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Delta vs Wye primary

Delta vs Wye primary

Delta vs Wye primary

(OP)
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.

RE: Delta vs Wye primary

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

RE: Delta vs Wye primary

(OP)
Ive read it, not much help in this casesad. 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.

RE: Delta vs Wye primary

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.

RE: Delta vs Wye primary

(OP)
Thanks! smile 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.

RE: Delta vs Wye primary

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

RE: Delta vs Wye primary

(OP)
That's what Im leaning toward.

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

RE: Delta vs Wye primary

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.

RE: Delta vs Wye primary

(OP)
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.

RE: Delta vs Wye primary

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

RE: Delta vs Wye primary

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

RE: Delta vs Wye primary

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.

RE: Delta vs Wye primary

(OP)
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.

RE: Delta vs Wye primary

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

RE: Delta vs Wye primary

(OP)
Thanks. This is other utility owned transformers.

RE: Delta vs Wye primary

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.

RE: Delta vs Wye primary

(OP)
@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?

RE: Delta vs Wye primary

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.



RE: Delta vs Wye primary

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.

RE: Delta vs Wye primary

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

RE: Delta vs Wye primary

(OP)
@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?

RE: Delta vs Wye primary

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.

RE: Delta vs Wye primary

(OP)
@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?

RE: Delta vs Wye primary

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.

RE: Delta vs Wye primary

@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.

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