parallel feeders 2

[tito103]

Hello,
I was infrared scanning a 2000 amp service 480 volts. The cables ran from a 2000 amp transfer switch too a 2000 amp main circuit breaker. The feeders are in cable and installed in cable tray. Approximate cable length 80ft. There where 5 parallel 600mcm feeders per phase.The problem I was having is a couple of the feeders are drawing twice the current then the other feeders in the same phase? Does this mean that the cables drawing less current have a higher contact resistance?

 

[itsmoked]

Or maybe your meter is suspect? Twice the current is um... a lot!

Keith Cress
Flamin Systems, Inc.- <

http://www.flaminsystems.com>

 

[dpc]

Did you use a clamp-on ammeter to measure the current?

And what did the thermograhpy show?

 

[tito103]

I noticed a 40 degree rise between cables. I clamped on my meter and noticed one cable had approx.560amps, the other four cables had approx 200 amps. The swichgear current meter was showing around 1300 amps total current per phase so the total current reading makes sense. This is happening on all three phases.

 

[wareagle]

I had this problem before with compression bolt connectors.
Remove the hi current connector and brush and clean it. It probably has a higher resistance connecton.

 

[wareagle]

Let us know what you find.

 

[dpc]

It's possible for contact resistances and other factors to cause some surprising differences in current. The problem can be worse in shorter feeders.

You might also want to verify that the cables are basically the same length and are routed in a similar fashion.

 

[tito103]

I am waiting for a shutdown schedule. This is a critical load feeder. I will post results after shutdown. Thanks for the info!

 

[waross]

One couple of observations.
First

I had this problem before with compression bolt connectors.
Remove the hi current connector and brush and clean it. It probably has a higher resistance connecton.

This is entirely possible. I would check the connections.

On the one hand, I have had problems with clamp-on meters being affected by strong fields from adjacent cables, BUT I understand that you detected a temperature difference between the cables and your total currents correlate well with the switch board meter.
With that much difference in current a bad connection would be developing heat. If you were infared scanning you may have noticed it.
On the other hand, it is possible that there is an impedace problem upsetting the distribution of current. This conclusion is supported by the fact that you did not report overheated connections and by your report that one cable in each phase is overheating.
How are the cables arranged in the tray? Improper phase arrangements in cable trays can cause unequal, division of currents.
I'd give it about 50/50 between bad connections and impedance problems due to improper cable arrangements.
You may want to try disconnecting the overloaded cable to see if the currents remain balanced on the other phases.
yours

 

[itsmoked]

I would expect warmer connectors at one or both ends of the lower amp cables. Do you see this?

What kind of structure do the cables terminate on? A buss bar? ?

Keith Cress
Flamin Systems, Inc.- <

http://www.flaminsystems.com>

 

[davidbeach]

Remove the hi current connector and brush and clean it. It probably has a higher resistance connection.

Nope. Just the opposite, high resistance connection would lead to low current. Somehow one cable has significantly lower impedance than the other four.

Are the five runs identical in geometry? Are the conductors grouped in groups of three with one of each phase in each group? Is there any magnetic material that forms a loop around some of the conductors but not all of the conductors? Something is very wrong, and given four lower current conductors and one higher current conductor, I would concentrate on the geometry of the installation, including all hangers and appurtenances and leave the connections for later. (If it were four higher current and one lower current conductor I would immediately go after the connections on the lower current conductor, but to say that 80% of the connectors are suspect is a bit too much.) Include the configuration of how the conductors exit the transfer switch and enter the main breaker enclosure in your analysis of the geometry of the situation; are there an equal number of conductors of each phase in each hole?

 

[tito103]

Hey Guys, just for the record the equipment is pushing 35 years. I know another testing company had scanned this equipment 4 years ago, and did not mention anything about this in there report. Another thing I noticed they bunched all the same phases together on the cable tray. The cable is tied down with plastic tie-wraps. The fields are not interupted with any ferrous material. And the cables enter the enclosure in one large rectangular slot.

 

[waross]

Hello tito103
Bunching each phase together is probably the cause of the unbalance.
yours

 

[wareagle]

david beach
Thanks for that correction. I thought about it some time after the post and realized what I had said. You need to check the low amperage connector.

 

[davidbeach]

So you have 15 individual conductors, five each of three phases, plus any neutral and ground conductors; and for those 15 conductors, there are three bundles, each with five conductors of the same phase. Is that correct? And in one of those phases, one conductor is carrying 560A while the other four are each carrying 200A, for a total phase current on that phase of 1360A. Is that also correct? Which phase are these readings from? What are the currents in the individual conductors of the other two phases?

The cable inductance would be lower if the cables were grouped such that each group had one conductor of each phase, plus a neutral if there are also neutrals. The segregated bundles will have a higher inductance, and therefore a higher total impedance, but unless each bundle very carefully maintains its configuration within the bundle the entire run, each conductor should have nearly identical impedances, a 5% difference in cable impedance would be extremely high and no where near enough to cause what you are seeing. I would expect more difference in impedance due to differing lengths of conductors than from geometry. On the other hand, if the configuration is maintained with absolute consistency, you might get a noticeable difference between the inner edge of a bundle, where it is nearest to the bundle of an other phase than the outer edge, where there is no adjacent phase. But even that wouldn't be enough for more than a few 10s of amps difference if it could even be that much. Certainly not what you are describing.

You say the installation is pushing 35 years of age; I suppose it is possible that connections could be going bad, but I'd feel a whole lot better about that possibility if you had five different readings spread randomly across the 200A to 560A range, rather than four at the low end.

There is something very wrong here, and it is difficult to come up with anything that seems reasonable without a lot more information. No promise that any reasonable suggestion come from further information, but the information will certainly help.

 

[itsmoked]

How about some pictures middle, ends?
They would be pretty useful at this point.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[waross]

Good morning davidbeach;
I understood from the OP's second post that the problem was similar on all phases.
I keep thinking that this sounds backwards. If the bundles were consistent we would expect fairly even currents except for the conductor in the center of the bundle which would have a LOWER current because of skin effect, not HIGHER.
I am wondering if this installation may be;
One bundle A-A-A-A-B, one bundle B-B-B-B-C, and one bundle C-C-C-C-A.
Comments?
BTW congratulations on regaining you customary position on the MVP list. Your quality has overtaken my quantity as it should.
respectfully

 

[PWR]

I have seen this problem on many installations. It occurs due to unequal reactance when the conductors are randomly place in the tray and when they are neatly installed with all the phases grouped together. The latter insures the worst case scenario. As an example I documented some years ago:

40 inch wide tray, 7 - 500kcmil per phase (21 conductors total) arranged (from the source) left to right 7 - A then 7- B then 7 - C conductors. Using a clamp on ammeter the C phase conductors (from the source) read 730, 350, 230, 185, 165, 220, and 410 amps respectively left to right. Note that the left most conductor has the lowest reactance because it has the most flux linkages from the A and B phases and therefore carries the largest current.

 

[davidbeach]

PWR, your numbers make a whole lot more sense than tito103's numbers. My guess is that you also had a run that was a whole lot longer than 80ft run considered here. I would not have expected as much variation between high and low conductor, but enough length of run could make that happen.

 

[PWR]

Actually, the run I documented above was only about 30 ft total length. The length only has to be long enough to establish a significant difference in the total impedance of each conductor. The current is going to divide based on the difference in the impedances even if the total impedance magnitude is quite small in each conductor.