Y-Y WHY?

[brian john]

We have a site that at present has a delta-wye 12470-208/120 VAC Transformer, the transformer is maxed out and the utility is upgrading to a larger transformer, the new transformer they are providing is a wye-wye 12470/7200 to 208/120 VAC.

My question would be why, the site is a large warehouse converted to refrigeration storage, they have maybe 500 super freezers (my term) that keep products at -35 F. I believe all the compressors are 208 single phase (maybe 208 3 phase), if this is important I can get the actual motor specs). I do not believe there are any large harmonic rich loads.

In reading about wye-wye there seem to be some issues that you do not have with delta-wye transformers.

 

[jim dungar]

In reading about wye-wye there seem to be some issues that you do not have with delta-wye transformers.

If this is utility owned equipment, there is little reason to worry if you will be solidly grounding your side of the wye.

Most of the literature you have read likely deals with customer owned wye-wye transformers. Customer premise wiring and protective device selection is rarely the same as utility grid wiring and protection. Different systems-different issues.

 

[T.M.Haja Sahib]

We have a site that at present has a delta-wye 12470-208/120 VAC Transformer, the transformer is maxed out and the utility is upgrading to a larger transformer, the new transformer they are providing is a wye-wye 12470/7200 to 208/120 VAC.
My question would be why

It is just a matter of money:a wye-wye transformer has lower insulation requirements and so less costly compared to a delta-wye transformer of the same rating.

 

[rbalex]

I agree with Jim - and emphasize you must be solidly grounded on "your" side of the transformer. (It's already a code requirement anyway. [250.21(B)(1)]) Delta-Y and Y-Y differ in the way faults, especially ground-faults, reflect between primary and secondary, but being on the "load" side usually makes little difference.

 

[mivey]

In reading about wye-wye there seem to be some issues that you do not have with delta-wye transformers.

I agree. I have never understood why there seems to be such a move to wye-wye either on utility distribution systems. When I ask, most do not know why but it is just what they have been doing. If I had to guess, I would say the fault lies with some sales rep or management person.

 

[rbalex]

I guess I should have acknowledged TM on this one too. Y-Y are a bit less expensive since insulation near a grounded neutral can be reduced somewhat. That said, at the "user" end of a utility supplied transformer, everything including faults look pretty much the same assuming the transformer impedance is the same.

 

[jim dungar]

One advantage to utilities is the ability to reconfigure a transformer bank as the needs of their grid changes.
We had a local POCO reconnect a grid from 14.4kV up to 24.9kV. They took all of their delta connected transformer banks and reconnected them into a wye. For 15 years, primary service customers had been told this could happen and therefore it would be advisable for their equipment purchases to accommodate this change.

 

[CONTROL FREQ]

I agree. I have never understood why there seems to be such a move to wye-wye either on utility distribution systems. When I ask, most do not know why but it is just what they have been doing. If I had to guess, I would say the fault lies with some sales rep or management person.

This is something I?ve never taken the time to understand. I used to work with wetblasts. (slurry of sand and water often used to clean or prep metal). The motors/pumps were always wired wye/delta through a basic reversing contactor? It was explained to me that we started the motor in a wye config, due to the high torque needed to bring the ?settled? sand into the slurry, then after a timer ran out, the wye circuitry kicked off and the delta took over (less torque? Less power consumption?) I never really understood it? Always assumed it was a torque/normal running cost factor???:angel:

 

[templdl]

Check out this link which includes a fairly good discussion on this subject. http://www.eng-tips.com/viewthread.cfm?qid=88701
Here's another article: http://www.mikeholt.com/technical.p...erroresonance and Its Effects On Transformers


Please note that a Y-Y transformer can not be built with a conventional 3 legged core as a D-Y is designed with because of problems with ferroresonance. A Y-Y is commmmonly designed with a 5 legged core. Westinghouse had a propietary 4 legged design which may have transfered to ABB when they purchased ththe Westinghouse business. As such a Y-Y transformer is more expensive.
Ferroresonance has to do with the magnetic path taken around the core of the transformer. Remember that it is not as simple as reconnecting the primary winding into a wye.

With smaller transformers 3-legged cores are commonly used but add a tertiary winding .
This I took from Yahoo "Ask" Website: A tertiary winding is to provide a closed-circuit path for zero-sequence currents/fluxes. That is most typically the case with transformers that, for system reasons, must be connected wye-wye (star-star for those in the IEC world). With a wye-wye transformer, the only path for zero-sequence flux is the tank of a transformer (and if the three-phase bank is comprised of three single-phase units, even that does not exist). The tank is a high-impedance path, so the result is that the zero sequence impedance seen looking into the transformer is very high. Also, the circulation of zero sequence current through the tank will cause the tank to heat and lead to paint failure. A tertiary winding presents a low impedance path to zero sequence currents, thereby reducing the zero sequence impedance presented to the outside world, while avoiding the problem of tank heating.
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I have attache and old ducumant that I have on file which illustrates the proble with Ferroresonance in the core design of a transformer.

 

[ron]

Google the Whyes of the Wyes - GE Industrial Solutions

It has some interesting perspectives and issues.

 

[Hv&Lv]

If a single line of a three phase system gets torn down with a delta bank downstream, there is backfeeding and grounding the line for safety can be tricky. With single phase reclosers, you can keep much more line on, but you can't install deltas behind these reclosers. With a Wye-Wye, the single phasing doesn't create so much havoc. There is still the small problem with common core Wye-Wye padmounts, there will still be some "phantom" voltage on the down line, but it will ground quickly and with no problem.

 

[T.M.Haja Sahib]

My question would be why, the site is a large warehouse converted to refrigeration storage, they have maybe 500 super freezers (my term) that keep products at -35 F. I believe all the compressors are 208 single phase (maybe 208 3 phase), if this is important I can get the actual motor specs). I do not believe there are any large harmonic rich loads.

You may check up if any HV neutral line is present and connected to the primary neutral of the wye-wye transformer provided by the POCO now or there is any delta connected tertiary winding in it.It is because, in the absence of both,the secondary voltages would become distorted to cause deleterious effects on your loads.

 

[Besoeker]

It is just a matter of money:a wye-wye transformer has lower insulation requirements and so less costly compared to a delta-wye transformer of the same rating.

I'd be interested to know how you conclude that to be the case.

 

[T.M.Haja Sahib]

I'd be interested to know how you conclude that to be the case.

See

I guess I should have acknowledged TM on this one too. Y-Y are a bit less expensive since insulation near a grounded neutral can be reduced somewhat.

 

[T.M.Haja Sahib]

I'd be interested to know how you conclude that to be the case.

In addition to reduction in insulation cost,the number of turns in the primary of Y-Y transformer is less and so copper used less corresponding to phase voltage instead of line voltage in the case of delta primary.

 

[Besoeker]

I guess I should have acknowledged TM on this one too. Y-Y are a bit less expensive since insulation near a grounded neutral can be reduced somewhat.

I shouldn't have thought it would make a great deal of difference.
I'm currently bidding a 12-pulse variable speed drive. I'm offering two options for the drive transformers - low voltage (690V) and medium voltage (3300V).
Configuration is Dy11d0 in both cases. The transformer prices are within 1% of each other. The MV inverter is more expensive but the additional cost will be offset by the reduced cost of the cabling.

 

[Besoeker]

In addition to reduction in insulation cost,the number of turns in the primary of Y-Y transformer is less and so copper used less corresponding to phase voltage instead of line voltage in the case of delta primary.

Fewer turns but higher current by the same ratio.

 

[T.M.Haja Sahib]

Fewer turns but higher current by the same ratio.

If the transformer is delta connected on the primary,the line current is Sq.Root(3) times the phase current.Here the primary is star connected and the line current is equal to the phase current.So the line current is actually less than that due to delta primary for two wye-wye and delta-wye transormers of same rating.

 

[T.M.Haja Sahib]

I shouldn't have thought it would make a great deal of difference.
I'm currently bidding a 12-pulse variable speed drive. I'm offering two options for the drive transformers - low voltage (690V) and medium voltage (3300V).
Configuration is Dy11d0 in both cases. The transformer prices are within 1% of each other. The MV inverter is more expensive but the additional cost will be offset by the reduced cost of the cabling.

I am afraid this is not relevant to this thread.

 

[T.M.Haja Sahib]

Fewer turns but higher current by the same ratio.

Yes.You are right.I committed a mistake in post#18.Sorry.