132/11kV transformer neutral cable

[eejacky]

Dear all,

Why the neutral cable gland shall be insulated from the cable box on the 11kV side of 132/11kV transformer? Also, why the 11kV neutral cable sheath shall be insulated? Is it related to the neutral current transfomer.

Thank you!!

 

[ScottyUK]

eejacky,

Is the neutral intended to be solid earthed, or impedance earthed? If the neutral were not isolated from the cable box, it would not be possible to use impedance earthing, as this would cause the transformer cable box and everything attached to it to rise above ground potential during a fault or imbalance.

Assuming the neutral cable is a normal XLPE/SWA/PVC type, the sheath keeps water off the XLPE insulation. XLPE is hygroscopic, meaning it absorbs water over time. When it absorbs enough water, the insulation fails. The sheath also stops the SWA corroding. It isn't really 'insulation', although it has obviously got some degree of insulating properties.

 

[RajT]

I assume that the 11kV side ( N that is) is earthed, either solidly or via a NER.
I also assume that by cable "sheath", you mean the cable armour.

If my assumption is correct then, conterary to what you say, you would be wise to ensure that the cable armour is bonded to the cable box gland plate via the cable gland, ie making sure that the armour is indeed earthed and not insulated.

The current transf ( which I presume is fitted in the N term box) has nothing to do with the treatment of the cable armour earthing.