from the power department...
earth current with Shield current of a 63Kv line
 Posted by masoud on 1 August, 2012 - 10:01 am
Dear Friends

each shield of a 63Kv Line (single core) with In = 450A has current amount 300A to Earth. is it normal? Each end of cable is connected to ground. First of all; this current must be omitted? then if it is possible to omit, how? plz help me.

Thanks

 Posted by Phil Corsp on 1 August, 2012 - 2:07 pm
Masoud...

No! Current of this magnitude will severely damage the shield. Normal metallic shield (or screen) currents, for a fully loaded cable, is in the order of 5-15A.

1) What is shield material?

2) What is effective area of shield in mmq?

3) Is the system neutral floating, that is, not intentionally earthed?

4) Is the system neutral low-resistance earthed?

5) Is the system neutral solidly earthed?

6) Is the system neutral earthed by means of an inductive impedance?

7) How long is the circuit?

8) Are the shields connected at each end?

9) Are the shields earthed at each end?

10) Are there intervening joints?

Regards,
Phil Corso

 Posted by Phil Corso on 1 August, 2012 - 5:25 pm

11) Are the single-core cables arranged in a flat (side-by-side) configuration, or trefoil configuration?

Phil

 Posted by masoud on 2 August, 2012 - 12:16 am
> 11) Are the single-core cables arranged in a flat (side-by-side) configuration, or trefoil configuration?

the cable cores are arranged flat at a cable tunnel floor separately.

 Posted by bob peterson on 1 August, 2012 - 5:23 pm
My first question is how did you measure it? It seems likely that this much current on a shield would damage it in fairly short order, so I suspect a measurement error since you did not indicate any obvious damage.

--
Bob

 Posted by masoud on 2 August, 2012 - 12:26 am
> My first question is how did you measure it?

the current was measured by a clamp ammeter that it connects to earth wire of cable shield. each phase has this current and same. it was checked for several times and the current was changed when the current of load has changed. in fact it is depend on load.

 Posted by Phil Corso on 2 August, 2012 - 4:50 pm
Masoud...

12) Is the ratio of Line-current to Shield-current always the same?

Phil

 Posted by masoud on 3 August, 2012 - 12:30 am
> 12) Is the ratio of Line-current to Shield-current always the same?

yes it is about %80In and changed when the line-current is changed.it was checked at 0->In of line-current.

 Posted by Phil Corso on 3 August, 2012 - 9:40 am
Masoud...

13) Have you checked for inadvertent connections between phase-conductors and the sheath, with the cable de-energized?

Phil

 Posted by masoud on 3 August, 2012 - 12:39 am
can I open the earth connection in one side and install a surge arrester then the ground at this side is connected to surge arrester? at this way the current of ground is deleted but at line-ground short circuit; i should have protection.

 Posted by Phil Corso on 3 August, 2012 - 9:29 am
Mamoud... but, you said it's continuous... therefore not a surge Phenomenon!

Phil

 Posted by Phil Corso on 3 August, 2012 - 1:19 pm
Masoud... There is something very wrong about your observations/measurements. There is no way the shield(s) can carry the current you measured!

Can you provide some detail about the installation?

Phil

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