Corner grounded delta transformers theory

[patm72]

I am currently working on a project where the equipment is to be fed by an MCC powered by a corner grounded delta transformer. I was aware of the many configurations transformer windings could have, but I have a hard time grasping the idea of the corner-grounded delta... Grounding a phase, it just sounds wrong to me. Why does it work without busting anything in the first place?

 

[magoo2]

Grounding a phase is wrong with a grounded wye system because it will result in a phase to ground fault. Most folks are more familiar with this connection.

With a delta connection, the phase to phase voltages will be constant and that is where the load is connected to. The phase to ground voltages with a delta connection, unless it is grounded, will be a bit strange since they depend on the capacitance to ground so they won't be constant. By grounding a corner of the delta, you help stabilize the phase to ground voltages and the phase to phase voltages will also be stable. With the corner grounded, you'll wind up with two phase to ground voltages equal to the phase to phase voltage and the third will be equal to zero.

 

[waross]

Corner grounding is cheaper than grounding with a zig zag transformer or a wye delta transformer and better than not grounding. Ungrounded delta transformer banks may experience a fault condition known as a discontinuous ground fault which may see a common mode high frequency, high voltage superimposed on the phase conductors. This may cause widespread burn-outs through out a plant.
If you wish to limit ground fault currents you may corner ground through a resistance. Avoid grounding through an induction coil as the coil may not suppress possible high frequency voltages resulting from a discontinuous ground fault condition.

Bill
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"Why not the best?"
Jimmy Carter