Delta connected 35 kV transformer

[daviandgo]

A customer recently requested primary service from our 35 kV totally underground distribution system. At this voltage our standard connection is solidly grounded wye-grounded wye transformers to reduce the risk of ferroresonance. The customer already has a 2500 kVA, delta (35kV side)-grounded wye transformer and wants to use it. The three-phase circuit breaker protecting the transformer will be about 40 ft away and connected to the transformer by underground cable. All transformers on this circuit are connected grounded wye-grounded wye. Should we allow this customer to connect to our system.

 

[jghrist]

The three-phase breaker should prevent ferroresonance, which can occur during single-phase switching operations. If you have any single-phase devices ahead of the breaker, you could still have a problem.

It's possible to have ferroresonance with grdY-grdY transformers at 35 kV. See "Check Your Ferroresonance Concepts at 34 kV" by Paul Locke, Philadelphia Electric Co., Transmission & Distribution, April 1978.

 

[busbar]

Delta-primary distribution transformers are common, perhaps more so in industrial applications than for utility systems. I wouldn’t see a problem especially with breaker protection on the transformer primary using the routine compliment of relaying. One advantage of a delta-wye bank is that zero-sequence current is not passed through to the other winding. A disadvantage is that primary insulation cannot be graded as in the grounded-wye case, but that is the transformer-owner’s concern. Surge protection ratings are similar for delta versus wye cases.

Salient reference materials may be the proverbial §1 and §7 of IEEE C57.105-1978 Application of Transformer Connections in Three-Phase Distribution Systems and §2 and §3 of IEEE C62.92.4-1991 Application of Neutral Grounding in Electrical Utility Systems, Part IV—Distribution

 

[daviandgo]

Thanks for your response. The upstream device is the substation breaker. However, there are a few taps off the main circuit feeding fuse protected transformers.

 

[jghrist]

The taps off the main circuit won't be a problem because they won't be able to single-phase the proposed delta-wye transformer.

 

[jbartos]

Suggestion: Normally, the network transformers are connected in grounded-wye primary to grounded-wye secondary. Is your customer power distribution of network nature?

 

[daviandgo]

No. All customers on our 35 kV system are fed radially.

 

[jbartos]

Suggestion: Normally, the radial power distribution toward load is supposed to have transformers delta-wye...delta-wye...etc. to the downstream loads. This is to break the zero sequence path at each transformer secondary voltage level and enable individual system grounding on each transformer secondary.