Distribution Transformer

I probably shouldn't attempt to reply to questions like this, but it is rather intriguing. I'm trying to guess why you might ask the question--maybe you have a (new?) 20 kW Hydro Power Plant that you want to use both to power some of your own load and connect to the utility with the thought of selling your surplus power to them, and drawing power from them when the Hydro Power Plant is not sufficient?

A transformer is inherently a step up and a step down device--from the perspective of the low voltage side, it is a step up device, from the perspective of the high voltage side, it is a step down device.

A better way to look at it is from a power flow perspective:

• If the hydro station is generating 20 kW, and your local load needs 24 kW, 4 kW will be drawn from the utility and will be stepped down through the transformer to supply the load that the hydro station cannot.
• If the hydro station is generating 20 kW, and your local load needs 20 kW, the transformer will neither step up or step down any power.
• If the hydro station is generating 20 kW, and your local load is only using 16 kW, the excess 4 kW will be stepped up through the transformer and flow to the utility / other customers of the utility.

I've ignored a few things:

• The transformer will use some (small) amount of power to keep its windings energized in all three cases.
• In making a connection like this to a utility, the utility will have various requirements for things like metering and (safety) relaying--I have no idea what those requirements might be in Germany (I mean, obviously, if they prohibit such a connection, what I described is null and void).

Oh, the star-delta makes no difference to what I described, but the utility or regulators may have requirements that force the secondary side to be star (for example, if they require a solidly (or resistively) grounded neutral on the customer side of the transformer).

Hope this helps!

Thank you

To step up the supply you feed the 0.4kV winding & take supply on the 11kV side. To step down - it is the reverse feed at 11kV take supply at 0.4kV. I have never heard of anyone ever doing it similtaneously. If you have a need to get 20kW to the power plant then you need to look at the following:

1. what is the protection on bothe neds of the cables.

2. What is the cable size you are using to feed the load,

3. After working out these & their losses (the load they take) - add that to the 20kW (plus starting current) & you can size the transformer needed.

4. You will then need to look at the cable voltage drop so that the tap setting on the transformer is correct eg output voltage may be 420V to give you 400V at the plant end.

Note the cable size will need to be determined by the voltage drop across the length, the starting current, overload conditions etc. this is the critical part before you do anything else.

If you look up the Schneider website they have links to calculations & technical papers on these things. Also most cable manufacturers have the data sheets on the cables & voltage drop per metre. They will help you if there is a slae in it for them.

simultaneously; means happening at the same moment or at the very same time interval and in consideration of your query my answer is NO.

Nevertheless the transformer can be step up or step down periodically dependent on the conditions imposed for the nature of duty.