cable distance transformer to switchgear

[mrbj]

Hi,
Can anyone tell me if there is a minimum allowable cable distance from the LV output terminals of a MV/LV transformer, say 1MVA, 11kV/433V to the CB, and what regulations/standards will apply in the UK.
Thankyou.

 

[mrbj]

Oooops,
that should be "maximum alllowable cable distance......"
Regards, and thanks in advance.

 

[Marmite]

There isn't a minimum distance, in fact many are close coupled, mounted direct onto the transformer tank. There is a maximum length of cables between the transformer and the C/B, based on whether they can be adequately protected by the 11kV O/C protection, as the 11kV winding is generally delta, LV star.
Regards
Marmite

 

[dpc]

Minimum or maximum?

David Castor

http://www.cvoes.com

 

[ScottyUK]

The LV windings can be protected very well using unit protection, but it's a fairly expensive scheme for a small transformer and requires an 11kV source capable of accepting a trip signal, which rules out a simple fuse feeder. It certainly removes the distance limit though.

What's the reason for the question?


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

 

[mrbj]

Thanks to all.
The reason I ask is that a couple of years ago I was involved in a scheme where the 11kv/433V Dyn11 Tx (No onboard isolator) was installed and the ACB was some 40-50m distance away. The DNO made us install a local ACB within 15m (cable length) of the Transformer LV terminal box. I cant remember the reason why, but am thinking that there must be some standard/reg/code that states the limits for the length of the LV cables in such circumstances.
Or is just down to upstream protection settings etc..
I am currently at the start of a similar scheme and would have liked to have clarified the need (or not) for a local isolation point as above.
Your views much appreciated.
Many thanks.

 

[mastergate]

I'm not sure there is a direct indication for a maximum distance between transformer and ACB, but i know there are some limitating factors:
- Protection: there are limitations for cable lenght depending on neutral system earthing
- Voltage drop which increases with cable lenght and so leads to oversizing cables. A big number of cables is a bad issue considering limitation in connection possibilities for both sides (transformer secondary and ACB terminals)
For economic side: i think it's better to extend MV cable than to add more LV cables.

**for 1MVA for exemple:
Primary current: 52.5 Amps --> 1 cable 3x35mm.sq Copper(assumption)
Secondary current: 1333.4 Amps --> ~ 9 cables 240mm.sq (3c/phase) (assumption)
For erection, it's very difficult to lay and to connect such a number of cables.

BEN.

 

[Miguel Villarroel]

I think DNO rules about a voltage drop border at ACB you can´t exceed, one should be the one who set the voltage profile to be used, but rules are rules

If there was no electricity there would be no internet. Good point, don´t you?

 

[rbulsara]

The DNO rules appear to mirror NEC (NFPA 70/ANSI/IEEE) requirements for conductor protection on secondary side of a transformer. NEC Article 240 gives a good reasoning too, which is the fact that conductors on the secondary side of a transformer (other than 2-wire or delta-delta units) are not considered protected by transformer primary overcurrent device. This is particularly true true for ground (earth) faults which are not correctly sensed by primary side devices.

NEC does permit limited (10 feet or 25 feet) unprotected secondary lengths for practical reasons and with some other restrictions.

In general NEC requires conductor overcurrent protection at its source. I would think rules elsewhere would be very similar.

Rafiq Bulsara

http://www.srengineersct.com

 

[mastergate]

r.Bulsara: do you have an some information for IEC equivalent, please?

 

[rbulsara]

Sorry, no, but engineering logic remains the same in this case.

Rafiq Bulsara

http://www.srengineersct.com