Circulating current in armoured single core cables

[Moncey]

In one of my Project we have used 630 sqmm/XLPE/AWA/Cu cables for Chillers installed in roof from MDB's in ground floor.For each Chiller of 1600Amp 9 runs of 630 sqmm cable is laid (3 runs for each phase),each phase cables are grouped together and cleat ed with trefoil cable cleats in trench and riser. Each length is 170 mtrs and its both ends are gland ed with aluminium cable glands in aluminium gland plate and for each group one 1Cx300 PVC Y/G cable is pulled.

But last week local authority put one comment "all single core cables to be non armoured" to avoid any current circulating in the armour.
Cable pulling and termination is completed so we cannot change any cables now, so can any one can please advice how to solve this issue technically.

 

[Marmite]

I presume that your cables are run as 3 trefoil groups of three phases, ie 1. RYB, 2. RYB, 3. RYB?
The easiest way to resolve the circulating current issue would be to change one of your gland plates to paxolin, or some other insulating material, so that the armours are earthed at one end only.

Regards
Marmite

 

[Moncey]

Dear Marmite,
The cables are run in 3 trefoil groups as 1.RRR 2.YYY 3.BBB and both ends it is aluminium gland plates.

 

[ScottyUK]

The cable arrangement is bad.

You can get isolating glands from the likes of Hawke and Raxton which are designed to isolate the armour from the gland body which might be easier than replacing the glandplate. An insulating glandplate will need to be fairly substantial to withstand the electromagnetic forces during a fault: glass-filled epoxy or glass-filled polyester would be more suitable than paxolin, but would likely be at least 16mm thick.

You really need to get the conductors into the correct formation as described by Marmite.

 

[waross]

I second Scotty's comments. The cable arrangement makes a bad situation worse. Look for isolating glands and reconnect the cable groups to RYB.
The present connection scheme will have higher circuit reactance than YRB grouping with greater circuit impedance and conductor voltage drops. The worst case voltage drop may be during motor starting when you can least afford it.
Under Canadian code it may be acceptable to derate the cables if you allow sheath or armour currents to flow. That may mean adding a foutrth group of cables (and changing all connections so that all trefoil groups are RYB). You will then be paying each month for the losses occasioned by the sheath or armour currents.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[7anoter4]

I agree with the opinion a suitable order of phase conductors in a trefoil formation it will be recommended in order to balance the current in parallel cables of a same phase.
But, in my opinion, still the induced voltage in the armor remains elevated. See [for instance] IEEE 525- IEEE Guide for the Application of Sheath-Bonding Methods for Single-Conductor Cables and the Calculation of Induced Voltages and Currents in Cable Sheaths. Appendix D. Calculation of Induced Voltages.
For one or 2 parallel three-phase systems the induced voltage may be calculated following this standard. For more parallel three-phase systems the induced voltage has to be calculated following general induction rule:
Ep=Sum[Epq=ω*(Iqa+jIqr)*ln(Spq/dp)]|q=1 to n,q<>p| where:
p is the considered parallel single-core cable in which is this induced voltage calculated and q is the inductor cable.
I don’t think the armor could be in contact with the gland plate since the cable is PVC jacketed. Just in case of jacket damage a contact may be present.

 

[ScottyUK]

"I don’t think the armor could be in contact with the gland plate since the cable is PVC jacketed."

Armoured glands most definitely would connect to the armour. That's the whole idea of them. I guess a stuffing gland sized to pass the cable outer sheath could be used and the armour just left cut short inside the board, but that's a really rough and ready way of terminating the cable and I wouldn't allow it on my site.

 

[Power0020]

I wonder if the armoring meant to be in multi-core cables is the SWA not AWA? - Sheath materials are very common with aluminum, copper and lead for single core cables (non-ferro-magnetic materials). AWA is used as mechanical armoring with single core cables to avoid eddy current losses in SWA cables.

If you but each group together i.e. RYB1,RYB2,RYB3,...etc, the sheath induced voltage and accordingly circulating current, will be reduced greatly.

Isolating gland will help eliminate the circulating current however
e
If you open one end, the induced voltage should be checked with normal and fault current situations to be safe for personnel. , a figure of 65 V is usually quoted for normal operation and 5 kV for faulty cases.

 

[7anoter4]

Thank you, Scotty.I got it. Using an old lead covered paper insulated cable with armor and serving the serving and armor was cut before entering the box or panel. But this was many years ago. Happy New Year!

 

[ScottyUK]

And to you 7anoter4.

There's still a lot of the old stuff in service in the UK, and probably will be long after I retire too.