Delta-Delta Transformer

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Grouch

Grouch

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New York, NY
Hi al,
Just wanted to make sure I follow these sections correctly: 250.20(B) and (D). If I have a delta-delta transformer, 3 phase, 3 wire delta connected (no 4th neutral wire originating from a phase winding that is center-tapped on the secondary)... I do NOT have to ground the transformer is that correct?... such as corner grounding. It is optional if I wanted to ground it?

Thanks again.
 
You still need grounding electrode connections and a SSBJ per 250.30(B) and you will need ground fault detection per 250.21(B)
You will not have a SBJ.
 
augie47 said:
You still need grounding electrode connections and a SSBJ per 250.30(B) and you will need ground fault detection per 250.21(B)
You will not have a SBJ.
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ah ok. So connect the GEC from the metal enclosure of the transformer to the nearest electrode, such as structural steel. and no SBJ, so no bonding between grounding and a neutral (since we don't have a neutral).
 
Grouch1980 said:
Hi al,
Just wanted to make sure I follow these sections correctly: 250.20(B) and (D). If I have a delta-delta transformer, 3 phase, 3 wire delta connected (no 4th neutral wire originating from a phase winding that is center-tapped on the secondary)... I do NOT have to ground the transformer is that correct?... such as corner grounding. It is optional if I wanted to ground it?

Thanks again.
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The only difference in the bonding and grounding between a grounded system and an ungrounded system is that the ungrounded system will not have a system bonding jumper. If you make it an ungrounded system, you will have to provide a ground detector device per 250.21(B), and marking as required by 250.21(C) and/or 408.3(F)(2).
 
Just done one this weekend, I always corner ground it so I don’t have to use a ground detector. It was a backfed step down used to step up, but now the plant has 480, so I had to reverse everything.
 
infinity said:
Any particular reason why you wouldn't use a Wye secondary?
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Actually I didn't specify it. A vendor recommended it for their equipment (all motor loads), to use a step down delta-delta transformer, from 480 volts down to 240 volts. The motors operate at a nominal supply voltage of 220 volts, 3 phase. They do have a +/-10% tolerance however. Would you still recommend a delta-wye transformer, with a secondary voltage of 120/208 volts?
 
Grouch1980 said:
Actually I didn't specify it. A vendor recommended it for their equipment (all motor loads), to use a step down delta-delta transformer, from 480 volts down to 240 volts. The motors operate at a nominal supply voltage of 220 volts, 3 phase. They do have a +/-10% tolerance however. Would you still recommend a delta-wye transformer, with a secondary voltage of 120/208 volts?
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Not a lot of people understand ungrounded systems, so unless the people who will work on the system have a good understanding of what an ungrounded system is, I would recommend a grounded system.
One reason an ungrounded system is sometimes specified is continunity of power as a single ground fault does not trip an OCPD, however you can also do that with an impedance grounded system, and eliminate the extreme high voltage that can be created when there is a restriking ground fault on an ungrounded system. That issue is one of the reasons many old ungrounded systems were converted to corner grounded systems.
 
don_resqcapt19 said:
Not a lot of people understand ungrounded systems, so unless the people who will work on the system have a good understanding of what an ungrounded system is, I would recommend a grounded system.
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My feeling as well. Electricians get confused when there is no voltage to ground while testing and that can lead to someone getting injured or worse because they think that the system is de-energized.

Grouch1980 said:
Actually I didn't specify it. A vendor recommended it for their equipment (all motor loads), to use a step down delta-delta transformer, from 480 volts down to 240 volts. The motors operate at a nominal supply voltage of 220 volts, 3 phase. They do have a +/-10% tolerance however. Would you still recommend a delta-wye transformer, with a secondary voltage of 120/208 volts?
Click to expand...
Since you do not need the neutral you could use a 240Y/139 volt secondary on the transformer which would be a grounded system.
 
infinity said:
Electricians get confused when there is no voltage to ground while testing and that can lead to someone getting injured or worse because they think that the system is de-energized.
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Some of them get confused when the come across anything they've never seen before. I'd say a great many average resi/basic-commercial folks have never even heard of high-leg delta, corner-grounded delta, 2-phase, or 240/139y (even though they're in the code book). That's not their fault, but then saying "This is all messed up" is.
 
zbang said:
Some of them get confused when the come across anything they've never seen before. I'd say a great many average resi/basic-commercial folks have never even heard of high-leg delta, corner-grounded delta, 2-phase, or 240/139y (even though they're in the code book). That's not their fault, but then saying "This is all messed up" is.
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Yeah, it’s more light industrial than anything. Some convenience stores use them, but it’s mostly in small industrial parks because of the foreign machinery. The Exxon down from @readydave8 shop is a delta.
 
Hey now, I've got a 4W open delta feeding my shop and I love it. Ive got three voltages 120 208 and 240, not to mention both 3 phase and 2 phase. Now I just need a couple transformers so I can have 480
 
Joethemechanic said:
Hey now, I've got a 4W open delta feeding my shop and I love it. Ive got three voltages 120 208 and 240, not to mention both 3 phase and 2 phase. Now I just need a couple transformers so I can have 480
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You would not have 3-phase and 2-phase unless you had a T connection not a 4 wire delta. Effectively our industry counts phases by the number of Line-Line voltages the system has, while ignoring the lines neutral ones. This is not a perfect definition because a 2-phase system, especially a 5 wire one, could have more voltages if you counted the cross pair voltages like L1-L2'.
 
jim dungar said:
You would not have 3-phase and 2-phase unless you had a T connection not a 4 wire delta. Effectively our industry counts phases by the number of Line-Line voltages the system has, while ignoring the lines neutral ones. This is not a perfect definition because a 2-phase system, especially a 5 wire one, could have more voltages if you counted the cross pair voltages like L1-L2'.
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A-C is 240 volts, N-B is 208 volts. They are 90 degrees apart. That's good enough to run a two phase motor if I have to. If I wanted to get fancy I could even out the voltage with a boost/buck transformer
I have no need for 5 wire two phase here. All my normal loads are taken care of by my 4W open delta high leg.

I know exactly what a Scott-T is. One regular pole pig and one pole pig with the primary tapped at the 86.6%. I'm from philly,
 
jim dungar said:
Yeah but it is not really 2-phase, if it was there would be no need for a Scott-T.
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The voltages are unequal, but it's still two phases 90 degrees apart. And if I add a boost on the 208v phase I can get them equal to within a few volts. The load doesn't care. Frankly it's just for testing some antiquated machine tool that I might see once every 5 years if that.

Although I've seen 2 phase machine tools run that way in smaller shops with 4W high leg services. Typically those motors are never loaded at nameplate HP for long.

And if you notice 208 is 86.6% of 240
 
Joethemechanic said:
The voltages are unequal, but it's still two phases 90 degrees apart. And if I add a boost on the 208v phase I can get them equal to within a few volts. The load doesn't care. Frankly it's just for testing some antiquated machine tool that I might see once every 5 years if that.

Although I've seen 2 phase machine tools run that way in smaller shops with 4W high leg services. Typically those motors are never loaded at nameplate HP for long.

And if you notice 208 is 86.6% of 240
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In a Scott-T transformer the 86.6% tab is used on the primary side not the secondary. Your arrangement, using on open delta, causes two sets of current current to flow through the A-N windings of the transformer. These issues are mitigated because your 2-phase motors present relatively little load on the 3-phase transformer.
 
Grouch1980 said:
Actually I didn't specify it. A vendor recommended it for their equipment (all motor loads), to use a step down delta-delta transformer, from 480 volts down to 240 volts. The motors operate at a nominal supply voltage of 220 volts, 3 phase. They do have a +/-10% tolerance however. Would you still recommend a delta-wye transformer, with a secondary voltage of 120/208 volts?
Click to expand...
I think Don mostly answered this but you need to either corner ground that secondary or if you want to leave it ungrounded must still install GES and EGC's for all non current carrying parts plus a ground detection system.
 
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