High Cable Currents and Temperatures

Well it definitely sounds like you have circulating currents in your shield wires caused by the current running in the other phases in close proximity (this is a normal phenomena). It also sounds like your existing cable layout arrangement is to blame.

http://www.olex.co.nz/media/docs/Section-Two-eda6ac7b-d64a-4018-8d74-06503640c2d7.pdf

Have a look at page 7 of 16. Altering you cable arrangement should minimise the circulating current and heating problem to an acceptable level.

can someone advise me on how to install 10 nos of single core copper conductor Aluminum armoured cable in each phae.Total no of cables in the circuit would be 10*3+5 for neutral.We need equal load sharing in cables of same phase.

On the net I found That the Canadian Electric code Rule 12-108 (2) Appendix B mentions about it but I do not have any access.

I already have with my link in post #1. The examples are for different arrangements up to 4 conductors per phase, scale this up to 10 conductors per phase. With regard to the neutral, read the text on page 7 as it is explained there.

For further information you will need to consult your local electrical regulations (the information should be in there too). If you don't have a copy you will need to buy one.

For 3 single core cable in a phase the configuration available is "Trefoil".But at most occassions it is not possible to lay in Trefoil formation in air.If we lay straight parallel then what should be the configuration/order of cables.Any picture available?

See link in post #1 again (the straight parallel cable arrangement pictures I provided you). As I and others have said, if your are trying to run all the individual phases together in bunches, you won't solve anything no matter how you lay the cables (that's the whole point of symmetrical cable arrangements, each phase in close proximity to the other two minimises the effects you are seeing).

From the link

Non-symmetrical arrangements result in different impedances and hence unequal current sharing between parallel legs of the same phase.

If you still don't understand the basic principles of electricity and magnetics, have a look on the internet for some guidance and further training.

I have connected the cable as advised by Jack and i have solved the problem of Un equal current in cables of same phase.But there is circulating current in the armour as it is earthed at both ends.I want to know what could be the maximum permissible current that may be allowed to flow in the armour of 300 sq mm copper conductor single core qith aluminum armouring.Is there any data available?

Obviously you want to keep the circulating current as low as possible. The effect of the circulating current in the screen is that it causes cable heating requiring you to de rate the cables maximum current carrying capacity.

As a general rule if the circulating current is less than 5% of your normal current thru the cable, then you shouldn't need to de rate the cables. Your local electrical regulations and standards will have the exact figures, as may the cable manufacturer who supplied you the cables.

I agree with Jack. You post said

Quote "We have connected 10 nos of 300 sq mm copper conductor Xlpe insulated Aluminum Armoured Pvc sheathed L.V. cable in each phase to carry a current 2000A from main panel to sub panel.

The cable has been layed with cables of R phase lying adjacent to each other followed by Y phase and then B phase."

This is likely the problem. You should have the cables grouped R,Y and B together. You have all the R's together, Y's together and B's together. I am also questioning whether the armour should be grounded at both ends. Maybe someone else could confirm this.

Only grounding the screen at one end would solve the induced current problem but creates an induced (standing) voltage problem where the screen could momentarily reach a dangerous potential under fault current conditions.

Whether or not single point grounding can be used will obviously depend on local electrical regulations and standards, but does not seem to be the normal preferred practice (at least down here anyway).

I remember running in tri-foil ? when running parallel feeds from gen-sets to main transformers

so you would have to run each group together, I can't remember but there were spacing guide lines as well for between each group i think.

hope that helps a bit :)

Jack of all trades is right. There is a problem in the layout of the cables. Grounding the screen at both ends is also good. The unbalance load in each cable, considering the described configuration, is due to the skin effect created by the alternating current in cables.

"3)The cable has been layed with cables of R phase lying adjacent to eachother followed by Y phase and then B phase." This is the worst lying method .

A solution would be to have a symmetrical arrangement of the three phases;the equilqteral triangle arrangement is the best known layout for cables; also twisting cables in each phase would equalise the currents that run in each cable,but difficult to maintain.

Nikolay and jack of all trades have pointed out correctly, the conductor laying could be the major issue for the current in armature, as the magnetic lines produced are not been canceled, as ideally they should be and should be even in all the phases.

The variation in current from 20 Amps to 200 Amps in a single phase is caused by 1. Skin effect and 2. The proximity affect, this results in uneven distribution of current within the conductor.

For example if there are 3 conductors carrying same phase A B C...

Conductor B shall get heated up maximum, because of the effect of other two conductors and hence shall carry lesser current compared to other two.

Grounding the Armour from both ends shouldn be a problem, it is supposed to be a ideally preferred solution, at least upto 500 Amps. Do not know above that.

And as laying the cables as shown by Nikolay is difficult in practical terms, laying a R, Y and B together should solve the problem to a good extend, as the ,magnetic lines are cancelled by each other.