11000 volt transformer

[333rocky333]

At work today we had some bods come to work on a Hv transformer.

EDF came and isolated it at there one first.

The EDf bloke locked it off and said

"He had fitted Earths"
Any one know what he meant by that last bit and what they did.

 

[bernardgreen]

He had connected earths to the phase conductors of the isolated section of the 11Kv circuit to reduce the risk to people working on it from arc overs at the isolation points or some other un-expected re-energisation of the section.

 

[RF Lighting]

Presumably the incomming phases will have been shorted to earth to remove any induced voltage.

 

[333rocky333]

Thanks, thought it was something like that, but could not see what he did.

Worryingly the other guy (NOT edf) worked on the other one by testing with a multimeter. No test lamp, proving unit to be seen

 

[holmslaw]

..

 

[333rocky333]

So why would the store have there own fitted 20 metres away , rather than be fed straight from the EDF one.

The only advantage I see is having there own air breaker, that they can access themselves.

 

[333rocky333]

But on restoring power, the above procedures are reversed including getting matey boy to confirm his work is complete ie permit to work signed off.

And then test to confirm ok to switch 11kv back on.

Yes , there was some paperwork and a few phone calls before he switched on.

 

[bernardgreen]

When an insulated failed on an 11Kv pole route in the field behind us the crew hammered in earth rod and croc clipped heavy earth cable onto all three phase lines.

 

[mfarrow]

The store won't have their own transformer I wouldn't have thought - just a 400V panel with incoming isolator.

The reason for the earthing in a field is probably because they're a long way from the switchgear - no nice local Ring Main Unit to switch and earth down.

 

[333rocky333]

The store won't have their own transformer I wouldn't have thought - just a 400V panel with incoming isolator.

Its 11000v alright, most of the stores have them and EDF wont touch them as there not theres, just wonderd why they need there own one as opposed to the Edf one closeby

The 800amp 415v output then feeds into the store panel main acb inside the store

There big stores.

 

[RF Lighting]

Maybe it was cheaper for the stores to supply and install their own private 11kV transformers, rather than getting the DNO to do it.

IIRC Spark123 has some experience of privately owned 11kV stuff, and may have a few ideas?

 

[efiste2]

The earth in the field would be called a DRAIN EARTH, being local to the point of work. There would have also been at the very least one PRIMARY earth usually applied at the Ring Main Unit and listed on the safety document/permit to work. As for the EDF guy, more than likely he will have ISOLATED the LV Transfomer links or ACB and the then ISOLATED & EARTHED the HV side of the Transformer via the HV circuit breakerm and the either issuing a Permit to Work or an ISOLATION and EARTHING document to the local engineer. As mfarrow says the store will have an LV DIST panel and ACB.

 

[thetman]

Most large stores have thier own transformer which they own. The need qualified maintenance guys to operate this stuff. The local DNO supply a ring main unit into the HV ring.

If the store takes and HV tarrif it's cheaper if the demand is great enough.

This is how Asda stores work.

 

[333rocky333]

Thinking about it, I suppose the store would also have a constant supply from the HV side and therefore not be affected if the edf one went down.

 

[efiste2]

To add to RF's reply, most larger supermarkets and stores do indeed have thier own 11000/415 transformer, which are either fed from a DNO 11kv switchpanel board or via an 11kv RMU / 11kv Metering Unit combo. I know in SP netowrks, Any consumer declaring over a 1MVA supply has to have thier own HV transformer.