 23 March 2004

As a starting point and working back from the ACB size to a transformer rating of around 1500kVA, then you should be thinking about 3 x single core XLPE/AWA 500mm2 copper conductor cable per phase and neutral - so basically 12 cables into the ACB compartment glanding chamber which it appears to be designed for

Lots more information required including position of the N-E bond and any requirement for restricted earth fault protection, unit protection schemes and intertripping to HV - these will all influence the solution based on the selected protection CT's

As a minimum, make sure the trefoil groups are properly arranged and contra rotating in terms of lay up - put the neutrals between the groups

Other than that, as Mike's post, lots more info required - are you the designer or installer ?

Regards

OMS

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Let the wind blow you, across a big floor.
 12 April 2018 01:22 PM
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ToniSM

Posts: 375
Joined: 21 November 2006

We used either 500mm² cambric or 4 sector 600mm² PVC/PVC singles per 500kVA. The runs between transformer and switchgear would be as short as possible but they were still clamped in trefoil formation.

This may be worth a watch https://www.youtube.com/watch?feature=player_detailpage&v=HmnrJa7Zk0A#t=84s

-------------------------
The reports of my death are greatly exaggerated.
 12 April 2018 02:48 PM
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callumcox

Posts: 10
Joined: 10 April 2018

Hi oms I am just the installer but trying to get the knowledge as this job seems a bit all over the place.Its a 1000kva transformer ,existing cable are doubled on transformer around 95mm size and drop down a concrete duct into the old panel,I worked out the amperage to just over 1400 but got told you can set acb to 1600amps?.
 12 April 2018 02:59 PM
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callumcox

Posts: 10
Joined: 10 April 2018

Hi Mike it's a 1000kva transformer the cable run is about 5mtrs total ,it's ran in a concrete duct with the cable forming just two 90 degree bends.
 12 April 2018 03:30 PM
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mapj1

Posts: 10249
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1000kVA is equivalent to 3 single phase loads of 330kVA - or 330/230 =~ 1.4kA per phase
A breaker of 1.6kA rating, that presumably can be wound down to be less if required, seems about right.

If the old cable really is just 2 runs of 95mm per phase, then presumably the old transformer is a lot smaller (or of course not 230/400) 95mmsq as a single AWA is good for 300-350 A depending on the mechanical details and cooling.
So maybe the old TX was half a megawatt or so, or the wiring was just undersized.

some current ratings for AWA to compare

Take that 1400 Amps and divide by 3, if there are 3 cables per phase, and you probably could go with something a size or so thinner than the 500mmsq OMS suggested up top, as he was assuming a 1500kVA transformer.
Equally over such a short distance the cost is not a great decider. (from rather dodgy memory last time I looked 500mmsq single AWA was ~ £20 per metre, but that was many ages ago and copper prices move up and down like the proverbial thing...)

Arguably having oversized cables that run cool is more economic in the longer term, compared to a design that 'sweats' and needs cooling.

Note also the weights - several kilos per metre, so if you had said 20m long, or if you thought you wanted to try only one cable per phase, you would not be able to move it into position by hand.

There is never any harm in asking questions even if its not your design, especially if you have areas of uncertainty - at least you will know roughly what to expect and what size crimper and spanners to bring in advance.

-------------------------
regards Mike
 12 April 2018 04:03 PM
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callumcox

Posts: 10
Joined: 10 April 2018

Hi Mike thanks for the info,I was a bit vague on the cable size ,still on site working ,typing. My reference to 95mm was a approximate circumference I would expect a 95mm 4 core sea,obviously a single core would be a bigger value.
 12 April 2018 04:35 PM
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callumcox

Posts: 10
Joined: 10 April 2018

Looking at your chart mike 2xawa of 400mm would be adequate,not sure if the transformer would take 3 cables parallel up as the cover has not been off yet. Does this seem about right with the information given?
 12 April 2018 04:45 PM
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OMS

Posts: 22503
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OK - sounds like 185mm2 single core

A parallel pair would be OK for about 1100A - or a bit more if you run them a bit warm and they are installed in a well ventilated concrete trench on cable ladder (or laid loose)

It all sounds a bit undersized for a 1MVA transformer to my mind

Of course, they could be 240mm or 300mm depending on how accurate the "95mm circumference" value is and if they are XLPE or PVC insulated

I strongly suspect they are actually 300mm as that would be a pretty standard parallel pair solution for a 1MVA distribution transformer so you could max out at about 1600A long time operation and a bit more if you were pushing towards emergency operation

Regards

OMS

-------------------------
Let the wind blow you, across a big floor.
 12 April 2018 06:11 PM
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callumcox

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Joined: 10 April 2018

Many thanks OMS ,Mike ,Toni,such a great bunch of information,loving this forum already
 16 April 2018 10:13 AM
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williamjohn

Posts: 196
Joined: 22 November 2010

With short runs of three single core cables in parallel, lay them in trefoil. If not in trefoil, different magnetic paths near single cores in parallel can cause different impedances on each core. This upsets the current balance causing one cable on each phase to carry more than its share of the current. I learnt this the hard way.

If the load is balanced, you may be able to use one single for the neutral (3 per phase plus 1 for the neutral is a common arrangement)

Edited: 16 April 2018 at 11:18 AM by williamjohn
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