Conceptual Question Transformer Z

[wirenut1980]

Hello, I'd like to compare the impedance (in ohms) of 2 different transformer configurations.

1) Three phase transformer, let's say pole mount types, 3 single phase transformers banked together in wye-wye, all with equal nameplate impedances = %Z

2) One 3 phase transformer, wye-wye, let's say a padmount type, also with nameplate impedance = %Z (same as the single phase transformers).

My question is will there be any difference in impedance (in ohms) between the two different configurations? If so, approximately how much, and why would there be a difference?

Thanks and Merry Christmas to all!

 

[bcorbin]

They should be the same impedance. After all, a three phase transformer is nothing more than three single phase transformers all housed in one package.

 

[wasasparky]

Three phase transformers can be three separate sigle phase transformers, or one common core with three coils, so I don't think the answer is necessarily a simple one...

 

[bcorbin]

Three phase transformers can be three separate sigle phase transformers, or one common core with three coils, so I don't think the answer is necessarily a simple one...

That's true, but if it has a matching nameplate impedance, like he said, they're going to be equivalent.

 

[LarryFine]

My question is will there be any difference in impedance (in ohms) between the two different configurations?

I would also say there will be no difference, because any physical difference was "taken into account" before the final impedance rating was acheived.

That's like saying that a pound of iron and a pound of popcorn weigh the same, because the difference in density is factored in with the volume.

The impedance, like the weight, is a result with all contributing characteristics already factored in.

That's true, but if it has a matching nameplate impedance, like he said, they're going to be equivalent.

Exactly.


Added: Now, armed with this information, one might be curious enough to "reverse engineer" the two transformer configurations, and discover what physical differences there might be, such as total core mass, turns ratios, etc., that end up with the same rating.

 

[jim dungar]

Theoretically there it should be possible to build a single core transformer bank with the same impedance as one built of 3 single core units.
But, historically "pole" type oil-filled single phase transformers are built with a much lower %IZ than are dry type units. The result is that a three phase bank of single transformers typically has a much lower (I have seen 50%) impedance than a single core three phase dry unit.

 

[templdl]

wirenut1980,
I'm just curious to know what intregues you about the Y-Y transformer?
Are you considering this for an actual application?
Y-Ys are requested for specific application reasons.
We have some customers that request them simply because they have a Y souce such a 480Y/277 or 208Y/120 which is normally not a good reason.

 

[LarryFine]

wirenut1980,
I'm just curious to know what intregues you about the Y-Y transformer?
Are you considering this for an actual application?
Y-Ys are requested for specific application reasons.
We have some customers that request them simply because they have a Y souce such a 480Y/277 or 208Y/120 which is normally not a good reason.

He could also be referring to utility transformers with 480Y/277 and/or 208Y/120 secondaries.

 

[ramsy]

There is a 3rd option the OP ignored, when comparing impedance between 3-phase xfmrs.

3) Two transformers, both pole mounted open delta, can supply 3-phase loads.

 

[wirenut1980]

wirenut1980,
I'm just curious to know what intregues you about the Y-Y transformer?
Are you considering this for an actual application?
Y-Ys are requested for specific application reasons.
We have some customers that request them simply because they have a Y souce such a 480Y/277 or 208Y/120 which is normally not a good reason.

This is an existing utility application and I work for a utility company. The application is several substations serving a manufacturing facility of car parts. The reason I got involved in this application was because one of the single phase substation transformers had been replaced about 6 months ago. Recently, the new transformer has developed a small oil leak. The 2 original transformers have an impedance of 5.7%, and the new transformer has an impedance of 2.5%. The discusions were regarding how much impedance mismatch was acceptable for a group of banked transformers. We are certain that 2.5% is too much mismatch.

Here are the particulars:

3 single phase 833 kva transformers, 7200 V-480/240 V banked wye-wye for 12,470Y/7200-480Y/277 V, maximum measured demand with kw and kvars is around 2 MVA, load factor of 0.5.

We're not convinced that the leak was caused by the impedance mismatch, but it still is a concern.

Anyways, the question I posed originally, was just a curiosity more than anything else.

 

[templdl]

Interesting.
I'll run that past on of my engineers.
Wye-Wyes are always intriguing applications. We make wye-wye transformers, the smaller onus made with 3 leg cores, as they get larger we add a tertiary winding to deal with magnetics that circulate in the core and 4 or 5 legs for larger transformers.

I your case you have (3) separate transformers configured a a Y-Y which is different that if they were on a common core. Other than the obvious difference in impedance how are the Neutral treated? Are they bonded together and then grounded?