Importance of Sheath in EHV Cable 1

[ravindranathan]

Understand HDPE Sheath in a EHV Cable is purely for mechanical protection.Then why so much importance is given to check electrical integrity of HDPE sheath in EHV Cable?Why do we need graphite coating to test the electrical integrity of HDPE sheath .

 

[Kiribanda]

Graphite is meant to be a semiconductor. Therefore, the sheath test can be used as a check to confirm that
the sheath has not got damaged when it was on the drum or during the installation.

 

[ravindranathan]

Thanks Kiribanda...what you have mentioned is understood.
However am not clear why do we need to check electrical integrity of HDPE sheath.What damage will it do the cable if there is a micro hole in the sheath ? HDPE sheath is purely for mechanical protection ,then why do we need to check electrical integrity by applying 5kV DC across sheath and copper wire?

 

[RRaghunath]

Ravindranathan,
HDPE pipe is meant to protect the cable against mechanical damage and, also against water entry.
You may be aware water is not good for XLPE as its presence leads to treeing in the insulation and consequent damage to the insulation.
So, a hole in the sheath needs to be identified and attended to on top priority.
This was the requirement even for those XLPE cables with PVC outer sheath.

R Raghunath

 

[Kiribanda]

All HV & depending on the rated voltage some MV cables do have protection from water getting into the cable.
Because XLPE & water are "enemies". You can read a lot about it.
Therefore,
1) Radial water sealing is achieved by using a metal-PE laminate. The metal is normally aluminum. Copper may also be used.
2) Longitudinal water sealing is achieved by using a water swelling material at the copper wires or swelling powder between the screen wires.
That means before commissioning we have to verify the integrity of the outer sheath.