Currents in Cable Support Structure

One possibility might be if the phases are run in separate cables, and one or more of them wraps around part of the structure.

yes the phases run in separate cables and almost all of them wrap around the structure

That would be it, then. The assembly has become a transformer.

Seen something similar with a Unistrut type support system that had been constructed in a square around the singles cables. The designer saw a simple support system. The reality was the support system became toasty warm. Another one to watch for is scaffolding built around singles cables. As some of the previous people have siad you are making the secondary turn of a transformer and then shorting it out.

It is still not clear:

1) Are 3.3kv cables single-core non-shielded cables?

2) are they grouped in 3 phases- cables touching [in equilateral triangle] or are ungrouped ?.

3) The cable tray is in trench [open trench?]or somewhere hanged in air?

4) Is it a metallic [steel or aluminum] cable tray?

5) Where the "insignificant" grounding current was measured?

Usually 3.3 kV system is an ungrounded system-or zig-zag transformer [ngr] grounded- that means the secondary transformer neutral is not grounded but only 132 kV side neutral is grounded.

Far more information is needed.

What is the amp load on each of the 3.3 KV cables and what are their physical sizes by gauge or cross section area and how hot is "hot"? ? ?

13 amps flowing through a large metal frame will not cause any measurable resistive heating effects being that the potential voltage is likely in the millivolts levels across any point in the frame.

More likely hot cables means that too much current is flowing through them for extended periods.

Enclose in steel pipe.

I think, the cables are installed in a flat profile; in this case an electric field from each cable will generate a significant value of eddy currents circulated in the structure support. To overcome the phenomenon; 1- Sheath of the cable at ends is to be earthed, 2- Single phase cables can be rearranged to be installed in a trefoil profile to vanish the effect of three phases generated electric field.

Isnt eddy current supposed to circulate inside conductors??? And not in surrounding structures??? The sheath of the cables is already earthed at one end and at this point it is not possible to arrange their layout in a trefoil manner.

Use insulators in the cable support if possible. You want only 1 earth to avoid the shorted loop mentioned. But heating in the support is probably not causing the cables to be warm, it's probably the current in the cables from the load. Are they the proper size wire? How much warmer are they than expected?

How warm is warm. Although not desirable the cables are rated to run at a temperature that you would not want to get hold of without a burns team on standby.

We calc up to 90degC. Even at half that temp its going to feel hot to touch.

This is a case of induction. The voltage in the cables are irrelevant. The currents and the cables carrying capabilities are the relevant factor. When measuring and reporting them, the in-phase effective current is important. But, equally important are circulating (out of phase, nonsellable) currents, as they heat the cable equally well.

There is one more factor, you in all likelyhood did not take into account. The high current harmonics from various power drives. You would benefit significantly studying Holzfeller's notes on that particular subiect.

Both radiate heat to the surrounding mechanical structure. Additionally, both do function in a transformer fashion to directly induce current in the support structure. These two current values are essential to start a meaningful dialogue (plus the harmonics, and their phases).

If you want to go to one more factor, you need to break out the old school books on the magnetic field strength outside of a conducting cable. From the top of my head, you calculate the surface B, and let it decay exponentiall with the distance.

Since I like to bet on the outcome:

Circulating currents, and

Large harmonic currents

Plain overload (distant third)

Are the cause of the heating trouble. Magnetic coupling ought to be taken care of in the code adequately.

You mention that " recently we are facing a problem....." . Were the cables working OK before?. How old is the system? as it was requested before, give us for each phase the load currents, cable size and temperatures before and after the problem arose.

What did you change that created a problem? increased the load? changed or added equipment? etc...

I need to rephrase. Recently we detected a problem would me the more appropriate statement. There has been overheating of the cables for the last 6 months for reasons still unknown to us. The connected load has remained the same. While trying to find out the root cause we happened to detect this phenomenon of currents circulating in the cable support structure. Cable size is 1000 sq mm single core armoured aluminum conductor. There are three numbers of cables per phase. Load per phase is approx 1200Amps.

Well I see a possible problem right there in that you are carrying a near continuous current of some 1200 amps through a 1000 Sq mm cable inside a conduit.

My books say that the maximum continuous current for a cable that size inside a conduit should be no greater than 600 amps and 1100 amps in open air.

Too be honest the fact you are only getting 13 amps of induced currents in your framework at those current levels is rather impressive really. Which if you do a simple voltage test of the same components you will likely see that 13 amps is only at a few 10's of mili volts potentials equating to less than a few watts of actual electrical energy being lost whereas given the approximate resistance of a cable that size per KM you are loosing around 120,000 watts per phase per km to cable heating!

All the framework modifications in the world are not going to cure the real problem you have of simply having your system way overloaded and the only fix for that is to put in bigger cabling or run multiple cables in parallel for each phase.

Basically you are wanting to chase fleas, the few watts of inductive loss in a system carrying some 12 Mega watts base load with a 360 Kw base loss to heat per KM on top of that, when your problem is the heard of elephants they are sitting on.

What is the problem PC...? You got a nice set of answers. Now it is your turn, as you are obligated. Did the cat got your tongue?

I agree.. the amount of heating of the cables due to the currents in the structure is negligible. I was looking for a way to niullify these currents, got plenty of ideas here..thank you people...for starters we are planning to change the bottom (gland plate) of the cable junction box which is a MS plate now with a aluminum plate...coz its non magnetic and my guess is that these magnetic flux is originating at that point where the cable glands are. Lets hope it works.

Dear Mr. pc1981,

Can you furnish some more data/detail.

1. LV SIDE CABLE - Is it 3 core.? or SINGLE Core.

2. If it is 3 Core, does all R Phases grouped to gether.? Y phases grouped to gether.? B phase grouped together,.? (OR ALTERNATIVELY) each 3 core is clubbed and used for per phase.? phase. If alternative is adopted, Cable Heating will occur, on account of Inductance.

DHAYANANDHAN.S

the standard way to reduce circulating currents in the cable sheath and in a metal conduit would be to earth one end only.