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.

 

[HamburgerHelper]

Try tracking down GE's "The Whys of the Wyes" paper.

 

[Mbrooke]

Ive read it, not much help in this case. From my understanding wye vs delta primaries significantly effect the behavior of the network under normal operation and faults.


Thus far I have decided to add a delta winding either in the primary or secondary of the unit to prevent zero sequence currents passing through. Currently most of the units are wye grounded-wye grounded which presents a relaying challenge since a fault involving ground also gets 'seen' in the transmission network.

 

[davidbeach]

If you presently have two winding wye-wye transformers you have zero phase shift from high to low. If you start replacing them with two winding transformers with one delta and one wye winding you'll have some odd multiple of 30 degrees of phase shift between high and low. You will be unable to parallel the low side of a new transformer with the low side of an old transformer.

In many cases that may be a much more significant consideration than the possible benefits of a new configuration.

 

[Mbrooke]

Thanks! That is one concern I did look over. There is indeed a 30 degree phase shift however that doesn't appear to be a great concern since both transformers are in for replacement and the buss tie breakers are run normally open (non paralleled operation) to limit available fault current. An understandable concern is closed loop restoration of 33kv circuits between different substations, however it is usually avoided in that even if phase displacement is identical significant current can still flow between substations from transmission line impedances(distribution trying to balance transmission loss).

The major driver behind having a delta connection in the transformer is to eliminate zero sequence elements from passing and to allow for harmonic/transient circulation.

However, the big question is having it either on the primary or secondary.

 

[davidbeach]

I'd go delta high side, ground wye low side. Seems like pretty common practice among the utilities I'm familiar with.

 

[Mbrooke]

That's what Im leaning toward.

Just to ask:
Can a delta primary power transformer be energized without load?

 

[jghrist]

If you went with delta secondary, you probably would need to add a grounding transformer to maintain an effectively grounded system. This would be important if you have any wye connected loads or equipment (like arresters) not rated for full line-to-line voltage. Also if you have existing ground fault protection.

 

[Mbrooke]

A fault duty rated zig-zag grounding transformer will be added to create an artificial neutral point for fault detection purposes. Most likely in both cases the system will be bonded to the ground mesh (grounded) with a 2 to 10ohm MV resistor to limit currents. The 33kv lines all feed delta-wye (33 to 480v & 33 to 13.2kv) step down transformers, fortunately as a result the slight neutral shift under faults should not cause secondary secondary voltages to rise. No phase to neutral laods exist on the line.

 

[luffy2011]

Yes, it should be fine to energize an unloaded transformer with delta primary as long as you use 3-phase, 3-pole circuit breakers.

http://www.powerqualityworld.com

 

[Mbrooke]

Ok. My fear with a delta primary is Ferroresonance. Does a circuit switcher do the same?

 

[magoo2]

A circuit switcher has a close or tight enough closing span between the phases that I wouldn't worry about the possibility of ferroresonance. We use circuit switchers with our 115 kV and 230 kV delta wye transformers where the low side is 34.5 kV and we haven't had a problem.

 

[luffy2011]

Yeah, any device that closes the 3 phases simultaneously.

http://www.powerqualityworld.com

 

[Mbrooke]

Ok, got it. I guess a delta primary is the way to go.


Just one quick question. Is a delta delta connection even possible with a 115 to 33kv transformer? Just wondering.

 

[luffy2011]

I think it all boils down to your intended appication. If you use DELTA-delta, I see no concern as long as it is what you need, and have considered other factors such as need for grounding. However, if this is a utility serving common customer loads, then, a DELTA-wye is the typical connection.

http://www.powerqualityworld.com

 

[Mbrooke]

Thanks. This is other utility owned transformers.

 

[prc]

In India except in one state,connection used for these sub transmission transformers is YNyn (star/star).Earlier(30years back) we used to provide a stabilising tertiary winding in such transformers.In one state 110kV delta is used as primary.Eventhough they want to switchover to star, parallel operation concerns prevent them. Remember a delta primary with OLTC on primary will be costlier by 10-15 %. Unlike in the star case, the tap changer will be always at 110 kV potential with delta connection, a weak link from maintenance concerns.

 

[Mbrooke]

@PRC you bring up something I thought about. In a wye primary tap changer insulation can be reduced as well as winding insulation as windings get closer to the grounded neutral point.

What are the reason in India for primary wye?

 

[Power0020]

A delta on primary is much common with star on 33 kV level, this is cost - effective limiting the winding requirement on HV side, and removes the requirement for a zig-zag on 33 kV side. (given you will have a 115 kV star point somewhere in the system, for GF protection)

NER on 33 kV seems a good solution to limit the fault current on 33 kV side and the GPR as well.

 

[prc]

Mbrooke, Frankly Idont know the reason for star primary. This is the universal practice here and several thousands of such transformers are ordered every year. Even with such system, for large factory loads it used to be delta/star with resistance earthing. But nowadays such applications are also using star /star with resistance earthing on secondary.