Sheath fault current 1

[Power0020]

"The two point bonded cables will have sheath circulating current equal to the phase current" - This statement is widely used but I do doubt it a bit.

I understand that the circulating current is a function of the induced voltage (depends on cable mutual inductance) divided by the loop impedance (includes remote end grounding grid impedance and sheath impedance), others look to the cable as a 1:1 transformer, where the magnetic balance will force the sheath current to go to a value equal to the phase current (a situation if true will mean a very high mid of cable sheath voltage to ground in heavy current situations).

a similarity exists with GIS and IPBD enclosures, where the induced currents are about 90-95% of the phase currents, I can accept that when the CSA of the enclosure if so big even compared with the main conductors.

Other point is that magnetic field does exist outside the cables even if two point bonded.

Any clue?

 

[jghrist]

If that were true, you would need a shield with the same current-carrying capacity as the conductor. Clearly the currents are not nearly equal.

 

[Mbrooke]

I would think its highly dependent on length. Along enough cable will produce charging currents even more than the full load current.

If wanting to limit such current often the practice is ground one end only.

 

[ScottyUK]

Mbrooke,

It's not capacitive charging current, it's an electromagnetic 'image current' which flows in the outer conductor of a coaxial busbar. If you ever get a look at a power plant generator, check out the isolated phase bus between generator and GSU transformer and note the massive bonding conductors between the three coaxial bus ducts. On a correctly operating IPB the image current is very nearly equal in magnitude and antiphase to the main busbar current resulting in near-perfect flux cancellation. It's normally only significant in ultra high current applications such as a generator main terminal connections.

In the case of a cable the sheath impedance is too high and the drive voltage too low to allow an equal image current to flow.

Power0020,

You have pretty much got it. I have some references stashed away somewhere although I can't lay my hands on it right now. I will have a look at work.

 

[Mbrooke]

Sorry, I read through to fast. I was thinking of an underground XPLE cable with a concentric shield.


In terms of the coaxial buss bar this is more along the lines of induced current? Magnetic induction creating circulating currents in the metal coaxial?