Delta Delta transformer
- Thread starter GLSA
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electrofelon
Senior Member
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- Cherry Valley NY, Seattle, WA
Well if they did not ground one of the phases, then it IS an ungrounded system (unless it has a center tapped winding that is grounded). IF it does not have center tapped winding, you may leave it ungrounded but you would need fault detectors. For most general applications, I would say it is simplest to corner ground it.
- Location
- Placerville, CA, USA
- Occupation
- Retired PV System Designer
There are a very limited number of possibilities here:
1. The output winding is a high leg delta, in which one of the 240 windings is center tapped and the center tap is grounded.
2. The output windings are simple delta, and one of the three corners is grounded. It need not be the B phase, but that is customary.
3. The output winding is totally ungrounded, with the whole set of windings free to float at any voltage with respect to ground while maintaining a constant 240V from terminal to terminal.
There is no way that it could be grounded other than 1 or 2.
However, in the case of 3, the output terminals may not appear to be ungrounded.
There will be a certain amount of capacitance between each winding and ground, and between the wires connected to the secondary and ground.
These capacitance values will be roughly equal under normal circumstances, so they form a wye voltage divider of capacitors with the center point firmly grounded. (The raceways and the transformer shell are grounded.)
That means that if you use a high input impedance meter to measure between phase terminals and ground you will get approximately 137V on each of the three.
But that 137V cannot sustain a load, because of the high impedance of the stray capacitors. That means that if, for example, you connect a 240 or even 130V light bulb from A, B or C to ground it will NOT light.
If you allow for the effect of connected loads, it is also possible that one of them could also have the effect of trying to center the delta with respect to ground, and even allow more current to be measured.
- Location
- Placerville, CA, USA
- Occupation
- Retired PV System Designer
Certain types of arcing ground faults can pump up the delta to twice or more the nominal voltage with respect to ground. This could be more than the insulation in some wiring, distribution equipment and loads can tolerate.
Also, someone expecting no more than 137V to ground could be exposed to 480V to ground or more.
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