impedance corner grounds

[stevenal]

There has been much discussion in the forum regarding ungrounded delta versus grounded wye systems. Grounds provide voltage stability and fault clearing, ungrounded delta allows you to ride out the first ground fault. Impedance grounded wye systems seem to allow the best of both worlds. But why have I never heard of an impedance corner grounded delta? Seems like it would allow an existing delta system to stabilize their voltages with little expense. Can't find a prohibition in the NEC. I cannot be the first to have thought of this. Is such a thing ever used? Just wondering.

 

[jbartos]

Suggestion: The major drawback of the delta connection with corner grounded connection is that there will be higher voltages to ground than at wye with its neutral resistively or solidly grounded.

 

[stevenal]

Sure, but the existing ungrounded delta already has line to line rated insulation. Will impedance grounding a corner cause voltages to exceed line to line? One source told me yes, during faults. I am not yet convinced of this.

 

[jbartos]

Comment: The larger voltages to ground cause larger short circuit currents. These are detrimental and equipment bracing must be increased, which is costly.

 

[mvcjr]

jbartos,
I don't think there would actually be an increase in short circuit currents that the transformer would face when one corner of a delta is grounded. Sure, ground fault currents would be larger but a line-to-ground fault is just equivalent to a line-to-line fault to which the equipment is already braced for.

One advantage on the protection side is there would be one less fuse, breaker, etc. needed for a corner-grounded system than an ungrounded delta system.

 

[busbar]

It may work. Offhand, the only thing would be dealing with some continuous ‘grounding’-resistor heat dissipation from dissymmetry. It is sort of like one simplified means of ground detection—look at a single phase-to-ground voltage and monitor it for both under- and over-voltage.

 

[stevenal]

busbar,

This system would be unigrounded at the transformer. The grounded phase would connect on the transformer side of the resister. There would be no path through the resister for load current under normal conditions. The only non-fault continuous current the resister would carry would be due to capacitive coupling.

jbartos and mvcjr,

The resister or reactor would be sized to limit ground fault currents to acceptable continuous levels just like with an impedance grounded neutral system.

 

[davidbeach]

If you high impedance ground a corner of a delta system I would be very leary of using only two poles over-current on a three phase circuit. In a solidly grounded corner grounded system, the grounded phase is at zero volts to ground, and any other phase to ground is a line-to-line fault. In an impedance grounded corner grounded system a ground fault on one of the other phases will send the faulted phase toward zero volts to ground with the "grounded" phase raising away from ground by the amount of voltage drop caused by the vault current in the resistor. The limiting case is that the "grounded" phase would be full phase voltage away from ground.

 

[stevenal]

That's how I see it. Addition of the ground ensures ground potential stays at the corner, or moves away along one of the adjacent legs of the delta. Phase to ground voltage doesn't exceed phase to phase.