Delta vs Wye primary

[Mbrooke]

Several 115kv to 33kv step down transformers are scheduled for replacement, we have the option of either going wye grounded or delta for the primary. What are the pros and cons of each option? The units are between 15 and 60MVA.

 

[Power0020]

star star with NER on secondary is very dangerous for overvoltages on secondary in case of ph-g faults on primary,

 

[Mbrooke]

@power0020: Can you elaborate? Why would an NER result in overvoltages in star-star (wye-wye)? Wouldn't a delta winding be more expensive since the lowest voltage in the windings to ground are about 57.7% while a star primary reduces in potential as it comes closer to the neutral?


While a wye primary does have insulation economy, my concern is that it acts as a 'grounding transfomer' in that the delta secondary will try to 'balance' out differences. however, in theory, if all the primaries are wye on the 115kv system, wouldn't the issue be less so?

 

[bacon4life]

A couple of utilities in Washington State use wye-delta-grounded wye 115/12 kV transformers. I think the HV neutral is left isolated and the delta tertiary provides the grounding source for the LV side.

 

[Mbrooke]

@bacon4life: I have heard of those but never seen one in person. The primary is left floating but somehow a delta tertiary establishes a working ground reference in the secondary. Do these have a phase shift?

 

[mgtrp]

The phase shift is no different to a conventional wye-wye transformer - that is, no phase shift unless you go about deliberately messing with the connections to create one.

We (a utility) use star-star (both neutrals earthed) with a delta tertiary in instances where there is some risk of backfeed, so as not to have an unearthed system if the high-side opens up for some reason. Typical examples might be a mine with it's own generation that is capable of synchronising onto our system. In cases where there is no risk, we use a delta-star transformer with neutral earthed.

 

[Power0020]

@Mbrooke: if you have a fault on the primary of a star-start with NER (autotrafo / common NER), the potential rise on healthy phases will be about 73%, this potential will be transferred to the secondary (As the same neutral point).

If the primary is solidly earthed and the secondary is NER earthed, shouldn't be a problem.

you lose the economy virtue if the utility requires a fully insulated wye, this is usually required where non-effectively earthed systems are present as the neutral voltage can jump ( very true with NER on HV side)

The delta smaller current will have a smaller winding CSA by then, but you need to have another remote end with a neutral point so you can get good ground fault protection.