delta transformer question

[dnem]

dnem -
My comment was that I have never seen a corner grounded 240D. Has anyone else ever seen one? If so, I'd be curious why it was speced that way. The only time I ever see 240 D is for specific loads like K2500 posted.

K2500 -
Was the one you refered to a corner grounded 240D, 240V high leg grounded D, or un-grounded 240D?

... 1] Only need one voltage

I don't know what that means. There is only one voltage - 3 phase 240V delta. Corner grounded or not, that is the only one there is.

What it means all depends on what?s being compared.
Comparing 3? corner grounded delta to 3? floating [ungrounded] delta, both have only one voltage.
Comparing 3? delta to 3? wye, delta has one voltage, wye has 2. . My comment wasn?t focused only on just different types of deltas.

... 2] Smaller cheaper transformer ...

I don't see that either. I've never heard of a transformer mfg making a different transformer for corner grounded D as opposed to ungrounded D.

Once again my comment wasn?t focused only on just different types of deltas. . When you take a certain size transformer enclosure and wind a specific amount of insulated copper in its windings, you?ll get more KVA out of it if you configure it as a delta. . You need a bigger enclosure and more copper for the same KVA wound wye. . That was my only point.

... 3] Don't need ground detectors

That's true, But the cost of three transformered 240V pilot lights isn't much.

carl

If because of the single voltage, cheaper transformer reasons you choose delta instead of wye, then a corner grounded will operate the same as a floating delta. . The floating gives you the option of continued operation after the first fault. . The corner grounded is safer in some ways.

Most of the delta secondarys that I see are center tapped/high leg types. . Of the remainder, I see many more corner grounded than floating. . I?ve actually only seen 2 floating delta systems during inspections. . Most of my exposure came from my brother-in-laws floating delta in his machine shop.

I?ve got a ?rare bird? for you.

Have you ever seen a corner grounded system that uses the building steel as the corner grounded phase itself ? . It has 2 wires in a steel conduit [the combination of building steel and conduit functions as the 3rd phase / grounded phase]. . No fault current only equipment ground anywhere. . Each motor case has a bond wire to the building steel. . It was installed during WW2.

David

 

[coulter]

... When you take a certain size transformer enclosure and wind a specific amount of insulated copper in its windings, you?ll get more KVA out of it if you configure it as a delta. . You need a bigger enclosure and more copper for the same KVA wound wye. ...

240D vs 208W right? Are you sure they would have different kva? I think they will be the same.

carl

 

[coulter]

... . The corner grounded is safer in some ways. ...

I've never seen any evidence of this. Why would this be?

carl

 

[hardworkingstiff]

I?ve got a ?rare bird? for you.

Have you ever seen a corner grounded system that uses the building steel as the corner grounded phase itself ? . It has 2 wires in a steel conduit [the combination of building steel and conduit functions as the 3rd phase / grounded phase]. . No fault current only equipment ground anywhere. . Each motor case has a bond wire to the building steel. . It was installed during WW2.

David

I've heard it was to save copper during the war.

 

[dnem]

... When you take a certain size transformer enclosure and wind a specific amount of insulated copper in its windings, you?ll get more KVA out of it if you configure it as a delta. . You need a bigger enclosure and more copper for the same KVA wound wye. ...

240D vs 208W right? Are you sure they would have different kva? I think they will be the same.

carl

No, I intended to compare same voltage to same voltage. . For example, take a 480v delta compared to a 480v wye.

On the wye, when a phase conductor carries 100a, the attached phase winding carries 100a.
On the delta, when a phase conductor carries 100a, multiple phase windings combine to contribute to the current flow on that conductor and there is no individual winding that carries the 100a. . Less current on the winding means less copper needed, smaller winding, and possibly smaller enclosure.

I might be wrong about the KVA because the 480delta individual winding voltage would be 480v at less than 100a. . While the 480wye individual winding voltage would be 277v at the full 100a. . I'm not sure how the wattage [or KVA] would compare.

David

 

[dnem]

... . The corner grounded is safer in some ways. ...

I've never seen any evidence of this. Why would this be?

carl

At my brother-in-laws floating delta machine shop, I've scene his employees stick their hands in places that have energized parts/terminals without using any care. . In the past they've always done so without incident because you can touch any single energized part anywhere in the building that's supplied by the floating delta and not get electrocuted even if you're standing in water.

But if a fault occures, the only change will be in the ground detection lights on the main gear up near the office. . If the employee is unaware of the change in the lights or forgets that something has changed, that same action can result in his electrocution.

Plus the issue that I mentioned before about the electrician taking a wiggy reading phase to ground and getting no voltage might wrongly assume the power to be off and start his work on the equipment. . He could be hurt/killed by electrocution or sudden equipment startup.

David

 

[jcopeland]

3-Wire Delta

3-Wire Delta

I?ve seen Corner Grounded Delta Systems used on remote pump stations and rock quarries because it only uses 3 conductors.
Saves a lot of money over long distances. The grounded phase is usually bare.

With high impedance grounds in different legs of an Ungrounded Delta System can cause inductive and capacitive voltages much higher than the supply voltage causing destruction to electronic equipment. I saw this happen on 2 300hp A/C drives.