high voltage underground cable

[stanyadji]

I have a 150 kv UGC 3x1x400 sqmm XLPE insulated,beside these cable laid unscreened pilot cable.Someone said that it doesn't matter because the power cable already sheated and screened with metal screen.( base on Faraday cage theory )
Is anybody can help me in explaining this matter ? As I know the pilot should be screened too.

 

[slavag]

Xmmm. Something new for me.
screened cable as Faraday cage. Iz very intresting, and pilot cable isn't screened..nice to heard.
Give to this someone chance write report to boss about reasons of unwanted trips.
Best Regards.
Slava

 

[HSKabel]

Not so fast. If the HV cables are single core and are installed with some spacing between them and may in addition the screens are single point or cross-bonded, there is a significant residual magnetic field as a result of the superposition of the magnetic fields of the individual cables.
If, on the other end of the spectrum, the HV cables are laid in touching trefoil or in a triplexed 3-core cable and the screens may be solidly bonded (grounded at both ends), the residual magnetic field will be comparably very low.
Induced voltages in the pilot cables are a potential issue.
There is no Faradey cage here. Magnetic fields do permeate the sheaths or shields of power cables under load.

 

[RRaghunath]

Stanyadji,
The faraday cage principle doesn't apply in this case. The metal screen with 1-core cable would not have been earthed at both ends.
Had the metal screen been earthed at both ends, the circulating currents in the metal screen would produce flux that opposes and thus nullifies that due to main conductor currents.

 

[waross]

I thought that the Faraday cage applied to objects inside the cage, ot fields radiated outwards.
Surrounding a cable there are both electrostatic and magnetic fields. The shield limits electrostatic emissions and capacitive coupling.
Given the low level of current in the shield relative to the conductor current, I doubt that there will be much cancelling of the magnetic flux. The reason for heat buildup in the screen is the relatively high R, because the I² is not that high relative to the conductor current.
I am familiar with the concept of reducing magnetic field radiation with eddy currents. The magnetic radiation from air core reactors is sometimes shielded from the re-bar in nearby concrete with aluminum sheets. The sheets are relatively thick (1/4") so as to have a low R and facilitate high eddy currents to oppose the main field. These shields do not actually shield the magnetic field but reduce it. There is still a radiated magnetic field from the eddy currents in the aluminum. Part of the field reduction results from the magnetic energy being converted into heat energy in the eddy currents.

Bill
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