To shield or not to shield the cable...that is the question

[Tesla1971]

I work for an engineering company that does substation design. I wanted to know if there is a rule of thumb when to used shielded cables for power & control cables. I've heard anytime the voltage is 230KV or above, shielded is required. Can anyone verify this. Thanks.

 

[dpc]

Back when I was doing some substation design, substations 230 kV and above got shielded cables, FWIW.

 

[oldfieldguy]

Concur with dpc.


old field guy

 

[slavag]

Hi.
From my point of view:
GIS substation=only shielded cable, it's MUST ( AC/DC supply cables is Q's, possible not).
For AIS substation above 150kV it's preferd, but not must
in all cases current/voltages wiring is prefered.
If you will use newer digital relays, I would recommend use only shielded cables.
Power cables for all votegaes> 1kV only shielded.

Other Q's grounded both ends or only one.
Best Regards.
Slava

US guys-- wish good choice you today

 

[7anoter4]

According to NEC 2005 art 310.6 shield is already compulsory above 2400 V .See:
thread237-229637

 

[dpc]

7anoter4,

Correct, but I think we're mostly talking about the lower voltage circuits in a substation where the shield provides some additional protection from the electric fields associated with the bare HV conductors.

 

[7anoter4]

I see the point, thank you, dpc. I think that in High Voltage Substation all analog cables must be shielded, of course, and will run through steel conduits and through closed metallic grounded cable trays –segregated for this kind only. The digital signal, also shielded, may be routed in the same tray with low voltage power and control cables[or communication cables].Control cables-usually low voltage cables- does not need to be shielded or screened if they are routed inside the buildings. At outside high-voltage -AIS or GIS- the control cable has to be screened or shielded due to the arc flash energy hazard more than for EMI purpose. In my opinion, from 160 KV these cables are always shielded and grounded at the yard end.
Best Regards

 

[slavag]

7Another4, Im not so agree with you.
From our experience.
In the indoor GIS, MUST be used only shielded analog and digital cables and shield MUST be grounded on both sides.
Analog and digital MUST be run in the separate conduit or trays. Reason for this is overvoltage transients due the switchng of GIS elements. Outdoor substation is other issue, but we prefer use today only shielded cables ( analog and digital) and grounded on the both sides, but of course is depend on the ground grid of substation and distance between parts of substation.
We had a lot of false operation of digital inputs in the newer realys ( BTW, only before two weeks, false signal was only 2ms, due the lighting strike ).
BTW, old substation with EM relays was with not shielded digital cables many years w/o any problems.
Best Regards.
Slava

 

[lz5pl]

slavag, do you really ground both sides also in outdoor substations? I have quite negative experience with such grounding, especially in old substations (rehabilitation). Digital inputs of numerical relay protection of new 110 kV feeder, with 150 V threshold operated and stay high when circuit breaker on particular bay in 220 kV switchyard is open !!! We didn't succeed to find the reason. Probably grounding grid in the substation (25 years old) have been broken. The only solution was to remove grounding on the switchyard side of control cables.
After that case we ground only on control room side, by the way this is also typical practice in Bulgaria. The substation in question was not here.

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

 

[cranky108]

I don't see it talked about above, but with capacitor banks I would use shielded cables at the 115kv level. But not the whole 115kv substation.

 

[slavag]

Good Morning.
Lz5pl, you are right. Old SS is big big problem.
But, possible in the old too, we do it with separate 95mm^2
grounding conductor from LCC to control/relay room.
( BTW ,more and more I think about conversion digital to FO signals )
Actually it's so much variants.
Cables for AC of heathing/ventilation/lighting and DC circuit aren't need shielded for sure.
Of course, it's my opinion only.
Best Regards.
Slava

 

[7anoter4]

Hi Slava
I think the disagreement between us remains only in one point. We agreed that analog signal [4-20 milliamperes] will be shielded and routed through a dedicated [let's say "segregated as analog "K" cables] metallic raceways as conduits, closed cable trays and similar with no gaps or opens, the digital [1-5 V cables] also shielded but could be routed in a same cable tray with other low voltage cables and finally, power and control cables [110-220 V dc or ac cables], which could be shielded [or screened ] if they are routed to the outside switchyard rated above 150 kV .The only point we disagree is the one end or both ends grounding.
There are people which think the GPR is even all around the area. But in our experience –and mainly due to lightning strokes- the potential is very different from place to place. Some one measured thousands of volts from a distance of thousands of feet. So, importing such voltage from the site to the relay room is not a good idea. We prefer to ground the outdoor end in order to protect the sensor located there and to insulate the inner end. The live wire may transmit also dangerous potential and a surge protector is connected at the inner side. In order to avoid this phenomenon an "optocouple" is inserted in this circuit some time.
If there is no ground potential differences between the ends one may grounded both ends if it would be some reason.
I agree with you FO would be a better solution.
Best Regards