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When I was at work the other day, one of the techs showed me a delta connected power supply in which the phase to phase voltages measured 120Vrms. What didn't make sense to me was when he measured the phase to ground voltages and got 65V, 65V, and 77V. I asked him how the ground was connected to the delta system in order to measure these phase to ground voltages, but he didn't give me a satisfactory answer. It can't be a high-leg delta because the numbers don't work, so does anyone know what type of delta connection this could be?

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  • \$\begingroup\$ Are you sure it wasn't being fed from a transformer with wye (star) connected secondary supplying the power? \$\endgroup\$ – Transistor Aug 12 '17 at 16:31
  • \$\begingroup\$ Ground a center tap on one of the delta windings. \$\endgroup\$ – Optionparty Aug 12 '17 at 23:25
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schematic

simulate this circuit – Schematic created using CircuitLab

Figure 1. "Isolated" delta supply with ground-fault monitoring.

Without a wiring diagram of your supply it is impossible to say. One possibility is that there is a neutral connection and an unbalanced load. Figure 1 shows a common setup where three incandescent lamps are used to monitor the three phases:

  • If there are no earth faults on the system all three lamps will illuminate at the same brightness.
  • If an earth fault occurs on one phase the phase lamp will go out and the others will be brighter.

The lamps, in effect, create a weak pull to centre around the neutral. (Don't forget that neutral is connected to ground at the incoming supply point or at the local supply transformer.)

If you do a CAD drawing of an equilateral triangle with sides of 120 V and strike arcs from each vertex - one of 77 V and two of 65 V they should intersect pretty closely. That is your "neutral" point which has been pulled away from the true centre by an unbalanced load.

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  • \$\begingroup\$ In that diagram, I don't see how the bulbs can be lit at all. The neutral doesn't connect back to the delta-connected secondary, so where's the return path? \$\endgroup\$ – pr871 Aug 12 '17 at 17:02
  • \$\begingroup\$ The bulbs will light even if the neutral wire isn't there. They would just form a three-phase wye (star)-connected load. By connected to the primary neutral the lamps (or another load) will tend to centre the voltages around 0 V. I asked below your post to check that the feed is actually from a wye-connected secondary. If so the voltages are easily explained. \$\endgroup\$ – Transistor Aug 12 '17 at 17:07
  • \$\begingroup\$ Ok sorry, I see what you mean now. I'm not sure if the feed was from a wye-connected secondary. However, I do know for sure that the outlet from which the voltages were measured is meant to emulate delta power present on a naval ship. The outlet was unloaded when we took measurements, but maybe there was an internal load as you have described? \$\endgroup\$ – pr871 Aug 12 '17 at 17:23
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Sounds like a Zig-Zag transformer intended for harmonic loads or loads with asymmetric currents like half wave rectifiers to cancel out harmonics upstream.

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