MV Cable Termination Issue

You will definitely have cable armour heating up due to eddycurrents imposed by residual flux. Had the cable been loaded to three phases in the three cores, the load being balanced, there will not be any residual flux, as the phase fluxes cancel out each other. But, in your case, in cable -1, there will be two times "R" Phase Flux and one time "Y" phase flux; these wouldn't add upto zero and hence there would be residual flux and this causes circulating currents in the armour and armour heating. Same is the case with cable-2 too.

May be you can avoid it by earthing the armour only at one end and NOT at bothe ends. But, beware, there might be touch potential hazard at the non-grounded end armour, in case of an appreciable earth leakage/fault.

Thanks for reply .

As we have very limited space inside the MV terminal box we can't avoide the crossing of the cable . We have below solution with 2 core crossing i.e cable 1 Red core and Cable 2 Blue core crossing each other , although there is sufficient space in b/w what will be the impact in this case . Have any other option is available for termination or how we can avoide the leakage flux in this case. Both cable are tape armoured and armoured connected to one end i.e switch gear side .

Regards

ASD

Your best solution I think. The induction from the two cables (A phase) in number 1 conduit would induce a current in the single conductor. (B phase) It would be better to have all three phases in the conduit and have mutual induction, thus cancelling the effect. Don't worry about crossing them, as long as you don't come within the limits of clearance from the live parts. They are insulated, right!

Dear Mr. may2u

I don't agreed. The case is we have MV termination on both cables . After termination passing them to conduit and finally terminate on bus bar is not feasible . there is no way to pass the cable in conduite after MV termination.

ASD

Ah, let me think.... you just switch them at the buss bar/switch gear. Three (A,B,C) phases in one conduit and (A,B,C) phases in the other. Just a matter of using a little phase tape. Not TOO much or you could choke off the electrons!

This is the correct way to terminate eventhough there are crossing of phase cables.The effort should be on finding ways to allow sufficient clearences at the phase crossings.Complete set of termination kit must be used.That includes semi-conductive,insulative,stress control sleevings and earthing conductor.On the surface of the completed/correctly terminated individual phase conductors,there are capacitive voltages which can cause the breakdown of insulation and total cable failures.

The cable is supplying 6.6kv,~630 amp per phase to the 6MW motor.If you are at designing stage,please use one 400 sqmm 3-core copper cable(steel tape/wire armouring) or 3 numbers of 630sqmm single core alluminium cable(only alluminium wire armouring available) which will carry the required current and is more economical.

With 2 nos of 3-c 300 sqmm cables that has phase crossings,you can do the following methods:-

(1). This is only a small problem at 6.6KV. About 40 mm of final air gap between the phase crossings will be sufficient and very safe.After putting on semi-conductor and insulation sleevings (heat shrinkable termination kits ?) and before the phase cable is cut and termination sockets are crimped onto the phase conductors,bend and adjust the two sets of phase conductors to have some air gaps between the conductors of the different phases (at phase crossing).Crossing of conductors of the same phase can be in full contacts.If you cannot achieve this, follow (2) below.

(2). Make and add 3 small "U" or "Z" shape copper bars onto original termination points so as to allow two cable terminations.Whatever you do,always allow about 120mm air gap for phase to phase and 110mm air gaps for phase to earth.

(3). If you are under great pressure to complete the job (such as already in L.A.D. time) or not happy with the bosses or just lazy,you may use this method as it will work too and it may not last.Put about 5 to 6 inches(axial length) of extra insulation sleevings on the phase cables where two different phases are crossing and touching each other.This metod may last a few years.I have seen such cases that lasted more than 8 years.This is not a good method.

REgards,

Khor

Never terminate the 2 numbers of 3-core cables as shown in your original question.

Regards,

Khor.