Center Tapped Delta Transformer 1

[Will96]

I recently updated a surge protection circuit to include line to ground protection. Would equipment that is connected to a center tapped delta configuration cause this type of surge suppression to be stressed? The reason being is that now, the MOVs are failing where as before, when only utilizing the line to line protection, the equipment ran fine. Also when two MOVs are connected in series, will the energy absorption be doubled?

 

[busbar]

4W∆ [240/120?] is not a symmetrical configuration. Is the surge device clearly intended for the application?

 

[Will96]

This device was intended for a balanced 3phase 230VAC system. My question is why would the first MOV configuration(line to line protection only work and the MOV configuration with the added ground to line protection fail?

 

[resqcapt19]

A 4 wire delta 120/240 volt system will have two phase conductors that are 120 volts to ground and the third will be 208 volts to ground. The phase to phase voltage will be 240 for all combinations. The phase with 208 to ground may be causing your problems.
Don

 

[busbar]

Failure may be from the [acceptable-with-limitations but sometimes misunderstood] normally unequal line-to-ground/"neutral" voltages, applied to a device intended for symmetrical ø-ground systems. For plain-vanilla 4W∆, Bø-grd voltage is typically 1.73x Aø-grd or Cø-grd. At this point, careful review of installation instructions, nominal circuit voltages, device ratings and requisite overcurrent protection may be in order.

For series-connected MOVs, there could be damage from unequal per-device voltage division, whether under static or dynamic conditions. There are several PDFs on MOV application for AC circuits listed at

http://www.littelfuse.com/ASP/Search/Detail.asp?ID=380

 

[busbar]

A characteristic failure mode of misapplied MOVs produces significant and undesirable quantities of flame, smoke and radiated heat.

 

[jbartos]

Suggestion to Will96 (Electrical) Dec 23, 2002 marked ///\\I recently updated a surge protection circuit to include line to ground protection. Would equipment that is connected to a center tapped delta configuration cause this type of surge suppression to be stressed?
///Yes, it would, e.g. during switching.\\ The reason being is that now, the MOVs are failing where as before,
///The MOV may need higher ratings, e.g. energy dissipation rating.\\ when only utilizing the line to line protection, the equipment ran fine. Also when two MOVs are connected in series, will the energy absorption be doubled?
///It might, depending on the voltage rating of MOVs and surge wave. However, the paralleling of MOVs should double energy absorption.\\

 

[jwerthman]

For a 4-wire delta system, the voltage from ground to the opposite corner from the center-tapped ground will be 268 volts, not 208 volts. The voltages from the other two corners to ground will be 120 volts and all phase to phase voltages will be 240 volts. The "wild leg", as the lineman like to call it, is always a potential problem since it is higher than all of the other voltages and generally can't be utilized for anything. I suspect your MOV protection can't tolerate 268 volts continuously.

 

[Will96]

jwerthman: What is your reasoning/theory behind your statement that the voltage from ground to the opposite corner from the center-tapped ground will be 268V and not 208V?

 

[jwerthman]

will96: I stand corrected. Goes to show you the danger of a quick response. resqcapt19 was correct that the odd voltage to ground will be 208 volts, not 268 volts as I incorrectly stated. I'll check my math a little better next time. Thanks.