Optical cable in parallel with 110kV .

[lz5pl]

In a current project we have the following case:
Power cable 110 kV with length 800 m and fiber optic cable for differential protection will be buried in parallel directly under the ground. What minimal distance between them is required by the standards? In our country regulations I cannot find anything specified for that case, we have only requirement for 50 cm distance for telecommunication cables.
Any advice on IEC-world or North American standards or practices will be helpful.
I am concerned mostly about some voltages inducted in FO-cable armour, as fire hazards will be negligible on distance of 0.50 m .

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It may be like this in theory and practice, but in real life it is completely different.
The favourite sentence of my army sergeant

 

[Skogsgurra]

Is there really a metal armour on the fibre cable?

Otherwise, it can be put side by side to the 110 kV cable. Overhead 110 kV lines have fibre cables attached to them in many applications.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[lz5pl]

Yes, FO-cable is armoured. We work a lot with OPGW on 110 kV and above overhead lines, and based on that I suppose that grounding of the cable's armour should be enough.

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It may be like this in theory and practice, but in real life it is completely different.
The favourite sentence of my army sergeant

 

[7anoter4]

I think the main problem is the voltage build-up at the ungrounded end of the shield[armour].
1) If you may insulate the shield there is no problem.
Let's say the 110 KV cable overall dia is 80 mm and the Irated 1000 A and the optical cable shield dia is 20 mm.
2) If the optical cable is laid up above the 110 kV cables and more or less symmetrical then:
a) If the cables are close one to another than 500 mm distance to optical cable will be sufficient as the voltage at the ungrounded end may be limited to 30 V.
b) If the distance between 110 KV cables will be 150 mm the voltage may be 260 V.
3) If the optical cable is laid-up in-line with the 110 kV cables then:
a) If the cables are close one to another for any distance the voltage may be 700V.
b) If the optical cable may be protected in 4" steel conduit then the voltage at the steel conduit ungrounded end may be 140 V and if the distance between 110 KV cables will be 150 mm the voltage may be 260 V.
4) The grounding at both ends poses the problem of shield heating and may be thermal stability in the short-circuit case.
Regards

 

[slavag]

Hi.
It's must be special variant of FO cable.
Possible order FO cables with dielectric central member.
I like a MCA version of cabels with loose tube or minicable, is good for indoor and outdoor application. Of course type of jacket is very important too.
Best Regards.
Slava

 

[7anoter4]

Hi Slava
FO cable with steel armoring is used for underground [buried] laid cable, in order to avoid rodent attack. A corrugated anti-rodent steel armoring is provided under a polyethylene jacket. Some time the jacket may incorporate a sealed aluminum tape as moisture barrier, also.
Best Regards

 

[slavag]

Hi 7Another4.
You are right in your post.
Isn't power eng issue..but, instead your option:
Possible use MCA cable: MCA...V-D-VJP-G
V=minicable material PVC
D-FRP dielectric central member
VJP-jacket PVC,Dielectric armoring-Polyethelene.
G-Gel water blocking.
We have a very good expirience with this type of FO cable.
Best Regards.
Slava

 

[7anoter4]

Thank you, Slava.
Best Regards

 

[lz5pl]

7anoter4, thank you for your posts. We are still considering what to install - steel tape armoured cable, buried directly under the ground, or normal cable in HDPE pipe, also buried. It is a matter of price also, but we found that both options cost almost equally.
Your calculations about the induced voltages are very helpful. I haven't a tool to make them.
Actually normal load will be under 50 A only, so these voltages during the operation are not a problem. But short circuit level is quite high - 25 kA, and I am very concerned about insulation of the armour on ungrounded end.
So probably we will prefer to install second option (in HDPE pipe). This will simplify also the installation, as the ODF's could be installed directly in the relay panels on both ends.

Thank you and slavaq for your help.

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It may be like this in theory and practice, but in real life it is completely different.
The favourite sentence of my army sergeant