Unbalanced Parallel Conductors 7

[brhodes]

I'm a new engineer for an electric utility with limited experience and have stumbled on something that really puzzles me.

While infrared scanning a padmount transformer located outside of a plant we noticed overheating of conductors. Let me explain the setup.

This is a 500 kVA 208/120 volt transformer that feeds a 1200 amp main panel located inside the plant. There are 3 runs of 500 MCM per phase and Neutral to the main panel. The distance is approximately 30 feet from the transformer to the main panel. The cables are in 3 bundles. One cable per phase and neutral in each bundle(A,B,C,N per bundle). Each bundle is in its own underground conduit to the main panel inside the plant. The load is 80% 3-Phase.

While observing the transformer during the infrared scanning I noticed overheating of the cables on A-phase to the point of discoloring of the copper. The infrared camera showed a significant difference in temp on one conductor on each phase compared to the other two conductors on each phase. I used a clamp-on ammeter and on each phase I got an average of 200 amps more on one conductor than I did on the other two.(These results are from memory. I do not have the written values with me)

We checked for loose connections at the transformer and the main panel. The only loose connection was on one conductor on A-phase which showed signs of overheating (which probably only caused a problem because of the high unbalanced current. The connection was not significantly loose). After checking all of the connections I used a clamp-on ammeter and got the following results.

A1: 125A B1: 7A C1: 96A N1: 43A
A2: 178A B2: 395A C2: 216A N2: 158A
A3: 194A B3: 8A C3: 77A N3: 140A

Prior to making sure we had good connections A-phase showed similar unbalancing as B and C phases. The plant shut off all loads to that transformer and all conductors showed 0 current. The plant then turned on a small three phase load and the amperage was low on the conductors, but they still showed the same proportionality of unbalanced current.

I have read some threads on similar problems and the cause can be anywhere from cable positioning to loose connections. These figures just don't add up. Even the neutral current doesn't seem right. Can we have some circulating currents? Can the unbalanced conductors and high neutral current be related?

We plan on taking off the load tomorrow and verifying all connections are clean and tight on the transformer and panel.

Any help on this matter will be much appreciated!
Thank You

 

[waross]

Cathodic protection is a long shot and would be DC unless the equipment was damaged.
What kind of facility is it?
I have sometimes found clamp-on meters to be influenced by the magnetic field of adjacent conductors and so give errroneous readings. That may be a factor with the unusual neutral currents.
When you see neutral currents well in excess of the maximum phase unbalance, look for either harmanics or faulty current readings.
respectfully