New 11KV building supply

[johno12345]

Hi,

The factory where I work has recently had a new 11KV transformer installed and I am a little confused. I work in the IT department and we are having trouble with our mains supply. We have had someone in to monitor our supply to our dis board and they immediately noted that the phase rotation was wrong and there are harmonics on it. They looked at the new transformer and this is where I get confused. The REC wouldnt supply us with an earth on the 11KV supply. We have 10A 11KV supply to the transformer and on the other side there are 2 400V 3 phase supplys. Then we have a ground rod which has been bonded to the neutral with a large copper strap. I thought that we didnt bond earth and neutral in the UK. Is this a TT supply or is it PME as the earth and neutral is bonded? It just doesnt sound right to me!

The transformer is on tapping 1 which is 105% HV turns. This results in an average voltage of 250V but it aparently should be lower. They now think that there may be a problem on the 11KV supply.

I just dont want my IT stuff getting fried, or me for that matter!

The next thing they suggested is that I connect the Telco functional earth to a seperate bus bar in my dis board and connect the earths from all the outgoing circuits to that so all the end circuits have a clean earth. Wouldnt that cause problems with multiple earth paths? I am at my wits end as the maintenance dept dont seem to know what to do and are trying to ignore it all

Does anyone else work in an IT dept or have experience of how to wire IT rooms up proplerly?

Thanks (sorry to ramble)

Johno

 

[scoobydooby]

Chocky blocks are good, they never let me down

 

[Big_Spark]

Johno, the average voltage in the UK varies between 238.6V and 253.1V under no load conditions (of course there is variation in this). Remember that this is ONLY AN RMS value of the supply.

The odds of the problem being with the 11kV supply are small, though not impossible.

If you feel there are problems then your employer should call in a specialist contractor to undertake a full field test of the 11kV Tx.

I do hope that you are not entering the HV room in the building as that is actually an offence unless you are qualified. This is instant death and if there is a problem with the Tx then no-one should be entering the room without proper precautions. Any accident in that room will instantly involve the HSE and you can guarantee a prosecution will follow if unqualified people are in there or precautions have not been taken.

 

[Stoday]

We have had someone in to monitor our supply to our dis board and they immediately noted that the phase rotation was wrong

How do you know this? If you have three phase motors, they would run backwards and you wouldn't need a monitor to tell you. If you don't have motors, the rotation is irrelevant.

and there are harmonics on it.

Probably generated by your factory. In particular, if there are third harmonics they must have been generated in your factory because they can't pass from the 11kV through the delta connected primary of your HV/LV transformer.

They looked at the new transformer and this is where I get confused. The REC wouldnt supply us with an earth on the 11KV supply.

The DNO (used to be REC) don't have a local earth, so they don't give you one.

I thought that we didnt bond earth and neutral in the UK.

The neutral must be connected to earth at one point only, at the HV/LV transformer.

The next thing they suggested is that I connect the Telco functional earth to a seperate bus bar in my dis board and connect the earths from all the outgoing circuits to that so all the end circuits have a clean earth. Wouldnt that cause problems with multiple earth paths?

Separate earth to the transformer earth might be a solution.

 

[Stoday]

If you feel there are problems then your employer should call in a specialist contractor to undertake a full field test of the 11kV Tx.

I do hope that you are not entering the HV room in the building as that is actually an offence unless you are qualified...

He says the supply is 10A at 11 kV. That's only 190kVA, so unless his factory has other transformers, this transformer isn't his - it belongs to the utility (Other than in exceptional circumstances a factory is only supplied at HV for a much larger load - 500 kVA or more). And there won't be "an HV room".

 

[kevplumb]

The transformer is on tapping 1 which is 105% HV turns. This results in an average voltage of 250V but it aparently should be lower. They now think that there may be a problem on the 11KV supply.

that puts it 2.5 % under ??

 

[Big_Spark]

He says the supply is 10A at 11 kV. That's only 190kVA, so unless his factory has other transformers, this transformer isn't his - it belongs to the utility (Other than in exceptional circumstances a factory is only supplied at HV for a much larger load - 500 kVA or more). And there won't be "an HV room".

Stoday, I didn't see the need to go into a full explanation as you had, and kept my commentsd simple as the Guy obviously knows nothing about what he is discussing and has got himself involved in sonething he has no business being involved in. The TX, regardless of size, must be in an enclosure or room of some description to prevent mischievous hands investigating (if you know what I mean)

Further. I think his estimate of the loading is off and the 11kV supply is probably 100A or 1.1MW @ 400V

 

[johno12345]

Hi,

We can only get to see the transformer because it is outside in the yard, the only cable connections we can get to are the 400V ones after the transformer that feed the bus bars etc. The transformer is ours and there are several. I only mentioned this one as it is the one that feeds the computer room.

Aparently our electician alters the phase rotation as he wires up each machine to make it run right! My 3 phase stuff is not motors so it doesnt matter.

The monitoring people agree that the harmonics may be caused by the factory machines but they also think that most are caused by my UPSs there are about 30 in total.

I think we will pobably get the earth sorted out so it is seperate from the factory earth. The telco earth goes straight back to the main earth bar on 35mm cable.

Thanks for the replys, it makes a bit more sense now

 

[andy]

but they also think that most are caused by my UPSs there are about 30 in total.

ever considered gettin 1 very large UPS?

 

[johno12345]

yes, but we dont want 1 single point of faliure.... and they are very expensive