Why utilities use Wye-Wye Conneted transformer??
Why utilities use Wye-Wye Conneted transformer??
(OP)
Hello,
I had two questions
1) Why utilities are using Y-Y connected transformer for power distribution? The only advantage I see is the cost (less insulation). Are there any other advantage/ disadvantage of using this type of transformer?
2) What are the disadvantages if both the star points (primary & secondary) are connected to the same grounding grid?
I had two questions
1) Why utilities are using Y-Y connected transformer for power distribution? The only advantage I see is the cost (less insulation). Are there any other advantage/ disadvantage of using this type of transformer?
2) What are the disadvantages if both the star points (primary & secondary) are connected to the same grounding grid?






RE: Why utilities use Wye-Wye Conneted transformer??
RE: Why utilities use Wye-Wye Conneted transformer??
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I do not see any disadvantages. If the high voltage and low voltage gear is installed in a common room, you even have to connect your star pints to avoid dangerous voltages between the two separate grounding grids (according to the German norm DIN VDE 0100-442).
RE: Why utilities use Wye-Wye Conneted transformer??
Grounding both neutral points is the accepted practice and some transformers are built with the neutrals internally connected so that only one external grounding connection is required.
RE: Why utilities use Wye-Wye Conneted transformer??
1) Why utilities are using Y-Y connected transformer for power distribution?
///Y-Y transformers are used within power distribution networks on medium and high voltages since they are more advantageous than any other connections.\\\
The only advantage I see is the cost (less insulation). Are there any other advantage/ disadvantage of using this type of transformer?
///Other advantages are considered to be:
1. The protective relay settings will be protecting better on the line to ground faults when the Y-Y transformer connections with solidly grounded neutrals are applied. The Y-Y network transformer standardization leads to numerous advantages.\\\
2) What are the disadvantages if both the star points (primary & secondary) are connected to the same grounding grid?
///All harmonics will propagate through the transformer, zero-sequence current path is continuous through the transformer, one line-to-ground fault will trip the transformer, etc.\\\
RE: Why utilities use Wye-Wye Conneted transformer??
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Please elaborate, Jbartos.
I cannot see any reason why zero-sequence current path should be continuous trough the transformer.
There is no big difference if the star points are connected beneath the earth surface or above the earth surface, provided you have an acceptable low impedance of the earth.
RE: Why utilities use Wye-Wye Conneted transformer??
"1. The protective relay settings will be protecting better on the line to ground faults when the Y-Y transformer connections with solidly grounded neutrals are applied. The Y-Y network transformer standardization leads to numerous advantages."
To my mind, protective relaying is MUCH easier on a delta-wye transformer because ground faults on the secondary side are isolated from the primary, making coordination much easier. If there is upstream relaying on a delta-wye transformer, any zero-sequence current can be assumed to be from a primary ground fault, allowing very sensitive ground fault protection. On a wye-wye, a low-side ground fault causes primary ground fault current, making coordination more difficult. Actually, ground fault protection is one of the primary advantages of delta-wye units.
Utilities like wye-wye transformers, as near as I can tell, because they are cheaper, it allows them to standardize and they provide protection against that old boogeyman ferro-resonance.
If I am designing an industrial installation, I will always try to negotiate for a delta-wye unit.
Old literature will also mention wye-wye units have problems with telephone interference due to third harmonics passing through the transformer, but I'm not sure if this is really much of an issue anymore.
RE: Why utilities use Wye-Wye Conneted transformer??
It seems very much a regional thing, and is typical for multigrounded-neutral circuits. References are ANSI/IEEE C62.92.4-1991 …Application of Neutral Grounding in Electrical Utility Systems, Part IV—Distribution and IEEE C57.105-1978 …Application of Transformer Connections in Three-Phase Distribution Systems. In §7 of the second reference, Grd·Y—Grd·Y is listed as a configuration that will not be susceptible to ferroresonance. One place you may find it, where previously not used, is in a distribution-circuit uprating—say, from 12kV to 20.8kV. Overhead banks can be changed from delta to grounded-wye, as can padmount 3ø transformers equipped with a stick-operable primary oil switch.
Grd·Y allows for use of single-bushing-primary pole-top transformers.
RE: Why utilities use Wye-Wye Conneted transformer??
RE: Why utilities use Wye-Wye Conneted transformer??
You're right that consideration must be given to the delta-wye configuration when determining adequacy of transformer through-fault protection.
But the 58% figure you spoke of only applies for the condition of the the transformer primary protection being required to sense a line-to-ground fault on the secondary side. If you have secondary main protection, this 58% curve shift does not apply to the secondary protection.
The delta-wye transformer is just as rugged as a wye-wye, it's just that the primary protection does not see the full magnitude of fault current (on per unit basis) for a secondary ground fault.
RE: Why utilities use Wye-Wye Conneted transformer??
This is more and more important as non-lnear loads are used. A wye/wye transformer has a high impedance to all zero-sequence currents (triplens) and will cause excessive voltage distortion with lots of third, ninth etc harmonics. A delta/wye transformer behaves a lot better in this respect. Delta/wye is used in most new installations in northern Europe.
RE: Why utilities use Wye-Wye Conneted transformer??
In close coupled applications I don't worry too much about the line-to-ground fault damage line as seen from the primary. But when the secondary main breaker is remote from the transformer, a cable fault in the secondary feeder has to be cleared by the primary device. Even in the close coupled case though, the wrench that was left resting against one bus bar, between the transformer and the secondary main, has to be cleared by the primary device. (Good work practices and mandatory tool inventory help here too.)
RE: Why utilities use Wye-Wye Conneted transformer??
2) What are the disadvantages if both the star points (primary & secondary) are connected to the same grounding grid?
jbartos (Electrical) answered on Mar 2, 2004
///All harmonics will propagate through the transformer, zero-sequence current path is continuous through the transformer, one line-to-ground fault will trip the transformer, etc.\\\
skogsgurra (Electrical) wrote on Mar 2, 2004
jb has a point in pointing out that zero-sequence loads behave differently in wye/wye and delta/wye transformers.
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skogsgurra,
JatTiw didn’t ask about the different grounding of wye/wye and delta/wye transformers, he just wanted to know, what is the difference between the disconnected and connected grounding grids in primary and secondary star points.
The zero sequence current will be continuous through the transformer even if the primary and secondary star points are connected to different grounding grids, IMHO.
RE: Why utilities use Wye-Wye Conneted transformer??
„All harmonics will propagate through the transformer, zero-sequence current path is continuous through the transformer, one line-to-ground fault will trip the transformer, etc.“
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Please elaborate, Jbartos.
///See for example Reference:
Stevenson "Elements of Power System Analysis" for explanations.\\\
I cannot see any reason why zero-sequence current path should be continuous trough the transformer.
///I cannot see any reason why zero-sequence current path should be discontinuous in the grounded Y-Y transformer on the both sides.\\\
There is no big difference if the star points are connected beneath the earth surface or above the earth surface, provided you have an acceptable low impedance of the earth.
///About right.\\\
RE: Why utilities use Wye-Wye Conneted transformer??
Manufacturers often embed a delta tertiary winding in wye-wye transformers to overcome their inherent problems. Maybe this makes such a transformer more acceptable, but given a choice I would always choose a delta-wye unit. The only time I would consider a wye-wye transformer is when phasing dictated that choice.
RE: Why utilities use Wye-Wye Conneted transformer??
„I cannot see any reason why zero-sequence current path should be discontinuous in the grounded Y-Y transformer on the both sides.“
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Yes, in the grounded on both sides Y-Y transformer you have a zero sequence current in the primary and secondary winding.
But in this case there is no difference if the star points are connected to the same grounding grid or to separate grids.
That was my point.
RE: Why utilities use Wye-Wye Conneted transformer??
I hate to be like this, but I need to correct you about what JaTiw really asked about. He wrote:
JatTiw (Electrical) Mar 1, 2004
Hello,
I had two questions
1) Why utilities are using Y-Y connected transformer for power distribution? The only advantage I see is the cost (less insulation). Are there any other advantage/ disadvantage of using this type of transformer?
I answered question number 1. And what I say about distortion is right. There are also other reasons, like possibility to load the phases differently (talking about transformer without primary earth or neutral, which is how our tertiary lines are built over here).
RE: Why utilities use Wye-Wye Conneted transformer??
RE: Why utilities use Wye-Wye Conneted transformer??
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skogsgurra,
I am sorry if I misunderstood you. Though English isn’t my native language I had the impression that Jbarts commented on question Nr. 2.
JatTiw wanted to know: “ What are the disadvantages if both the star points (primary & secondary) are connected to the same grounding grid?“
As you see, JatTiw didn’t ask about the difference between grounded and ungrounded starpoints of Y-Y-Transformers, he just asked about the disadvantage of connection to the SAME grounding grid.
RE: Why utilities use Wye-Wye Conneted transformer??
Some German utilities are using YY transformers with a D-tertiary winding (for example 110 kV/20 kV), the primary star point is grounded, but not in every substation.
You have to reduce the fault current to 10 kA (to reduce the fault voltage on the grounding grid).
If you ground your star points you need a reliable interconnected grounding grid with a low impedance, otherways you get a dangerous fault voltage on your grounding grid.
This can be considered as a disadvantage.
RE: Why utilities use Wye-Wye Conneted transformer??
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skogsgurra,
What do you mean saying „to all zero-sequence currents“. I guess there is only one zero-sequence current, though there may be many harmonics in the fault current. Only the 3xNx50/60-Hz-impedance is high in a YY transformer, the fifth harmonic (which causes a lot of problems to utilities) cannot be filtered out in a D-Winding.
The zero-impedance is low in a YY transformer with a grounded star point.
If you have an additional D-tertiary-winding, this winding will make your zero impedance even lower.
If the star point isn’t grounded you will have a very high zero impedance.
RE: Why utilities use Wye-Wye Conneted transformer??
A title vaguely similar to brupp’s reference is Rusch, R.J.; Good, M.L.; Wyes and wye nots of three-phase distribution transformer connections, IEEE Transactions on Industry Applications, v26n4, July-Aug. 1990
It may be worth mentioning that the application will vary partly depending on winding voltage level, such that there may be different reasons for Y·Y application at transmission and subtransmission versus distribution and lower voltages.
RE: Why utilities use Wye-Wye Conneted transformer??
jbartos said:
"1. The protective relay settings will be protecting better on the line to ground faults when the Y-Y transformer connections with solidly grounded neutrals are applied. The Y-Y network transformer standardization leads to numerous advantages."
To my mind, protective relaying is MUCH easier on a delta-wye transformer because ground faults on the secondary side are isolated from the primary, making coordination much easier. If there is upstream relaying on a delta-wye transformer, any zero-sequence current can be assumed to be from a primary ground fault, allowing very sensitive ground fault protection. On a wye-wye, a low-side ground fault causes primary ground fault current, making coordination more difficult. Actually, ground fault protection is one of the primary advantages of delta-wye units.
///I think that you are referring to the power distribution system from substation to Clients/Consumers. In medium and high voltage power distribution networks, the delta connections are less used since the bushing would experience higher stresses, the wye connection solidly grounded provides lower impedance ground return path for the line to ground faults, causing the protective relay more positively trip. The high voltage and extra high voltage power distributions mostly use wye connections\\\
Utilities like wye-wye transformers, as near as I can tell, because they are cheaper, it allows them to standardize and they provide protection against that old boogeyman ferro-resonance.
If I am designing an industrial installation, I will always try to negotiate for a delta-wye unit.
///Please, notice a distinction between distribution transformer and network transformer.
Reference: IEEE Std 100 Dictionary defines:
Distribution Transformer (power and distribution transformers) A transformer for transferring electrical energy from a primary distribution circuit to a secondary distribution circuit or consumer's service circuit. Note: Distribution transformers are usually rated in the order of 5-500kVA.
Network Transformer (power and distribution transformers) A transformer designed for use in a vault to feed a viable capacity system of interconnected secondaries.
Please notice that I referred to Y-Y network transformers.\\\
Old literature will also mention wye-wye units have problems with telephone interference due to third harmonics passing through the transformer, but I'm not sure if this is really much of an issue anymore.
///When it comes to transmission lines terminated via Y-Y connections, the telephone lines are usually far away from the medium and high voltage transmission lines.\\\
RE: Why utilities use Wye-Wye Conneted transformer??
I think that we mean the same thing. You say 3xNx50/60 Hz (N must also be odd) and those frequencies are what I call triplens (150, 450 etc Hz). And I think that I mentioned about the non-existant neutral. Or did I forget about that?
RE: Why utilities use Wye-Wye Conneted transformer??
I answered your posting from Mar 2, 2004, in this posting you didn’t mention the non-existent neutral.
I don’t have any doubt that all participants of the discussion are experts in power supply.
The only problem we have in this discussion is that nobody reads the postings of the opponents (including myself).
RE: Why utilities use Wye-Wye Conneted transformer??
Can you verify the publication number GET-3388B is correct? I am in contact with GE personals & they could not find this publication number.
Regards
RE: Why utilities use Wye-Wye Conneted transformer??
http://www.beckwithelectric.com/powerlines/articles/i37/wayneart.htm
RE: Why utilities use Wye-Wye Conneted transformer??
Unfortunately, given the 37-year age of GET3388, finding a senior career-GE person with it maybe next to impossible. It is unlikely that more current discussions in transfomer connections will have any glaring disagreement with it.
RE: Why utilities use Wye-Wye Conneted transformer??
http://www.joepower.com/ref/transf.htm
for:
Connection - The Whys of the Wyes GE GE GET3388B 1967 December -
RE: Why utilities use Wye-Wye Conneted transformer??
RE: Why utilities use Wye-Wye Conneted transformer??
RE: Why utilities use Wye-Wye Conneted transformer??
Nice bibliography, but how do you get to the documents?
///Visit
http://www.joepower.com
http://www.joepower.com/ref/intro.htm
http://www.joepower.com/about/contact.htm
etc. for more info including a contact email address\\\