AC Single core cable armour grounding one end only???

[ctpt]

I posted this few days back but received no reply in the forum. Trying my luck again....

Spec. of one of the projects I am working requires that "Armour of all single core AC power cables shall be grounded at one end only”. I understand it is to do with circulating current but can't visualize real problems for power cables if laid in trifoil formation.
The project comprises of 6kV and 400V system and both systems are high resistance grounded.
Does anybody have any similar experience of armor grounding?

 

[jerryjointer]

hi there i have just joined in order to reply to your post
i am currently working on a 33kv single core set up system
of 200mw plus our role was to install out door terminations
and cone type plugs for the main contractor.
these to be installed as instructed by the switchgear builder.
they asked for single point grounding at the plug end and armour and screen bond at other but not to earth the result is four failures in six months i suspect due to current circulating in the armour and screen.we are working on a solution that may involve sheath section joints to limit induced voltages that can be present on
single core systems.
you must get it right or failures will happen
good luck

 

[Kiribanda]

ctpt,

Normally we specify the single core cables with "NON-MAGNETIC STEEL" wire armour, so that no induced currents and we can connect both ends to the grounding system.

If the cable is having a screen/shield, then the copper screen/ shield can be bonded to the grounding system only at one end. But MV single core cables are available without screens too.

Any comments please.

Kiri

 

[jerryjointer]

hi
any single core cable will induce a voltage in its metal armour/screen , should a closed circuit be present ie earthed both ends then current will flow causing heating of the cable the greater the load the greater the heat
if single point earthing is used on long ccts the un earthed end will have a voltage present
depending on system conditions several thousand volts may
be present at the un earthed end these may be controlled buy
sheath voltage limiters or by transposing the screens to cancel out the induced voltage
if both ends must be grounded derrating of the cable or larger cables may be rquired.
current flow in the single core has a magnetic feild radiating from it this feild is the cause of the voltage in the armour or screen irrispective of the screen material
the case i am working on at present has seen burning of armour and screen bonds the screens are copper and armour are alluminium
the client also has failures on 11 kv singles
hope this helps

 

[ctpt]

Hi Jerryjointer,

If cables are laid in a trifoil formation (in a 3 phase system), then the induction effect should mutually get cancelled and there should be no induced voltage.

 

[sslobodan]

Grounding cable is made to nullify effect of inducted currents in power line. If they are grounded on both ends you will get condensator between core and shield of a cable that will create reactive currents. On short power lines you should ground only one side (for. i.e. you have power cable (10.5 KV 3 MW) leading from generator to its connection cubicle on a system you ground it on cubicle end only)that is because the re is no significant voltage drop trough core co there will be practically no voltage drops and you can consider that other end is protected with grounding also. But when you have a large power lines like in distribution networks they are grounded on both ends. The reason for it is that if you let cable running for lets say 200KM and ground it on both ends the difference in potential may brake isolation due to over voltage (as someone mentioned above). Induction of lines is calculated so it can usually nullify capacitor effect of such system because of reactive currents of induction and capacity are compensating it selves (+jD -jD = 0) if they are equal.

 

[jerryjointer]

hi again check out shrink polymer systems website
page 26 under heatshrink joints in there catalouge
ansd this will tell you all you should require
the site i am dealing with at the moment has 33kv/11kv
single core failures on a monthly basis and puting it right is going to be very costly
regards jerry