Why utilities use Wye-Wye Conneted transformer?? 3

[JatTiw]

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?

 

[allmend]

„A wye/wye transformer has a high impedance to all zero-sequence currents“

---

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.

 

[busbar]

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.

 

[jbartos]

Comment on dpc (Electrical) Mar 2, 2004 marked ///\\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.\\

 

[Skogsgurra]

allmend,

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?

 

[allmend]

Hi skogsgurra,

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).

;-)

 

[JatTiw]

Hi Brupp
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

 

[tulum]

To the original post...FYI

http://www.beckwithelectric.com/powerlines/articles/i37/wayneart.htm

 

[busbar]

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.

 

[jbartos]

Suggestion: Visit

http://www.joepower.com/ref/transf.htm

for:
Connection - The Whys of the Wyes GE GE GET3388B 1967 December -

 

[davidbeach]

Nice bibliography, but how do you get to the documents?

 

[stevenal]

Priorities first. The pictures are there. I liked CT Suzie.

 

[jbartos]

Suggestion to davidbeach (Electrical) Mar 9, 2004 marked ///\\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\\\