3 Phase Dropped leg

[Sparky Adam]

I had a building I was in (120/208 3 phase) start acting weird. Some of the lights were working, the battery backup troffers were flashing, whole sections of the building not working.

After testing I found that A and C phase had about 65 volts to ground and B phase had 120. The power company said that we had lost a leg, but the symptoms looked more like a dropped neutral.

Can any of you explain this? It seems that if we lost a single leg that we wouldn't have any voltage on that leg to ground.

 

[Jraef]

You were reading the voltage through something that was connected phase-to-phase, like a coil on something that was switched on or a transformer primary.

 

[Sparky Adam]

You were reading the voltage through something that was connected phase-to-phase, like a coil on something that was switched on or a transformer primary.

Why would that show as half the voltage on two of the legs if only one of the legs had dropped out?

 

[synchro]

It appears to be exactly what you'd expect from losing one of the lines feeding the primary of a delta-wye transformer.
That would leave the delta primary winding around one leg of the magnetic core still fully connected, and so the corresponding secondary winding on that leg would provide a full 120V output on phase B. But the other two primary windings would be in series and sharing the voltage across the two intact inputs, with the common connection between these windings no longer connected to a line input. And so in this case each of these primary windings would share about half of the voltage, and so their corresponding secondary voltages would each be about half of the normal value.

 

[kwired]

Like syncro mentioned, you likely have a delta -wye transformer.

If you had wye-wye you probably would have seen normal voltages between two of the secondary lines and ground but the third one would just be non functional.

Wye-delta (with neutral conductor of the wye connected) you won't necessarily notice anything if a primary line goes down, particularly if load level is low, as it just will continue to operate as an open delta system in this situation.

 

[LarryFine]

Wye-delta (with neutral conductor of the wye connected) . . .

Isn't that a no-no?

 

[winnie]

Isn't that a no-no?

The utility follows different rules and has different needs then the building wiring normally discussed here.

When you have a wye:delta transformer, the voltage balance on the delta acts to force the star point of the wye to be exactly at neutral relative to the phase conductors.

If you connect a wye:delta transformer (with its neutral) to a wye service, then you can see very large circulating currents in the transformer and neutral as the transformer fights to correct phase imbalances. So in general simply don't connect the neutral of a wye:delta transformer.

But sometimes the utility wants this balancing effect. In this case the transformer needs to be sized (and protected) suitably for the size of the distribution system.

Jon

 

[Hv&Lv]

We don't solidly ground the wye side of our wye-deltas…

 

[winnie]

We don't solidly ground the wye side of our wye-deltas…

Never? Or hardly ever?

 

[Hv&Lv]

Never? Or hardly ever?

Well… hardly ever. Although im trying to think of one that is grounded and not floating wye.
Normally ground with a solid blade when closing in the primary fuses, then pull the solid blade to float the wye

 

[kwired]

Isn't that a no-no?

Single core transformer it usually is. When you make a three phase bank out of separate single phase transformers it doesn't have the same issues as a single core does and it is pretty common at least around here for pole mounted transformer banks by the POCO to have Wye primary connection that includes connection to the MGN. Open delta banks must use the MGN on the primary or else you only have single phase capability if only using two conductors for the supply.

 

[Sparky Adam]

Like syncro mentioned, you likely have a delta -wye transformer.

If you had wye-wye you probably would have seen normal voltages between two of the secondary lines and ground but the third one would just be non functional.

Wye-delta (with neutral conductor of the wye connected) you won't necessarily notice anything if a primary line goes down, particularly if load level is low, as it just will continue to operate as an open delta system in this situation.

Makes sense. Thank you.