Transformer Question

Kelter

Hi All,

I'm struggling with a transformer question which I really should know... but am not sure about. So here it is -

A site has a 10kV electricity supply. It then has a transformer to step it down to 400V for use on the site.

If you put a generator in the site which generates at 400V, and wanted to export (forgetting all the syncing and licencing issues etc), could the same transformer be used to step up the electricity to 10kV?

Firstly are they in practice set up to do that

Secondly is there losses, such that the export voltage would only be <10kV

Thirdly would that matter to the grid?


I'd really appreciate anyone's help on this

Thanks

fishdog

If you put a generator in the site which generates at 400V, and wanted to export (forgetting all the syncing and licencing issues etc), could the same transformer be used to step up the electricity to 10kV?

Yes

Firstly are they in practice set up to do that

As far as I know a step up and a step down transformer are exactly the same, so yes.

Secondly is there losses, such that the export voltage would only be <10kV

There are losses if you step up or step down. Tap changes can be made to compensate for this. The losses are copper losses (proportional to the load) and core losses (fixed).

Thirdly would that matter to the grid?

The losses? No. The quality/properties (voltage, frequency etc.) of your supply, yes. There are very strict conditions to be met.

I hope this helps.

DublinDilbert

+1 to everything fishdog says....

It depends on where the "smart" meter is located as to whether the losses would cost you, or the ESB money.

If metered on the 10KV side the losses in the trafo cost you money, when consuming and transmitting power.... If metered on the 400V side the losses cost the ESB money...

The major losses with be I^2 x R in the windings and iron losses in the core of the trafo.

fishdog

If metered on the 10KV side the losses in the trafo cost you money, when consuming and transmitting power.... If metered on the 400V side the losses cost the ESB money...

I never thought of that TBH. I thought that it would only be metered (by the ESB) on the secondary side (415V) through CTs?? Is this not always the case??

Kelter

Excellent, Thanks guys

Bruthal

These transformers are 98 or 99% or more efficient so paying for the losses in the transformer would`t really be a big factor, the copper losses are just the losses that happen in the windings which are the same losses that happen in any conductor, but these losses are paid for by the consumer if metered on the ESB side as stated.

Metering can be done at any point depending on the installation, Intel for example is supplied at 110kv so that certainly wont be metered at 400v, or semperit which was a 38kv supply.
I was workin in metering myself years ago and the metering was often done at voltages of 10kv or above on large installations

Tapping changes are made to compensate for load changes rather than losses, the losses will still be present, big power transformers have automatic on load tap changers, while smaller
10kv to 400v ones have fixed taps usually selected to suit the 10kv input supply actual voltage.

Bruthal

fishdog wrote: »

I never thought of that TBH. I thought that it would only be metered (by the ESB) on the secondary side (415V) through CTs?? Is this not always the case??

The copper losses would be present in a system even without a transformer, the iron losses are probably the only additional losses the transformer would present, because the nearer the transformer is to the load the less 400v cables that are needed and so less of the higher current at the secondary transformer 400v side to have IIR losses on after the meter,

The 10kv windings have a higher R value than the 400v secondary windings but a lower current flowing in them but the 10kv windings are also longer so the copper losses in each winding will be the same in general,

The nearer the utility metering is to the load the more it favours the customer, even a 9kw shower on a 100m run of 6 square cable will have a lower kwh reading if its beside the shower than if its at the other end of the cable at the supply end.

So metering on the 10kv side of the transformer is in effect adding a longer run of cable between the meter and the load and the IIR loses which are present in any cable run are being lost after the meter, whether theres a transformer or not.

Certainly moving the metering from the 10kv side to the 400v side results in a small reduction in KWH readings when consuming units, just like moving the meter from the supply end of that cable feeding the shower to the shower end would also reduce the meter reading

Any thoughts on it?