Open Delta Transformers

[Will96]

Could anyone tell me what would be the highest voltage that an open delta transformer's "wild leg" could reach? I have measured upwards of 350V to ground on a 230V system. Could this potentially be higher on other systems?

 

[DanDel]

Are you sure that you have an open delta?
Typically, a wild leg is used to refer to a 3-phase delta system(typically 230-240V phase-to-phase) with one winding center-tapped and grounded. The potential between ground and the two phases of the tapped coil are 1/2 the phase-to-phase voltage, while the potential between ground and the third phase(the wild leg) is higher by a factor of 1.73.

 

[cuky2000]

Hello all,

DanDel is right, in normal operating conditions, the higher phase-to-ground voltage is 1.73 time the lower phase-to-ground voltage for a transformer configuration with mid-tap grounded open delta. See also NEC fig 250-3, art 215-8, 230-56 and 384-3.

In a not normal operating condition, continuous overvoltage on the red-leg delta (wild leg) occasional could occur particularly on unstable open delta system. Some of those possible overvoltages are ungrounded connection, ferroresonance, harmonics, unbalance voltage, etc.

Unfortunately, there is not a cookbook to make a specific recommendation to determine the abnormal conditions. Consider checking first the obvious, transformer connection, balance voltage, regulation, neutral/ground connections, harmonic content, etc.

Good luck

 

[busbar]

Will—Look over the transformer-secondary configurations at

http://home.att.net/~benmiller/elecsys.htm

It sounds like you are describing a 3ø 3-wire system with no intentional ground connection to the serving transformer-secondary terminals. As Dan and cuky have suggested for a grounded four-wire delta system, you should not read any more than about 220 volts to ground—or not exceeding 255V for a “corner-grounded” system. If indeed no terminals on the secondary windings are intentionally connected to ground, it may be possible to read higher-than-normal ø-ø voltage to ground. This may be a case where there is primary-to-secondary leakage from the transformer-set primary side, which is almost always fed from a higher-voltage source. Also, a resonant or arcing condition in the 240V system could cause readings greater than nominal ø-ø voltage. As you might imagine, this can be stressful to electrical insulation on anything connected—with motors and transformers being the most easily damaged.