24940Y/600Y 3Phase Padmount Transformer Wiring 8

[elmatador82]

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.

 

[VTer]

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

 

[elmatador82]

VTer:

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

 

[wroggent]

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.

 

[davidbeach]

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.

 

[prc]

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.

 

[davidbeach]

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".

 

[waross]

"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

 

[prc]

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.

 

[davidbeach]

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.

 

[waross]

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

 

[stevenal]

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.

 

[prc]

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"

 

[waross]

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

 

[prc]

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.

 

[waross]

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

 

[davidbeach]

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