Wye-wye autotransformer

[Dave Colton]

On a 3 phase wye-wye auto transformer do I have to connect the neutral if the load doesn't require a neutral?

Thanks

 

[jim dungar]

On a 3 phase wye-wye auto transformer do I have to connect the neutral if the load doesn't require a neutral?

Thanks

Yes, you need the source neutral if you want the transformer to perform correctly during imbalanced loading.

 

[Dave Colton]

Thank you. So neutral on the primary and the secondary neutral should be left floating?

 

[GoldDigger]

Thank you. So neutral on the primary and the secondary neutral should be left floating?

To be clear (since I am not completely sure what you are saying):

1. The words primary and secondary are perhaps not the best to use for an autotransformer since there are no separate primary and secondary windings.
2. The source neutral must be connected to the wye point of the autoransformer. The output circuit is treated as any other grounded wye circuit under the NEC, namely if the neutral conductor is not used at the load it does not need to be carried to the load.
3. Since this is the furthest thing possible from an SDS, the output circuit EGC must be present and connected to the input circuit EGC and there will not be a ground-neutral bond at the autotransformer.

.

 

[GoldDigger]

This might also be confused with a situation that has been the subject of a lot of discussion in other threads, namely the wye primary, delta secondary isolation transformer. In that case, regardless of the load, the primary wye point must be left unconnected to avoid potentially catastrophic circulating currents in both primary and secondary. (Utilities can get away with connecting the wye point since they are in a position to tightly control the relationship of the three incoming phase voltages.)

The situation in this thread is just a delta load rather than a delta wired secondary, so none of that discussion applies.