Metallic Shield / Sheath requirement for 69kV cu / XLPE Cable 1

[leur2011]

Does any one have a reference for the metallic shield and sheath requirement of 69kV, 60Hz, 3 single 500mm2 conductor copper XPLE per phase, trefoil configuration, route in a 2 kilometer underground direct buried cable system (ductbank approx. 100 meters various locations) with high water level approx. 0.7 m below grade level, high corrosive soil, and high temperature 40 deg. celcius?

Most of the typical installation that I am aware of is that providing copper wires as metallic shield that serves as low impedance path for ground fault current back to source in case of short circuits is safe for such installations.

Is there any drawback to preclude the use of copper wires metallic shield only without metallic lead sheath considering the type of installations and soil conditions. If there is, what are the governing codes and standards to use both a metallic shield and sheath to follow for the safe installations and operations of the cables?

 

[krisys]

The sizing of shield rating depends on many criteria, namely,
- are you proposing to earth both ends of the cable or single end only.
- what is the short circuit current
- standing voltage in case of one side earthed
- return current if both sides earthed
- sheath voltage limiter sizing
These are the salient elements of the design of installation for HV Cables.

There is a good IEEE Guide available addressing all the issues pertaining to the HV cable installation. I do not recall the title and number.

 

[7anoter4]

I agree with krisys. I don’t think there is a national standard which states the cable shield type. Usually there are standards for cable building and tests like followings:
AEIC CS7-93 Specifications for crosslinked polyethylene insulated shielded power cables rated.
ANSI / ICEA S-108-720-2004 Standard for extruded insulation power cables rated above 46 through 345 kV.
IEC 60183 Guide to the selection of high-voltage cables.
The cable manufacturers recommend also how to use this. See-for instance:
HIGH VOLTAGE XLPE CABLE SYSTEMS Technical User Guide[Brugg].

http://nepa-ru.com/brugg_files/02_hv_cable_xlpe/03_web_xlpe_guide_en.pdf

Brugg recommends Copper corrugated sheath:
for "All installations in soil, especially in locations with shallow ground water level"
See-for instance-also:

http://www.nexans.com/eservice/Leba...0234111/Lebanon/files/High-Voltage-Cables.pdf

 

[leur2011]

Thanks krisys and 7anoter4 for your responses and links you provided, but my question was;

- What standards says, metallic copper wire shield will suffice, no need for lead sheath.

- or lead sheath only, no shield is required.

- I have read Part 6 of ICEA S-108-720, it says that a metallic shielding consisting of a SHIELD, a SHEATH, or a combination thereof shall be applied over the non-metallic semiconducting layer.

So, what standards governing the cases of installation above.

Thanks

 

[Marmite]

Lead sheath is not the best choice for ducted installations as the thermo mechanical movement can eventually cause fatigue cracking. (Unless you are going for a bentonite fill as thermo-mechanical restraint. Lead sheathed cables are better direct buried.
Regards
Marmite

 

[leur2011]

To answer krysys questions. Our system is;

- We will ground the XLPE 69kV cable in single end only basically at mid-point 1000 meters, however, the cable manufacturer to establish the quantity of link boxes (SVL), base on the results of induce voltage calculations.
- The short circuit rating at the source is 40kA. We specify cable short circuit current capacity as 40kA at 3s.
- Calculated sheath standing voltage is 3150kV per phase, we specified 5kV sheath voltage limiter.
- Circulating current is not yet calculated at 69kV GIS load terminal side (not sure if these is required to verify) as we are still determining the requirement of metallic shield or sheath or combination of shield on top of it is a sheath thereof.
- The load of the cable actually is 133MVA, 60Hz, 69kV Wye connected solidly neutral.

 

[leur2011]

Marmite,

If you say the cable is metallic lead sheathed, does that mean the metallic shield e.g copper wires is not required. If so, what standard we can refer.

Thanks

 

[leur2011]

I've found a forum that describes a cable as Cu/XLPE/Cu wire/Lead Sheath/HDPE which was posted by nightfox1925 ages ago. I guess the cable is AEIC CS9 specified compliant to ICEA S-108-720 standard. What do you guys think.

Here is the link.

Link

Suppose our Cu wire shield for instance a (Cu/XLPE/Cu wire/Lead Sheath/HDPE) met our utility 3-phase 40kA fault at 3sec. Do I still need to specify the Lead Sheath?

 

[7anoter4]

First of all, in my opinion, Marmite is right: it is not recommended to draw lead-sheathed cable
through ducts or conduit. In my experience the lead sheathed cable has to be laid directly in the ground and in tunnels on racks instead of cable manholes.
Second, if the short-circuit current is elevated it is indicate to use copper wire insulation screen.
I am not sure the lead sheath will stay grounded at one end only and then circulating current will be high.
However, there is something embarrassing calculating the shield short-circuit current through lead sheath versus copper wire shield.
In my opinion, on one hand the copper wire shield could withstand more elevated short-circuit
current[3-4 times more].On the other hand since the copper wire shield resistance is lower than
lead sheath resistance the short-circuit current will be high.
For instance, usually 500 mm^2 conductor XLPE insulated 69 kV single-core cable the lead thickness 1.7 mm and the overall diameter of 65-71 mm the lead sheath total cross-section
area will be 215 mm^2 and the copper wire 140 mm^2 .
According to EPRI EL-5036-V4 the lead sheath resistance at 210 oC will be 335 microohm/ft [] and respective 69 microohm/ft for copper wire. If the cables laid in trefoil formation then Xm=24 microohm/ft .From the formula: Ish=Ic*Xm/sqrt(Rsh^2+Xm^2) where Ic=40 kA
for lead sheath Ish=2.8 kA but for copper wire 13 kA(!!).Following IEC 60949 the allowable short-circuit current for 3 seconds will be:
lead sheath: 3 kA ,copper wire 10.8 kA.
It seems to me lead sheath is better. It could be?