what sequence to install Trefoil cables underground

[cobrauk]

We are to install 2 x 630mm single XLPE CU cables per phase + 1 x 630mm for the neutral can anyone lay their hands on the what sequence to install Trefoil cables direct underground as a official PDF please as we are having a debate at work and its been years since I had to do this.

I think its
..L1 ................L1
L2 L3.... N .....L2 L3

800mm down 300mm apart is that correct ?

 

[Knobhead]

How long is the run? For long runs the phase sequence should cross over along the length.

How are the singles to be restrained?

Have a look for UKPN’s PDF’s about this. I’ve got them somewhere, god knows where.

 

[snowhead]

http://www.eon-uk.com/downloads/cable_laying_requirements.pdf

May be some help

 

[uksparks]

630mm? What are you connecting?

 

[Knobhead]

At 433V about 1000KVA.

If as is most likely a transformer feeding switchgear I’m more interested in how the cables will be restrained in the event of a fault. Buried at 600mm won’t stop the violent magnetic reaction.

 

[Engineer54]

And there was me thinking the days of the reduced neutral were over!!! I hope all the design calculations have been well and truely proven and that load harmonics have been taken seriously. I'm assuming this to be an industrial or a largish commercial supply...

 

[Knobhead]

A type “K” transformer needs equal neutral conductors. To install the 2[SUP]nd[/SUP] neutral on a conventional transformer is a waste of money. One of the reasons the transformer manufacturers only fit a half length neutral termination.

Hospital projects such as you work on, harmonics are going to be a problem and require a transformer with a substantial neutral connection to the star point. Hence the equal neutral.

For bog standard supplies a 3½ core is ample.

I[SUB]n[/SUB] = √((I[SUB]a[/SUB]²+I[SUB]b[/SUB]²+I[SUB]c[/SUB]²)-((I[SUB]a[/SUB]*I[SUB]b[/SUB])+(I[SUB]a[/SUB]*I[SUB]c[/SUB])+(I[SUB]b[/SUB]*I[SUB]c[/SUB])))

If it is a “K” transformer the cables should be run in quadfoil.

 

[Engineer54]

Harmonics have fast become one of the biggest problems for the regional DNO's, especially on TX's that are supplying both industrial/commercial and domestic consumers. Every household these days are generating substantially more harmonics via electronic equipment and gizmos etc than they were just 20 years ago, and it only keeps on increasing. Many of the older DNO reduced neutral cables are at, or close to their limit, not to mention additional currents circulating in the now undersized distribution TX's...

 

[Knobhead]

As per usual that’s down to lack of investment.

I live in the heart of student land, in the three years I’ve been here two new transformers have been installed to meet the demand making four MVA in a half mile area. They’ve all been cobbled in to the existing distribution system using the original 3½ core. The only thing to change is the number of link boxes, basically they’ve halved the lengths of the cables enabling twice the power to be delivered down the same cable.

 

[cobrauk]

Thanks for the reply's all,
Yes its 1000kva transformer feeding 2 industrial units its split 750kva & 250kva between the 2 units, the 250kva is being supplied direct by UK Power to the unit via a normal CT meter and cut out but we have to install the 750kva under ground cable direct to the transformer ourselves which is the first time this has happened, they said some new policy ? has anyone else come across this ?

49meter run to the transformer.

 

[Knobhead]

At that length of run they’ll want some form of OCPD at the transformer otherwise you’re relying on the MV OC protection. Are UKPN providing an ACB at the transformer?

You need to talk to UKPN, they’ll send you their specification. It gives the dimensions of the outgoing terminal chamber. You will be responsible for terminating to the ACB, they will specify the type of termination. I think for a 1200A supply it will be sweated thimbles in to expansion clamps.

How are you proposing the restrain the singles? I say restrain as magnetic repulsion can throw the cores apart with a massive amount of force. Enough force to throw ¼” decking plates in the air and slice an electrician in half. It happened.

PM sent.

 

[Engineer54]

Personally, i've never installed direct buried singles, either MV or LV cables, if we have used singles it's always been a ducted installation and most of our site's buried ducts (which are multi duct systems) have been cast in concrete within the trenches.

 

[Bobster]

I've always installed ladder rack in concrete trenches when using singles like that.

 

[Engineer54]

I've always installed ladder rack in concrete trenches when using singles like that.

The OP though, is talking about burial, and direct burial at that, which frankly i've never seen in the 40 odd years that i've been in this game...

 

[Bobster]

I've seen direct burial of 11KV. It was jointed about 20m from the substation from 2 x 3 core to 6 x single core.

Wasn't the neatest job, and the joint ended up failing. Unsure of how long it had been in service, but it was XLPE so it can't have been all that long!