Problem: High cable sheath current for 34.5kV distribution system

[jhonwyn73]

Greeting,
In 34.5kV distribution system, the cable distance between 34.5kV CB (upstream) and 34.5kV/13.8kV transformer is about 300meter. Unfortunately, each phase (single core cable) has high sheath current (AWA current = 100A for B, 80A for R&Y). Cable is single core 500mm and at terminals the AWA at CB & primary of transformer are solidly grounded. The transformer vector group is Dyn1.
This high AWA current causes frequent hot spot at CB terminals, and hence lead to repeated repair work.

The question: How to eliminate the high sheath current?
Is it a must to solidly ground both ends of cable?

Appreciate your help to minimize/reduce this sheath high current with no standard violation.

 

[davidbeach]

The sheath does not need to be grounded at both end. You could remove the ground connection at one end and insert an appropriately rated arrestor across the open. Under normal conditions the arrestor will function as an open while during fault events it may conduct to limit voltage rise on the sheath.

The other approach would be to interrupt the sheath at about 100 and 200 meters and cross bond the phases. That will help if the sheath currents are induced from the phase conductors, but would not be as much help if the driver is a difference in ground potential between the two ends.

 

[jhonwyn73]

Many thanks for the fast response.
Can you help for standard reference? This will help to convince others and expedite the corrective action.

 

[jghrist]

If you reduce the spacing between phases it will reduce shield currents. Using a higher resistance shield will also reduce shield current.

 

[7anoter4]

Try this one:
ANSI/IEEE Std 575-1988
IEEE Guide for the Application of Sheath-Bonding Methods for
Single-Conductor Cables and the Calculation of Induced Voltages and Currents in Cable Sheaths

 

[qman5]

If you move to a single ended bonding solution, bear in mind that you will need to install suitable sheath voltage limiters (SVLs) and possibly a link box at one end of the cable run. To avoid unknown voltage rises and induced voltage on adjacent cables, you'll also want to run a grounding continuity conductor from one end of the cable run to the other.

Cross bonding with cables at this distance wouldn't be economical.

 

[7anoter4]

In my opinion,qman5 is right.For accessories see [for instance]:
Tyco Electronics Energy Division Cross bonding and sectionalisation for high-voltage cable systems

http://www.coronabd.com/download/Raychem 132KV LinkBox.pdf

 

[jhonwyn73]

Many thanks to all for the helpful information and advise.
But the unbalance in the sheath current is really strange. The sheath current for two phases = 75A/phase, while for the third = 95A.
Is there any explanation for that?

 

[davidbeach]

There is an explanation, and it is with the unknown geometric arrangement of the three conductors and the resulting mutual impedances.

 

[7anoter4]

In my opinion, if you could ground the armor in the middle -keep insulated both ends-the built-up voltage would be less than 25 V at the ends at 500 A load current per phase. However, in the case of a short-circuit, you will need SVLs [1.5-2 kV may be].
The armor [aluminum wires] current it could be measured at the connection with the shield [copper tape] and the total current could be affected by this connection [one link is missing, for instance].

 

[jghrist]

If you don't ground both ends, you may need a separate grounding conductor for ground fault currents.

 

[Kiribanda]

jhonwyn73,
You have mentioned that the cable is having an AWA (Aluminium Wire Armour). If the armour is Aluminium then how there are induced currents due to the line current flowing in the main conductor?
Also could you please mention how many runs per phase and the cable laying configuration (whether tre-foil or flat formation)?
But I suspect due to AWA bonding at both ends, the Aluminum armour may be a good path for other ground currents (such as third harmonic currents) prevailing in that area?