Auto-Transformers

[alingstone]

Here's the picture. An 11kV 3 phase overhead system is supplied from a 33/11kV Dyn transformer. All along the route of the 11kV line are Dyn connected distribution transformers 11/0.4kV. A few kilometres down the 11kV line is connected an auto transformer with ratio 2:1 to step the 11kV voltage up to 22kv. The neutral point of the Autotransformer is connected to earth, thus effectively making the 11kV line a 4-wire system by the use of the earth. On the 22kV side of the aut are Dyn connected distribution transformers 22/0.4kV. All distribution transformers be them at 11kV or 22kV are either single phase connected or 3 phase delta connected. My question is, if the loads are unbalanced on the 22kV side will the neutral point of the autotransformer pass current back to the neutral point of the 33/11kV Dyn transformer? My theory is that it won't as the loads are all single phase or 3 phase delta connected. Can someone elaborate?

 

[Kiribanda]

alingstone,
When there is an unbalanced loading on the 22 kV system, the pos & neg seq. components have no effect on the neutral connections.Also since there is no common path on the 22 kV network, the zero seq. components cannot exist.Therefore,during unbalnced loading on the 22 kV network there is no impact on any neutral-earth connections even on the 11 kV network.
But during an earth fault on the 22 kV network the zero seq. components can pass to the 11 kV side and if there are any relays sensing zero seq. components on the 11 kV side, they might trip depending on their pick up settings.

 

[alingstone]

Thanks Kiribanda you've confirmed what i had theorised. We have a relay on the 11 side which covers 11 & 22, i will set the earth fault to a sensitive value of 5 Amps now that i don't have to worry about unbalanced loads on th 22.

 

[jghrist]

If by single phase connected, you mean Ø-Ø, then no, there will be no zero-seq current in either the 22 kV or 11 kV. If single phase connected means Ø-n, then there will be zero-seq currents in both.