Benefits and Problems with Yg Yg Transformer

[eeprom]

I am accustomed to using a delta-wye for distribution transformers. But I have recently encountered a grounded wye to grounded wye connection. I've been told that this is a very common connection (but not common to me). I understand the reason for the connection is for ferro resonance, or the avoidance of it. IEEE C57.105 says that this transformer connection is prone to tank heating for unbalanced loads, and that the 3rd harmonic multiples are not suppressed. I have many non-linear loads (numerous VFDs and a large UPS system) on this transformer. All of the system modeling was done using an assumed delta primary.

Are there any known issues with this connection in conjunction with non-linear loads? Am I likely to get nuisance tripping on ground currents? Any advice would be helpful.

thanks,
EE

 

[dpc]

Don't think non-linear loads will be much of an issue as long as you have a five-legged core, which is pretty much standard if it is a pad-mount. Ground fault current will be less than a delta-wye. This is a standard connection for virtually all three-phase service transformers used by utilities up here in the NW. I prefer delta-wye, but those are rarely seen in utility-furnished transformers. The wye-wye units are probably a little less expensive and they reduce the (supposed) issues with ferro-resonance to a large extent.

 

[Bronzeado]

eeprom,

The main issue on this kind of transformer connection is unbalanced loads, which produces a magnetic flux of homopolar characteristic.
In this case, the transformer tank will be path for this flux with the high currents in the tank heating the transformer.


Best Regards,

Herivelto S. Bronzeado
Brasília, Brazil

http://www.linkedin.com/profile/view?id=46319837&trk=tab_pro

 

[waross]

This is an issue with three legged core, three phase transformers.
In three transformer banks and five legged core, three phase transformers there is a return path through the core for unbalanced flux and tank heating is seldom an issue.

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

 

[Bronzeado]

That´s correct, Bill!

Best Regards,

Herivelto S. Bronzeado
Brasília, Brazil

http://www.linkedin.com/profile/view?id=46319837&trk=tab_pro

 

[oldfieldguy]

I can vouch for the problem with tank heating and the three-legged core.

old field guy

 

[prc]

In India Delta/star is the universal connection for distribution transformers. To me that is the best connection for that application.But from this forum I came to know from David that in US star/star and other connections are normal.Later I came to know that in China also they were using star-star DTs. But now adays they are also standardising on delta /star DTs.Bronzeado,what is the practice in Brazil?

My understading of the issue is different from those of experts as above.You will not have much problem with 3 phase 3 legged YN-yn transformers ie problems of neutral inversion,ferroresonance etc. Of course slight tank heating will be there when met with unbalnced loading.But that is less harmfull than other consequences. Let me quote from classic, Transformer Engineering by L F Blume (GE) 1951 Page 205, " Case 7-Y-Y connection with grounded neutral in single phase (banks)and shell type 3 phase units .This Connection is dangerous on account of the possibilitry of resonnance in the third harmonic with the line capacitance.It should never be used."
In US,generally 3 phase DTs are with 5 limbed core as they are using wound cores.

 

[Bronzeado]

prc,

We can not "generalize" the practice in Brazil about transformer connections.
However, in the transmission company I worked for 35 years, Chesf, Northeast of Brazil, the transformer connections are:

1. 500/230 kV, three-phase banks, Y-grounded/Y-grounded, auto-transformer with a delta tertiary (13.8 kV);
2. 230/138 kV, three-phase tranformers, Y-grounded/Y-grounded, with with a delta tertiary (13.8 kV);
3. 230/69 kV, three-phase tranformers, Y-grounded/delta. Earth reference made by zig-zag transformer;
4. 68/13.8 kV, three-phase tranformers, delta/Y-grounded;
5. 13.8/0.38 kV (distribution), delta/Y-grounded.

Best Regards,

Herivelto S. Bronzeado
Brasília, Brazil

http://www.linkedin.com/profile/view?id=46319837&trk=tab_pro