High leg on 120/240 open wye-open delta service

[dsgn]

I am atttempting to solve a power quality issue. Through the use of monitoring equipment, I have retreived data which indicated a dip in voltage on the high leg from approx 208v to 98v(measured phase-ground). At the same time, the other 2 legs remained relatively unchanged at around 126v(measured phase-ground).

This is a 120/240v 2 pot open wye-open delta bank with a grounded center secondary bushing on the lighter transformer.

 

[davidbeach]

I would hope that it was the larger transformer that had the center tap. But in either case, there was a voltage sag on the phase that feeds the transformer that does not have the center tap.

 

[stevenal]

A ground fault and subsequent clearing on the high side phase David mentioned would cause such a sag.
By the way "lighter" does not refer to the size, but indicates it is used for lighting (and other single phase) loads.

 

[davidbeach]

An other example of being divided by a common language. If dsgn had said the "lighting" transformer I would have understood immediately, but "lighter" didn't make any electrical connection and I didn't figure it had anything to do with hue or saturation. ;-)

 

[mc5w]

I think that he meant that the lighting transformer has the center tap. Commonly, the high leg transformer on these banks is rather small which adds voltage imbalance to a bank that already has voltage imbalance from other sources.

One source of imbalance is that even if both transformers are the same size and supplying balanced 3-phase load there is unbalanced inductive voltage drop in the bank. Also, when such a bank supplies balanced 3-phase load the primary neutral current is 1.73 times the current in the hot leads. If you are close to the supply substation this is not a big issue but if you are far away it could be a problem because it will add to voltage drop during starting. The primary neutral is often spaced 2 to 4 times as much from the hot wires as the hot wires are from each other.Also, one a wye primary system single phase and open delta transformers are limited by the phase to neutral voltage.

Some of the newer technologies such as lower loss insulators and lower no load loss transformers. Wound transformer cores using thin laminations have very low losses compared even to amorphous iron which is a more or less dead technology.

Part of the idea of an open wye open delta bank is to save on the no load loss of the third primary hot wire and transformer. Oftentimes the no load loss of that third hot wire and transformer outweighs the installation cost. Most of the cost of an overhead distribution line is in the poles and the first 2 wires. Adding 2 more hot wires involves a relatively small marginal capital cost but the no load loss is 3 times as much. If you have a load that uses lots of kilowatthours such as a goethermal heat pump then justifying genuine 3 phase power is easier.

What some farmers even do for the occasional operation of heavy 3-phase loads is to use a generator. After accounting for blackouts they get more utility from a generator for how often they need 3-phase. I have also seen a machine shop that used a natural gas powered 3-phase generator instead of the full fledged 3-phase primary line right out front. The generator was cheaper than paying Ohio Edison $15,000 to put up a 3 phase bank and then paying 16 cents per kilowatthour.