Harmonic current through a Delta-Wye 2

[sparky1944]

I am looking at a problem with a cable run on the primary side of a 480 delta - 208/120 transformer. The transformer feeds loads with relatively high harmonic current. The equipment ground wire on the primary side has severed from what looks like overcurrent but it is within NEC specs (#2 AWG for a 4/0 circuit).

Is it possible that 3rd order currents passed through the #2 ground. I can't figure out how, but I also can't figure out how the #2 ground could have overheated.

 

[jghrist]

The zero-sequence currents on the low side (fundamental or harmonic) will not flow in the high side of the delta-wye transformer. You may measure some 3rd harmonic current on the high side, but it will not be zero-sequence. Third harmonic currents are only necessarily zero-sequence if they are balanced. I have measured significant third-harmonic currents on the primary of a delta-delta transformer serving an arc furnace.

Could you have had a ground fault that damaged the grounding conductor? Another possibility is a ground loop formed by grounding the neutral in two places on an adjacent circuit serving unbalanced phase-to-ground loads. Some of the unbalanced current may be flowing in the 480 volt equipment grounding conductor.

 

[sparky1944]

Grounding the neutral in two places? It seems to me that would be difficult with a Primary delta. There is no neutral. I suppose a poorly grounded neutral on the secondary could allow for a ground potential rise but it would take well over 100 amps to damage the #2 AWG.


A little more info:
There was (eventually) a ground fault and that may have been what caused the neutral to overheat; however, the primary circuit was protected with 200 amp fuses. They should have blown before the #2 would have severed from overheating. I am somewhat puzzled. Any other ideas?

Thanks

 

[DanDel]

sparky1944, I think jghrist said a "ground loop formed by grounding the neutral in two places on an adjacent circuit". Note the "adjacent circuit" part. He's talking about a ground loop involving your ground connection which is passing current from induction. It's easy to check, simply repair the ground and measure current through it with a clamp-on ammeter(true rms).
You state that there was a ground fault, but the 200A fuses should have blown before the #2 wire. This is not necessarily true. Depending on the type of fuse, enough fault current could pass for enough time to damage the cable first, you will have to check the fuse curve and compare it to the cable damage curve.

 

[jbartos]

Suggestion to sparky1944 (Electrical) Oct 20, 2003 marked ///\\I am looking at a problem with a cable run on the primary side of a 480 delta - 208/120 transformer.
///Please, clarify which side is primary and which side secondary. Also, state what kind of system grounding is at the transformer and where it is located.\\ The transformer feeds loads with relatively high harmonic current. The equipment ground wire on the primary side has severed from what looks like overcurrent but it is within NEC specs (#2 AWG for a 4/0 circuit).
///Higher harmonic content might cause the severence.\\
Is it possible that 3rd order currents passed through the #2 ground.
///Yes, it is possible. It depends on the answers to questions asked above and one below.\\ I can't figure out how, but I also can't figure out how the #2 ground could have overheated.
///Please, what is kVA rating of the transformer?\\\

 

[rbulsara]

sparky:

Your second post is more helpful. It appears that you had a continuous ground fault of in excess of 100A but less than 200A (possibly close to 200A) which would not blow a 200A fuse, but heat up #2 wire. The heating gets only worse with loose ground connecitons. The groud fault must have had some impedance, a common occurance for a wire loosely touching ground or a winding fault.

By the way the secondary side neutral current or harmonics has nothing to do with the primary side EGC, specially if secondary neutral is effectively grounded to a groudning electrode (such as building steel) and transforemer frame.

 

[jbartos]

Suggestion to sparky1944 (Electrical) Oct 20, 2003 marked ///\\Grounding the neutral in two places? It seems to me that would be difficult with a Primary delta.
///Only over an added grounding transformer.\\ There is no neutral. I suppose a poorly grounded neutral on the secondary could allow for a ground potential rise but it would take well over 100 amps to damage the #2 AWG.
///If there is a poor connection, the damage may happen even at lower current than 100A\\\

A little more info:
There was (eventually) a ground fault and that may have been what caused the neutral to overheat; however, the primary circuit was protected with 200 amp fuses. They should have blown before the #2 would have severed from overheating. I am somewhat puzzled. Any other ideas?
///See my previous posting for questions. Please, disregard those questions answered in this post, e.g. where is primary and secondary. However, the system grounding still needs clarification. Normally, there is a neutral solidly grounded. The ungrounded secondary wye is not widely used because of high voltage transients during arcing on the secondary.
Third order harmonics may return through the ground since the ground currents are flowing from the ground (from wherever they leak into the ground) into the transformer neutral connection.\\Thanks

 

[sparky1944]

The secondary neutral of the 480 Delta to 120/208 Y is solidly grounded and is also connected to the #2 Ground. The operating curves for the 200 Amp fuses are WAY below the thermal damage curve for the #2.

I suppose that we could have a ground loop but that would be a hell-of-a loop to pump sufficient current to overheat/fail a #2 copper.

I'm stumped.....
Thanks
Ron

 

[jghrist]

If there were a fault to the grounding wire, the wire may burn open at the point of the fault because of localized arcing. Thermal damage curves do not take localized heating into account. You would expect to see damage to the phase wire at the grounding wire damage point, however. Is there a possibility that the phase wires were replaced after a fault but the grounding wire wasn't? Is the grounding wire damage at a point where it could have been exposed to a fault to uninsulated energized equipment?

Another possibility is that a lightning surge entered the grounding wire from the earth and followed the grounding wire to the grounding electrode.