Why shield of our HT cable heated up ?

[panthripu]

The HT cables(34.5kV) which are incoming to our plant from the substation have 5 cables each phase.The shields of the cables are connect to ground on one side. The other side shields are insulated and kept open.Length of the cable from substation to plant is around 250 mtrs.
Recently we observed , one of the open shields was overheated and the insulation was burnt ?
Can someone explain the reason ?

 

[Phil Corso]

Panthripu...

Because it's carrying current it shouldn't be carrying!

Phil Corso

 

[Phil Corso]

It may be inadvertently groundED somewhere else, other than at one point, which you stated is at the source! Burning indicates relatively high current!

Phil

 

[rlundsrud]

Perhaps moisture has wicked into the shield. This would be of fairly high impedance but could still create enough current flow to heat the shield. Have you checked the shield with a TDR to see if the impedance along the length is consistent?

 

[beanland]

Voltage Rise

Voltage Rise

The HT cables(34.5kV) which are incoming to our plant from the substation have 5 cables each phase.The shields of the cables are connect to ground on one side. The other side shields are insulated and kept open.Length of the cable from substation to plant is around 250 mtrs.
Recently we observed , one of the open shields was overheated and the insulation was burnt ?
Can someone explain the reason ? View attachment 21390

You indicate that only one end of the shield is grounded and the other is not. You also say it was one of the open shields that shows signs of damage. There will be standing voltage on the open shield due to magnetic coupling. If that voltage exceeded the level of insulation at that point, the insulation will track and carbonize, looking "burned." If the carbonized path is adequate, there will be current flow in the shield, leading to further damage.

The voltage rise is a function of the current. Even if the normal operating current does not induce enough voltage to cause damage, the voltage on the shield will spike during an overcurrent fault that passes through the phase conductors. It is generally a good practice to install a voltage limiter (surge arrester) on the open shield to prevent this kind of damage.

 

[Wire-Smith]

how is the cable run arranged, all circuit legs in each conduit(ABC and ground in each conduit)? what size wire?

 

[Wire-Smith]

also the shield could be broken in the run, tape shields require large bending radii, even when installing. if they had to bend it tightly to fit in manhole it could have been damaged and eventually burnt the rest of the way through and is now broken.

 

[Julius Right]

The transferred potential from source grounding grid could be very high in a short-circuit case and the insulation could break down. After then at this point the shield may be grounded through the burned insulation.