grounded line

[traction]

we have a 480/220 delta to delta connected transformer in our workplace.This supplies lighting and airconditioning units (window-type),occasionally we experience line being grounded(shorted ballast,strip wire insulation). How can this prevent this as we have measured line to line voltages reaching 400 v.

Incidentally we have a couple of newly installed equipment equipped with RCCD breakers which ussually trips.

 

[electricpete]

The delta secondary on the transformer creates an ungrounded system. Ungrounded systems may be subject to oscillation of the voltage to ground, particularly when insulation breakdown at one point in the system creates an intermittent arcing fault to ground.

I would expect you might measure high voltages to ground which might be destructive to the insulation. I would not expect that you would measure high phase-to-phase voltages as you reported.

If you search for ungrounded system in the electric power forum, I think you'll find more info.

 

[busbar]

I concur with electricpete's comments. Ungrounded-delta systems can be real headaches. I have a couple of suggestions but they apply to US standards. Is the system in the US? What is the kVA rating of the transformer? Does the bank have the center tap of one of the secondary windings brought out to a terminal or lead?

 

[jbartos]

Suggestions:
1. Use ground fault detector and remove ground faults. However, this would not prevent overvoltages, e.g. the mentioned 400V. This would need a suitable overvoltage suppressors.
2. Replace the trasnformer for 480V/240V with the star winding on its secondary, which may use a suitable grounding scheme, e.g. solidly grounded neutral, or high-resistance grounded neutral system.
3. Explore, whether the corner of the secondary delta winding grounding would be suitable. It is inexpensive solution with the following disadvantages:
3a. The necessity for positive identification of the grounded phase throughout the entire system.
3b. Instruments, meters, and overload relays should not be connected in the grounded phase.
4. Add a zig-zag dry type grounding transformer to establish a neutral.