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24940Y/600Y 3Phase Padmount Transformer Wiring
8

24940Y/600Y 3Phase Padmount Transformer Wiring

24940Y/600Y 3Phase Padmount Transformer Wiring

(OP)
Hi Guys,

I am new to the forum. I a WYE-WYE padmount three phase transformer 25kV/600V. The utility company requested our engineering firm to buy Y-Y style transformer. The high voltage utility line is a 4-wire service with the neutral grounded. The utility is only going to run three wires, i.e only connect the primary phase A,B,C and leave the neutral unconnected on the primary side of our transformer. Our secondary has a resistor grounded system (5A NGR). How would the transformer behave if the primary neutral connection is left disconnected? Does the HO bushing need to be connected to the cabinet ground? How would it behave if there was a line to ground fault on the secondary? Theoretically what would you measure for voltages on the secondary side between L-G and L-L? Does the HO bushing need to be tied to the main ground grid at the facility? Our secondary loads mainly consists of delta motor loads.

My apologies for the basic question but I am a junior engineer. Do you guys see any potential problems with the above setup? I kept asking them as to why we can't go Delta-Wye since they are running only three conductors but they wouldn't give me a technical answer and said that their standard requires a WYE-WYE transformer.

Look forward to hearing from you guys.

RE: 24940Y/600Y 3Phase Padmount Transformer Wiring

Check the transformer, it may not even have an H0 terminal, it may be that H0 and X0 are connected internally.

RE: 24940Y/600Y 3Phase Padmount Transformer Wiring

(OP)
The XO and HO bushings are available on the transformer and not internally connected.

RE: 24940Y/600Y 3Phase Padmount Transformer Wiring

You do not have to worry about the HO bushing at all. If your worry is regarding "floating neutral scenario" at the primary, it wornt affect you at the secondary as you anyways ground secondary through an NGR.If there's a line to ground fault on the secondary, NGR will limit the first ground fault. There will be a primary zero sequence component but it will not affect even you don't ground the HO bushing.

RE: 24940Y/600Y 3Phase Padmount Transformer Wiring

2
If the secondary wye has the ground being used for any reason,(even if it is for a high/low impedance resistor/reactor), it still requires that the primary wye also be connected to the grounded neutral from the Utility. Leaving the primary wye floating would create a severe voltage imbalance should a phase short to ground on the secondary side. In other words you would have an ungrounded primary wye with a ground wye secondary which is an extremely unstable transformer connection.

I somewhat doubt the Utility would leave the primary wye floating. Standard practice being always to connect both the primary and secondary connections to the Utility grounded neutral. This is done through the concentric shield on the underground HV cables which are used as a grounded neutral. They may leave the primary floating if they think you need the secondary floating and this is not your case.

Your best bet would be to fight with the Utility for a Delta-wye padmount. There is no reason why a primary delta will not work regardless of what the Utility thinks. It is the standard for industrial everywhere and European Utilitys as well. Chances are the Utility is not giving you a technical answer because even the tech does not know why it is specked as the standard. The few who do will bring up cost and ferroresonance. Cost is insignificant and ferroresonance is a rare condition that only occurs in large systems where the transformer is very lightly loaded and a single phasing condition on the primary has occurred. In your case a wye- wye would just cause way more headaches.

RE: 24940Y/600Y 3Phase Padmount Transformer Wiring

I agree with Mbrooke that it would be highly unusual for the utility to not ground the primary neutral. An ungrounded wye - resistance grounded wye connection does not furnish a zero-sequence path or zero-sequence source and would be unacceptable.

Reasons for the utility insisting on wye-wye would be standardization if they owned the transformer. If you own the transformer, the utility should permit a delta primary. Ferroresonance can be a real problem at 24.9 kV, but the installation can be designed to minimize the possibility. Ferroresonance could also be a problem with an ungrounded wye connection.

RE: 24940Y/600Y 3Phase Padmount Transformer Wiring

(OP)
Thank you for the response guys. We've already purchased the Y-Y transformer. We are in a bit of a bind here. I am still waiting to hear back from the utility. Would it still be acceptable if we bring 4-wires (A,B,C, and N) to the primary side of the transformer and solidly ground the H0 bushing on the transformer? The X0 would still be resistor grounded. Would there still be concerns in this setup?

RE: 24940Y/600Y 3Phase Padmount Transformer Wiring

Your plan sounds OK. Study the wiring diagram of the pad mount carefully though, many pad-mounts will have the neutrals bonded to the tank wall internally. Make sure the neutral from the secondary winding is separate from the primary winding neutral (the two are not connected together internally) and that the secondary neutral is brought out via a fully insulated bushing in the low voltage compartment. Remove any conductive strap from the X0 bushing connecting it to the tank. The low voltage neutral is to remain ungrounded until after the resistor. You might also have to run a grounding conductor from the pad to the facility along with the low voltage phases and neutral if local codes call for it.

As for the primary winding it does not matter if its neutral is connected to the tank wall or brought out via its own bushing; as long as its connected to the utility HV neutral your setup will work fine.

RE: 24940Y/600Y 3Phase Padmount Transformer Wiring

(OP)
Thank you Mbrooke for your response. I double checked with our transformer vendor as well as our schematics and the XO and HO bushings are completely isolated from one another. The X0 bushing is brought out via a fully insulated bushing in the low voltage compartment. The XO will be ungrounded until after the resistor.

We will run 4-wires from the HV utility to our primary and then tie the HO to the ground grid.

What would be the draw backs or concerns be with such a setup? Are there any potential safety concerns? Please let me know your thoughts.

RE: 24940Y/600Y 3Phase Padmount Transformer Wiring

Ground versus neutral; The terms tend to be used interchangeably in the distribution field, but you may want a connection to the system neutral. That is a connection to the distribution neutral conductor (which is grounded) rather than to a ground grid.
With the HO tied to the ground grid, any neutral currents resulting from any unbalanced loading or faults will flow to ground, through the earth and then through the system neutral grounds back to the neutral. Although this will work some would prefer to make a direct connection to the neutral conductor rather than using the earth as a conductor.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: 24940Y/600Y 3Phase Padmount Transformer Wiring

(OP)
Thank you for your response Bill. Your point does make sense. Would the following method work better to prevent neutral currents from flowing back to the H0?

- Bring 4-wires (A, B, C & N) from the high voltage line (system) and terminate them straight to the H0, H1, H2, H3 bushings on the primary side of the transformer
- Once terminated to the H0 bushing is it okay to solidly ground the H0 but keep it's solid ground/electrode independent of the plant ground grid? This way you you can prevent neutral currents from flowing back to the primary neutral? I am not even sure if this is even feasible in reality.
- OR Would it be better to just terminate to HV neutral wire to the H0 bushing and NOT solidly ground it and rely on the power line's neutral ground? The run from the pole to the primary of the TX is approx. 200meters. The only concern would be a break in connection in neutral, although unlikely.

Look forward to hearing back.

RE: 24940Y/600Y 3Phase Padmount Transformer Wiring

If the transformer is just a two winding wye-wye transformer (as opposed to having a delta tertiary) then the current into/out of the H0 bushing is limited by the grounding resistor connected to the X0 bushing. Look at the zero sequence diagram of a wye-wye transformer.

RE: 24940Y/600Y 3Phase Padmount Transformer Wiring

How do you plan to bring neutral to the transformer HO if the utility comes in with 3-wire conductors?

"Throughout space there is energy. Is this energy static or kinetic! If static our hopes are in vain; if kinetic — and this we know it is, for certain — then it is a mere question of time when men will succeed in attaching their machinery to the very wheelwork of nature". – Nikola Tesla

RE: 24940Y/600Y 3Phase Padmount Transformer Wiring

Since this is a padmount, perhaps they are running three UG cables as Mbrooke stated. The forth conductor is the three concentric neutrals.

RE: 24940Y/600Y 3Phase Padmount Transformer Wiring

(OP)
davidbeach - Thanks. I looked at the diagram and it makes sense. The amount of current circulating back to the primary will therefore be very small taking into account the turns ratio, 0.12Amps in this case.

VTer - I managed to convince the utility engineer to run 4-wires out to our primary. They will confirm if they are going to run concentric neutrals or a 4th conductor. Either way, they will run the neutral over to the primary. :)

RE: 24940Y/600Y 3Phase Padmount Transformer Wiring

So I am assuming you are going to have some sort of primary equipment for your transformer protection where the utility will terminate? Will this equipment also have a neutral bar in it or will the utility just land their fourth conductor or conc. neutral to the ground bus?
If the primary equipment sits on the same ground grid as the transformer, why couldn't you just solidly ground the HO to the grid as well as your ground bus in the primary equipment? This way you would not have to run a separate neutral conductor between them.

"Throughout space there is energy. Is this energy static or kinetic! If static our hopes are in vain; if kinetic — and this we know it is, for certain — then it is a mere question of time when men will succeed in attaching their machinery to the very wheelwork of nature". – Nikola Tesla

RE: 24940Y/600Y 3Phase Padmount Transformer Wiring

(OP)
VTer: Our transformer is 200 meters from the Utility Pole. The Utility Pole will have a gang operated switch with fused protection for the conductors leaving the pole. The pole is actually a 4-wire service. They have a neutral wire running on their high voltage line. The utility will land the neutral conductor onto their neutral line, not just to ground. On the transformer primary that neutral will land on the H0 bushing. The utility pole or in your words primary equipment does NOT sit on the same ground grid as the transformer due to the 200 meter distance.

RE: 24940Y/600Y 3Phase Padmount Transformer Wiring

elmatador82,

If your Y-Y transformer is a three-limbed core transformer, it will burnout some time after one of the HV fuses blowup. This happens due to the "zero sequence flux" created by the voltage imbalance that will induce high eddy currents in the tank, heating it up so the oil.

Best Regards,

Herivelto S. Bronzeado
Brasília, Brazil
http://www.linkedin.com/profile/view?id=46319837&a...

RE: 24940Y/600Y 3Phase Padmount Transformer Wiring

(OP)
Bronzeado:

We have wired the high high winding temperature back to our control system. We shut down all motor loads upon detecting this. Would this buy anytime on saving the transformer? I don't know the internal design of the transformer core. Is there something we could implement to prevent this from happening? Or perhaps generate an alarm for phase loss and shutdown the facility?

RE: 24940Y/600Y 3Phase Padmount Transformer Wiring

It is normal for secondary neutral currents to be reflected onto the primary neutral of a Y:Y transformer.
It is a characteristic of a three legged core that it forms a "phantom" delta and acts as if there was a delta winding.
It is a characteristic of a 4 wire Wye:Delta transformer or 4wY:D bank of single transformers that the delta will transfer power from healthy phases to a missing or low phase. In a transformer loaded 67% or more the current on the healthy phases will often be over the rated current. Leakage flux of a three legged core make may this somewhat inefficient, but you have replaced winding heating with tank heating, and heat is the killer.
I suspect that in the event of a primary phase loss the primary neutral current may approach the line currents of the healthy phases. Actually the vector sum of the line currents, but phase shifts and zero sequence impedance issues during an event may make a simple calculation based on pre-issue values less than 100% accurate.
BUT, any primary neutral current substantially greater than a reflection of the expected neutral grounding resistor current may be an indication of a primary phase loss.
Comments??

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: 24940Y/600Y 3Phase Padmount Transformer Wiring

I would suspect that this is a triplex or a four/five-legged core design. Any reputable manufacturer would not agree to provide a wye-wye three-legged core xfmr.

"Throughout space there is energy. Is this energy static or kinetic! If static our hopes are in vain; if kinetic — and this we know it is, for certain — then it is a mere question of time when men will succeed in attaching their machinery to the very wheelwork of nature". – Nikola Tesla

RE: 24940Y/600Y 3Phase Padmount Transformer Wiring

(OP)
VTer:

Yes you are correct. I just checked with cooper and our transformer is a 5-legged core design! Which is good news.

RE: 24940Y/600Y 3Phase Padmount Transformer Wiring

Quote (davidbeach)

If the transformer is just a two winding wye-wye transformer (as opposed to having a delta tertiary) then the current into/out of the H0 bushing is limited by the grounding resistor connected to the X0 bushing. Look at the zero sequence diagram of a wye-wye transformer.

Isn't this assuming there is no earth return path? I would think that in systems that don't have NRG that the earth path (between grounding grids) could be neglected because of the resistance of this path compared to the neutral is much higher. However, with the NRG the relative resistance of the earth path is not as high as otherwise and thus needs to be accounted for.

RE: 24940Y/600Y 3Phase Padmount Transformer Wiring

It's assuming that the zero sequence diagrams for wye-wye transformers in many standard references are correct. The zero sequence impedance from high side system to low side system is 3 *(HS NRG + LS NRG) + Transformer Z0. So, with a low side NRG, you have 3 times that impedance. With a solid connection on the high side there is no additional impedance in the zero sequence circuit. With an open on the high side, there is no connection of the zero sequence networks. Amp-turn balance is going to keep the neutral currents in tight relationship.

RE: 24940Y/600Y 3Phase Padmount Transformer Wiring

It is well known that with distribution transformers preferred connection is delta/ star. But subtransmission transformers are generally star/star with three limbed core and neutrals on primary and secondary solidly grounded.(10-100 MVA,say 66/11kV or 220/11 kV)When first time stabilising delta tertiaries were eliminated (some 35 years back)in such transformers, there used to be some tank heating whenever there was unbalnced loading on secondary. It never caused any problem in service eventhough there was some worry in the beginning.

How distribution transformer loading or performance differ from above subtransmission transformers with star/star 3 limbed transformer and solidly or resistor grounded neutrals?My query only because of the statement that "Any reputable manufacturer would not agree to provide a wye-wye three-legged core xfmr" All subtransmission transformers are like that only.

RE: 24940Y/600Y 3Phase Padmount Transformer Wiring

Quote (prc)

It is well known that with distribution transformers preferred connection is delta/ star.
Some places perhaps, but certainly not everywhere. Lots and lots of utilities that use wye-wye transformers for distribution (service) transformers. (12.47kV-480 or 208 in our case). I think that once you get beyond 3 phases of sinusoids with 120 degree phase separation running somewhere in the 50-60Hz range it becomes increasingly unwise to make general statements. Probably counter examples for most every "generally" or "well known".

RE: 24940Y/600Y 3Phase Padmount Transformer Wiring

"When first time stabilising delta tertiaries were eliminated (some 35 years back)in such transformers, there used to be some tank heating whenever there was unbalanced loading on secondary."
This would be in place of the circulating currents in the delta that the same conditions would cause.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: 24940Y/600Y 3Phase Padmount Transformer Wiring

1) Waross, I think the current in tank may not be that high as in tertiary because of the air gap between winding and tank, skin effect, and the increased metal resistivity faced by current as temperature from heating goes up.I wish I could make a measurement of extra KW in tank loss during loading with two phases only in a star/star transformer.Probably this can be done during load loss mesurement of transformer.My query was if we can tolerate tank heating in subtransmission star /star transformers why it is looked with worry in star/star distribution transformers with 3 limbed core?Or unbalancing of load will be more in distribution transformers?When we eliminated tertiary in subtransmission transformers, this was one of the issues of concern which turned out to be not that serious.

2) If someone got annoyed with my literary expression of well known, sorry ,I withdraw.I only meant well understood( by me of course!) I was always thinking that no one will consider my writing as universal truth and knowledgable, wise experts will correct me.

3) Now the preferred connection for distribution transformers- In India delta/star is the universal connection, not preferred one, as I have yet to see a star/star distribution transformer here.All industry and government standards call for only delta/star. In recent years,in some areas single phase transformers are used in line with US practice.Probably in UK and Australia also this may be the connection used as we were following UK practices.Scotty can guide me.

Of course we know US practices are different- "Many rural electric utilities do not have an established policy for three-phase commercial service...........The result is typically a utility system with different types of three phase transformer connections and voltages." -Wyes and wyenots of three phase distribution transformer connections:Robert Rusch and ML Good, IEEE on Industrial Applications ,Vol26 No4,July 1990,Page 683.

"This type of connection ie delta-wye with neutral grounded is the most popular connection in distribution substation banks"-James J Burke,Power Distribution Engineering (book) Chapter 2,page 49,Mercel Dekker,1994.Burke is exABB,USA and it seems even in USA, delta/star is the popular connection,even if not the preferred one.

In this posting jghrist also mentions"If you own the transformer, the utility should permit a delta primary" ie recommending delta/star.

RE: 24940Y/600Y 3Phase Padmount Transformer Wiring

The further removed from the customer load, the more balanced the system, so wye-wye without tertiary is likely to see less heating in a transmission application than in a distribution application.

The term distribution transformer is somewhat ambiguous. I think you're using it to mean what I'd call a service transformer, the last utility transformer between the utility system and customers taking primary service. What I'd call a distribution transformer is the one that separates the transmission system from the distribution system; the level of transformation above the service transformer.

The service transformer can be most anything. We don't permit grounded wye-delta as a service transformer without special protection. We have various (ungrounded) wye-delta banks in service for older delta services, but don't install new delta services. Most three-phase services use grounded wye-grounded wye connections, whether banks of 3 single phase transformers or the more common single three-phase transformer. For customers that take primary service we have no objections to their use of delta-wye transformers, grounded or not. I think the principle advantage of the wye-wye transformer is that it is less susceptible to ferroresonance as there are lots of ways to single phase the source.

For the distribution transformer (as defined above), nearly all of ours are delta-grounded wye. We're slowly getting rid of the few remaining "weird" connections, particularly as we eliminate distribution voltages that were historically derived from a delta source.

Within our system, all transformation between transmission/subtransmission voltages are grounded wye-grounded wye, and almost always autotransformers. We use transformers with tertiaries, but there are other companies in the area that have some autotransformers without tertiaries, but their newer transformers seem to again include the tertiary.

There are also subtransmission voltages in the area (but not on our system) that are derived from transformers with a delta power winding. Historically these were wye (usually grounded) on the transmission side and delta on the subtransmission side, but as those systems have required additional capacity I've seen a delta-grounded wye transformer paralleled with the older wye-delta transformer.

As with so many things in the utility power world the physics is common world wide, but the applications can vary considerably.

RE: 24940Y/600Y 3Phase Padmount Transformer Wiring

David; Am I correct in thinking that the service transformers tend to have the lowest PU impedances when compared to transmission and substation transformers? Where a 6% voltage unbalance may cause 100% of rated current to circulate in the delta of a grounded wye:delta with 2% impedance voltage, the same 6% unbalance would cause much less PU current in the delta. Also, as you mentioned, upstream transformers are less likely to see unbalanced voltages and phase loss is less likely.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: 24940Y/600Y 3Phase Padmount Transformer Wiring

Since their are ANSI standards for "Overhead-Type Distribution Transformers" "500 kVA and Smaller" "single phase or three phase", I tend to call those things scattered around town "distribution transformers" while the things behind the fences are "substation transformers" (no ANSI blessing here, these are just "power transformers" per ANSI). I agree the lingo is not at all universal, and service transformer is less ambiguous.

Perhaps for this thread we can agree to avoid the word distribution and speak only of "service transformers" and "substation transformers" (that feed a medium voltage distribution system). The first reference prc quoted above is for "service transformers", the later for "substation transformers."

Yes, their are lots of service transformer connections and voltages in use in the US. This is because of all the legacy equipment out there. It's kind of tough on the customers to be told they need to change equipment or install their own transformer. So utilities such as davidbeach's have more restrictive policies for new services, but continue to maintain all the old ones.

RE: 24940Y/600Y 3Phase Padmount Transformer Wiring

waross. I got little confused.We were talking about unbalanced loading of star/star transformers that can cause tank heating. The circulating current that you are mentioning is circulating current in delta winding due to difference in pu impedance of individual single phase units.

stevanal.You have the blessings of ANSI!!As per ANSI/IEEE C57.12.80 -2010 Standard Terminology for Power and Distribution transformers, the appropriate terms are Distribution substation transformer ( what I mentioned subtransmission transformer) and distribution transformer.Buke also uses these terms in his book.I could not find a term service transformer in ANSI.IEC is silent on the subject( IEC 50(421) Electrotechnical vocabulary on transformers and reactors)

Coming to legacy equipment-Quoting from the 1990 paper, "Many rural electric utilities do not have an established policy for three-phase commercial service.Instead, a request for service is made,and after several undocumented phone calls,the kVA size and voltgae requirements are finally determined. The result is typically......"This is different from our situation here where any new connection or load permit require clearance from powerful government Electrical Inspectors at each district and state.Probably this also helped in standardising the distribution transformers.Distribution substation transformers are either delta/star or more commonly star/star.

The first paper on transformer connections from US that I have is from 1903 at 20th Annual AIEE convention at Niagra Falls."Y or delta(greek letter delta was used at that time) connection of transformers" by F O Blackwell. " In transformers,therefore, have Y secondaries,it is desirable that the primary should be delta connected" (AIEE 1903, page 388).Aa per this, distribution trfs were with secondary voltage of 200/115 V and distribution substation transformers were with secondary voltage of 4/2.3kV, both with primary delta connected.

In 1975 IEE paper, " Select the right transformer winding connection for industrial power systems" by W C Bloomquist, Transactions on industry applications Vol 11 nO6, November,1975 page 641, "Delta /star connection is commonly used in industrial systems as it provides isolation at each voltage for ground currents.....Star/star connection is quite commonly used by utilities to serve general purpose single phase and three phase loads via 4 wire service,minimises the ferroresonance."Red book (IEEE 141-1993) Recommended practice for electric power distribution and industrial plants-Clause10.4.5-"Connections for the standard two winding power transformers are preferably delta -primary and wye secondary"

RE: 24940Y/600Y 3Phase Padmount Transformer Wiring

No, I am talking about the real, serious, destructive circulating currents that flow in any delta due to primary voltage and or phase angle errors in a four-wire-wye:delta transformer bank or transformer. This includes the phantom delta of a three wire core. These errors may be due to unbalanced loading or to primary issues.
I spent some years in an area where the four-wire-wye:delta connection was popular. When I became the acting system engineer of a very small island utility it took me several years to get rid of the four-wire-wye:delta connections on the system. Some of the issues were blown primary fuses, overheated and destroyed transformers and burned up refrigerator and freezer compressors.
Consider an open delta connection with two distribution transformers. The primary is star with the neutral connected. Now take a third transformer and connect it single phase on the remaining phase. The vector should be identical to the vector of the open delta.
BUT
If there are any voltage unbalances or phase angle errors on the primary system the vectors will not be equal. There will be an error of magnitude, angle or both.
If the third transformer is now connected into the delta bank as a four-wire-wye:delta connection then the error will cause a current to flow in the delta limited by three times the transformer impedance. With a low impedance distribution transformer a relatively small imbalance may easily drive more than 100% current through the delta.
When I see a four-wire-wye connection I want to be assured that there is no three legged core to develop a phantom delta.
Note: With four-wire-wye:delta connections on the system you become aware that single phase voltage regulators on distribution circuits may introduce phase angle errors and line to line voltage unbalance while keeping the line to neutral voltages equal. Great for long distribution lines with primarily single phase residential services but tough on three phase services and particularly three phase motors.
Disclaimers;
1> In North America I have seen a few legacy wye:delta connections from the days of delta:delta service. The primary neutral is left floating as the possibility of switching surges is much less of an issue than the four-wire-wye:delta connection issues.
2> The worst issues are on fuse protected distribution circuits. Transmission circuits have less likelihood of serious unbalances, often have controls to trip the circuit in the event of a phase loss and generally have higher transformer impedances.
3> It was noted and you will find the notes in old instruction books and texts dating from the time of the widespread change-over from delta:delta distribution to four-wire-wye:delta connections. NOTE: Severe faults may cause extensive primary fuse blowing throughout the primary circuit or system.
4> REA (RUS) recommends that four-wire-wye:delta connections NOT be used.
5> The phantom delta associated with a three wire core causes flux leakage which in turn causes tank heating. Notwithstanding, the phantom delta is transferring power from or to the bad phase as it reacts to and tries to correct the voltage and/or phase angle errors.
I could add a couple of pages of anecdotes and instances but you understand my concern with possible deltas with a four-wire-wye connection.
Yours
Bill
Ps: The four-wire-wye:delta connection is one of my buttons. Push it at your peril. Grin
Rant off.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: 24940Y/600Y 3Phase Padmount Transformer Wiring

Thank you Bill for the clarifications.I never wanted to push any of your buttons to get myself pounced. I remember your cautioning about wye-delta connection many times in this forum.I learnt it from you.I was under the impression that in US, distribution transformers have delta/star connection for power supplies and star/star connection for urban supplies. From your post understood there are other alternatives too.Thank you.
I recognise -to know that you dont know is also knowledge.Let me atleast acquire that knowledge.

RE: 24940Y/600Y 3Phase Padmount Transformer Wiring

Hi prc. I respect your knowledge and intelligence. When you question me, I take it as part of the conversation, not a challenge or putdown.
I took no offense. I just tried harder to make my explanation clearer.
In the part of Central America where I spent time, Three single phase distribution transformers connected in wye:delta was very common. It was also common to see one fused cutout hanging open on each bank. That made it effectively an open delta and the problems went away until the next change of duties at the utility and the new guy would have all the fuses replaced.
A little more explanation re: single phase regulators.
On a long rural three phase distribution system, it is common for the voltages to be somewhat unbalanced arriving at the regulator bank. Each regulator corrects the voltage and leaving the regulator we have three equal phase to neutral voltages. But, unbalanced loading implies a neutral current. As a result we will have a voltage drop on the neutral. Now draw a wye vector diagram to represent the voltages arriving at the regulator. Label it A, B, C and neutral. The voltages need not be equal but assume that the phase angles are 120 degrees. Now draw a vector to represent the neutral voltage drop. The origin will be the center of the wye point or the point labelled neutral. Now draw vectors from the displaced neutral to the original points, A, B, C. You will see that no matter what angle the neutral voltage drop vector is assigned, there is no way that the phase angles will be equal. Downstream of the regulator the phase to neutral voltages will be equal but the phase angles will not be equal. As result the line to line voltages will not be equal. This is no problem for single phase services but it raises issues with three phase motors and wye:delta transformers and transformer banks.

On the island I was able to eventually have all the wye:delta connections changed to wye:wye connections.

On the mainland I had no influence on the practice of the National Energy Company, but was faced with the problem of mitigating the issues and problems of my customers.
The most common issue was burned out refrigerator and freezer compressors.
Single phase switching was common. When maintenance was needed a section of distribution line would be isolated by pulling the fused cutouts.
When service was restored, one phase would come up but the other two phases would be backfed from any wye:delta banks at about 50% voltage. All the compressors on those two phases would try to start but with the low voltage they would all stall. When the second phase was energized the compressors on that phase would be pressure locked and continue heating until the thermal protection tripped. The third phase would also be energized by the back feed but the voltage would be a couple of percent down due to the regulation of the wye:delta banks producing the back feed. Then the third phase would be energized and by now all the compressors on two phases will be stalled and heating up, waiting for the thermal trips to take them off line.
Every time, somewhere in the system, one or two compressors would fail.
After about 15 years immersed in these conditions, I am wary of any distribution system with wye:delta connections, even wye:wye with phantom deltas.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: 24940Y/600Y 3Phase Padmount Transformer Wiring

At least you can't get the phantom delta when using a bank of 3 single phase transformers.

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