460/220V Delta Transformer

[Kiwiguy]

G'day Everyone,

Please excuse my ignorance (hey, I'm from New Zealand!) but I would like to know why you would select an unearthed 3-phase supply from a transformer(eg. 460/220V delta)over a wye-connected supply with an earthed neutral. The particular example I'm looking at is a transformer to supply 220V air conditioning units, which will give an umbalanced load across the 3-phases (the chances of ending up with A/C units of all the same size and in multiples of 3 are pretty remote!)
Coming from a country which uses an MEN system, unearthed supplies don't seem right!
Perhaps a 480/220Y is not a standard transformer?
Any advice in understanding why Americans do the things they do would be much appreciated!

 

[busbar]

Kiwi, assuming your are referring to US/ANSI systems, a [220 or] 230VΔ 3ø system is not all that standard—a 208Y/120V system is, and is commonly served by a 480Δ—208Y/120V transformer, with the secondary-neutral wye point solidly (and locally) grounded. Multi-motor (packaged) air-conditioning equipment would be nominally rated at 200V, 1ø or 3ø on the nameplate, or possibly labeled 200/230V.

480∆—240∆ transformers are available but not as common for the reasons you seem to imply. As far a commercial appliances are concerned, in the 1- to 5-ton [12,000-60,000 btu] ratings, 460V versions are not too common if avaialbe at all. Roughly similar to your MEN system, 480Y/277V is popular, although it seems the practice of using smaller utility or facility-owned transformers, serving one or a few buildings as opposed to a larger area and transformer. A 75kVA demand is an approximate breakpoint for 480- versus 208-volt building service, with 240V 1ø {and some 3ø} used for residences and small commercial buildings.

 

[jbartos]

Suggestion: The unearthed/ungrounded power supply system is used for dedicated three-phase loads that are well protected. It gives an edge in such dedicated application for allowing one fault before the maintenance remedy it. Generally speaking, it is not that bad idea for special applications.

 

[ausphil]

kiwiguy,

Your bowlers are currently cutting our batters to shreds in the super six game. Hopefully we will inflict some damage when we get the ball.

Back to the thread. Is this a NZ application? Just to get the picture in my head, is the transformer you are talking about a 11kV/480V or 22kV/480V transformer (or similar distribution tx), or is it a special LV to LV transformer?
Is the secondary of the transformer a delta connection, or is it a star connection but the supply to the a/c is just a 3 wire supply? Are the a/c units single, two or 3 phase units (and are they special purpose units)?

Quite often on 3 phase a/c units, they will just run a 3 wire supply to the unit, but usually run an earth with it too (or sometimes direct earth at the unit if sufficient). If the units are a 2 phase motor (ie around 415V supply) then the neutral will be redundant and not required (not unlike my tautology!)

Probably more questions asked than answered, but hope some of it's worthwhile.

ausphil

 

[Kiwiguy]

Thanks for the replies everyone.
I should have supplied more background information originally.
For Ausphil...
The application is at a US based manufacturing plant where the main distribution is a 460V. Transformers are then used to supply loads such as lighting and GPO's at other voltages. As Busbar pointed out we have 460/277V transformers for lighting, and 460/208/120V transformers for 120V lighting and GPO's.
The existing electrical system I've inherited has 460Vdelta/220Vdelta transformers giving an unearthed (ungrounded as the Americans say) 3-phase 220V supply. As jbartos pointed out, this supply should be for balanced 3-phase loads only (as commonsense would dictate). However, at this site there are many single phase loads at 220V which are connected to two phases of the supply. I had assumed that this must be standard practice but am now thinking that it is just due to poor engineering in the past.
P.S : I hope the Aussies don't resort to underarm bowling!
Now for everyone...
How should I best supply these single phase 220V loads? My answer would be to install a 460V/380V wye- (star- for Ausphil)connected transformer, but again this is probably not an on-the-shelf transformer in the US.
Note that it is common practice in homes here to run 220V single phase loads by using two phases of a 208/120V supply to the house. I'm not sure if this is common in the US mainland.
Your advice much appreciated.

 

[busbar]

Two points.

Operation of 220V 1ø equipment from 220V 3ø transformer secondaries should not be much different than that of 380Y/220V service. Given that smaller dry-type transformers are decentralized and typically limited to ~75kVA, [delta] ungrounded-system transient overvoltages would not be a common cause of insulation failure, particularly where the drytype is served from a solidly grounded 480Y/277V source.

Just don’t let the 60- versus 50-Hertz motor performance throw a wrench in the gears.

Also in ANSI regions, finding or waiting for a 480Δ-380Y/220V transformer may get agonizing compared to a "catalog-listed" 480Δ-240Δ unit. Also, for branch-circuit protection, 250V fuses could conceivably be used for either voltage class, but a typical North-American continent electrician might go into convulsions trying to figure out why the hell he's measuring an unbelievable 380-400V between secondary phases—let alone finding 380V stamped on a transformer nameplate. Copper savings with the higher-voltage system would likely be the least of his concerns.

In know if it were me on a west-coast afternoon like today—(1.) clockout, (2.) my pickup leaving the plant and {3.} a fishing pole would have to take precedence over some sniveling shipping clerk’s non-functional air conditioner. One brief stop at the bait store on the way to the river will yield a paper carton of live worms and suitable recreational libations.

 

[Kiwiguy]

Busbar, you are obviously a man of vast experience as you have your priorities set(in my view)correctly!
Although fishing isn't my preferred recreational activity, I can appreciate the importance of recreational libations being readily available at the bait shop.
Just one query from your last post - are you saying it would be acceptable to supply unbalanced loads from a delta connected transformer secondary? If so, I will throw in a 480V/220V delta transformer, stop by the local recreational libation specialty shop and then go somewhere quiet where I can contemplate the serenity of the tropical paradise I live on.
While doing that I will write to the compilers of the NEC2002 suggesting that the next edition should have a "US Electrical Stuff for Dummies (and Kiwis)" section.
(I will also recommend that the entry should state "Talk to Busbar at Eng-Tips&quot

Again, thanks for your help and may the fish continue to bite.

 

[BJC]

I think most of those systmes are left overs. Most utilities will not supply a 220/120 3 phase 4 wire system system. The neutral in such a system is a center tap on one side of the Delta. The voltage to either side of the neutral is 110 volts. Voltage to the phase opposite the neutral is 190 volts. NEC rules call for this phase to colored orange and the panel labeled.

http://www.itvss.com/pdf/v3di.pdf

these shouldn't be used any more. Utilities use to offer them because by using an open delta arrangement a 110/220 three phase 4 wire system could be made using two single phase ( cheap ) transformers.

 

[busbar]

OK, if I follow your question OK, the balancing isn’t absolutely critical as long as you don't exceed the transformer’s rated current on each phase. It’s best from a transformer-loading efficiency/temperature-rise aspect and voltage-regulation conditions (and voltage balance for 3ø loads) to reasonably balance secondary loads, but the same holds true for just about any 3ø power-distribution equipment.

Staggering single-phase loads so that final phase currents are within 20% of the 3ø average is a good rule of thumb, and that’s boilerplate in many electrical-construction specifications. But then, for air conditioning, say one area cools to the right temperature and shuts off, with others still running. Your once-balanced condition now gets shot out of the saddle, so to speak. For the mythical transformer with 208Y secondary, 75kVA works out to be 75,000/208/1.732 = 208.2 amperes per phase. So, finding one phase at 203 amperes and two at 176 amperes should be no big deal. It’s a good idea to also check voltages while you have the transformer or panelboard cover off, to verify that they are in line. A lot of dry-type transformers have several 2½% taps, allowing limited voltage adjustment.