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Sheath Voltage Limiter (SVL)

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Power0020

Electrical
Jun 11, 2014
303
SVLs are usually used to protect cable jackets from lightning and switching surges, the SVL conduction voltage should be coordinate with the through fault induced voltage to avoid conduction on fault with huge energy dissipation/explosion!.

I wonder in case of conductor-to-sheath fault, the SVL voltage may rise to very high values (neat to phase voltage, depending on fault location.

Would th SVL survive such fault? I think it will explode as part of the cable fault, I remember that some utilities require to replace the whole faulty section (till the first earthing point) as they don't trust energizing a faulted cable easily.

Any clues?
 
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In my opinion, the maximum [rms] of the shield potential in any location is the EPR [or touch potential] in this location. Usually is not more than 5 to 8 kV. The SVL has to withstand this voltage.

 
I have attached a sketch.

The SVL voltage is a function of sheath impedance (depends on location, voltage divider with system impedance)

in case of short sections, the SVL will be subjected to a lower voltage stress ,however, with HV and EHV networks with very low system impedance, a mid span or near to load end fault will cause sever power frequency voltage to appear on SVL.

The EEC will add to this voltage depending on the amount of current flowing through it (a current divider between both locations earth resistances).

a bit confusing to size the SVL!
 
 http://files.engineering.com/getfile.aspx?folder=378bd2a3-9c71-496b-a116-d14f1287e48b&file=IMG_20150120_134315.jpg
with VSL open-circuited, Ig2 will flow in ECC into Rg2 and back to supply (smaller than Ig1, unless Rg2 is much smaller than Rg1).

Sheath impedance can't be zero, ECC will transfer the EPR to Rg2, the SVL imposed voltage will be Ish.Zsheath (neglecting IECC & Ig2)
 
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