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Topic Title: A Quick Design Wiring Question..
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Created On: 08 January 2013 10:40 AM
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 08 January 2013 10:40 AM
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Sanft

Posts: 32
Joined: 22 August 2011

Good Morning Ladies and Gents,

I've currently be posed with a question, and I'm a little unsure of how to progress, to be quite honest with you I think that I've confused myself because the aspect that has got me stumped is quite basic.

The Scenario is as follows:-

An 11kV/400-230v Supply (Delta-Star Configuration) is being distributed to a complex compromising of Residential Buildings and an Office Complex. The Total kVA of the loadings is 974.6. I need to Calculate the current and size the protective device for the primary and secondary of the transformer.

Here is what I have so far:-

(1000 x kVA) / (√3 × U) = I (Amps) <-- This is the formula I am using for calculating current.

Primary Current:- (Star)

(1000 x 974.6kVA) / (√3 × 11,000V) = 51.153A (IL)
(51.153A / √3 = 29.533A (Ip))

Secondary Current:- (Delta)

(1000 x 974.6kVA) / (√3 × 400V) = 1406.713A (IL & Ip)


The only issue I have (and this is laughable, so feel free to poke fun at me!) is that I do not know what size protective device to use on the primary side, the secondary is quite obviously to be a big ol' 1500A Air Insulated Vacuum MCCB.

As for the primary I am torn between using a 32A or a 63A, as I do not have much experience in the field of distribution networks, I am unclear how to progress. My gut feeling is telling me that each phase is carrying 29.533A and therefore a 32A Protective Device (Probably use a BS88 or something of the like) should suffice, but this is where I am stuck, as there is quite a difference between the two.

If there is anyone who can help me out with some input, I would be very grateful,
 08 January 2013 12:02 PM
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broadgage

Posts: 1279
Joined: 07 August 2007

I dont think that BS88 fuses are suitable for 11KV
Leaving that aside, the modern tendancy is to use suitable circuit breakers to protect large transformers, not fuses.

On the LV side of the transformer I would use a large LV fuseboard with suitable cables to each building, generaly in line with DNO practice.
 08 January 2013 12:27 PM
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CPC

Posts: 32
Joined: 16 February 2011

Hi Sanft

Doesnt Star equal IP=IL & Delta equal IP=IL/root 3


Are you sure the primary side of the transformer is in Star? should it not be Delta?

Just a thought, i may be wrong. Someone please correct me if i am.

Many Thanks

CPC
 08 January 2013 12:53 PM
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Sanft

Posts: 32
Joined: 22 August 2011

You are completely correct, my apologies, all I did was put the wrong terminology in there, the figures are unchanged, it should be Delta Primary and Star Secondary:-

I can work out the kinks later, the only real issue is whether or not on the HV side to use a 32A protective device or a 63A Protective Device being that:-

Phase Current - 29~ Amps
Line Current - 51~ Amps

Thank you for the responses thusfar

Andy
 08 January 2013 01:03 PM
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AJJewsbury

Posts: 11349
Joined: 13 August 2003

I can work out the kinks later, the only real issue is whether or not on the HV side to use a 32A protective device or a 63A Protective Device being that:-

Phase Current - 29~ Amps
Line Current - 51~ Amps

It might be clearer if you think about where in the circuit the fuses will be connected - will they be in the line conductors or in series with the phase windings?

- Andy.
 08 January 2013 01:25 PM
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Sanft

Posts: 32
Joined: 22 August 2011

I think this is the issue as I'm not familiar with this type of work-

I'm not sure by practice where the protective device physically should be located, I'm terribly sorry for being inexperienced, is it viable to do it either way and it be safe?

I personally would have thought each phase would be individually fused at the secondary winding, due to the fact that if one phase was to encounter a fault, then it would be that phase alone that the supply would be lost to, and the other two would still be operational- but I'm a novice in this department,

What do you think?
 08 January 2013 02:32 PM
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AJJewsbury

Posts: 11349
Joined: 13 August 2003

Fair enough. You might have to wait for someone like Alan C to show up for a definitive answer on the HV side, but from what I've seen of pole-mounted transformers, the fuses on the HV side tend to be outside of the transformer casing, with the delta connections made inside (you just see three terminals on the HV side) - so the fuses are in the supply lines - so I would have thought they should be matched to your line current, rather than phase current.

You might want to add a bit for losses in the transformer, magnetising/surge currents and so on though.

- Andy.

(edited to correct typo)

Edited: 08 January 2013 at 02:39 PM by AJJewsbury
 08 January 2013 02:37 PM
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AJJewsbury

Posts: 11349
Joined: 13 August 2003

BTW - I suspect that there's a bit more "creativity" to sizing HV fuses - e.g. are they just providing overload protection to the HV windings, or are they providing some degree of protection to the LV side as well? (as the LV windings and some wiring will inevitably be before the LV fuses). Consideration of fault conditions and possibly selectivity (discrimination) with upstream protection too. I'm sure others here could enlighten you if it's of interest.

- Andy.
 08 January 2013 05:01 PM
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alancapon

Posts: 5759
Joined: 27 December 2005

HV fuses are typically for fault protection, not overload. They need to be sized to grade with the largest fuse you will use on the LV side of your transformer, which would usually be dependant on the size of the transformer. For a 1000kVA transformer, I would probably opt for an 80A HV fuse, as long as that graded with the rest of the HV network.

These days for new installations, circuit breakers are more common, as HV fuses will generally be oil-immersed in metal clad switchgear (with the corresponding maintenance and possibly environmental issues depending on site location). With a circuit breaker, you will have either a protection relay to set on the HV side, or time-lag fuses to choose. The manufacturer of the switchgear will often provide a table of suggested values against various standard transformer sizes if your company does not have a standard table of its own.

Regards,

Alan.
 10 January 2013 08:10 AM
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Sanft

Posts: 32
Joined: 22 August 2011

Thanks for all the help guys-

I opted to go for a 63A, Vacuum Circuit Breaker on the HV side, and an 1600 Air Circuit Breaker on the LV Side-
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