MV cable shield grounding 2

[JuanBC]

Hi,

Will someone please explain the advantages/disadvantages of grounding single or multicore MV cables (shield) at both ends?

Kind regards,

JBC
.......
"The more I read, the more I acquire, the more certain I am that I know nothing"

 

[7anoter4]

The short version: in case of grounding at one point at the other end shows an induced voltage, which depends on the current and the length of the cable. In case of grounding at several points a current will flow into the screen which will cause loss in the screen and will raise the temperature of the main conductor, thus limiting the ampacity of the cable. The complete version can be found in IEEE 575 IEEE Guide for the Application of Sheath-Bonding Methods for Single-Conductor Cables and the Calculation of Induced Voltages and Currents in Cable Sheaths.

 

[Power0020]

I think heating effects quoted for single core cables don't exist for multicore cables, they will be normally bonded at both sides to ground since no currents are assumed to circulate within balanced phase sheaths.

correct me if wrong!

 

[jghrist]

There will be circulating currents in the individual core sheaths of a three core cable if they are grounded at both ends. The current within each core sheath is not balanced.

 

[7anoter4]

I agree with jghrist. However, in my opinion, in the case of medium voltage systems where the neutral is isolated and the phase shields are in contact all the time, the circulation current can be neglected.

 

[JuanBC]

Hello,

Thank you all for your comments, they are really appreciated

There will be circulating currents in the individual core sheaths of a three core cable if they are grounded at both ends. The current within each core sheath is not balanced.

Are you sure about this? If phases are balanced, so will the magnetic field, and therefore, the sum gives zero. With magnetic field equals zero, there won't be induced currents





JBC
.......
"The more I read, the more I acquire, the more certain I am that I know nothing"

 

[davidbeach]

Each core has its own sheath and the magnetic field within that sheath isn't zero. A sheath around the overall assembly would come much closer to zero. When grounding both ends the sheaths are often cross bonded to reduce the circulating currents.

 

[JuanBC]

Thank you David!

JBC
.......
"The more I read, the more I acquire, the more certain I am that I know nothing"