35kV 750kCMIL vs Parallel 250KCMIL cable

[philly]

I'm evaluating options for installation of 35kV cable and aiming to evaluate most cost effective option. Application is underground in PVC conduit for approx 100ft between transformer secondary and MV switchgear. Transformer side transitions to overhead for connection on top bushings.

Plenty of termination space on both ends so not an issue.

Looking for any feedback from anyone with current pulse on copper prices and experience with install to determine if cheaper parallel 250MCM cable costs offsets any additional labor costs which would really only be extra cable pull and terminations in my opinion (2nd spare conduit would be run regardless).

Thanks

 

[petersonra]

If cost is an issue, why copper?

 

[ron]

We typically provide for a spare duct, so if you go with paralleled sets, you may need a 3rd duct.

Be sure the MV SWGR can accommodate paralleled sets. Some Metal Enclosed pad mount SWGR have trouble with multiple stacked elbows and a surge arrestor if required.

 

[JoeStillman]

That's some big load at 35 kV. 20 - 30 MW with 750 CU.

 

[ron]

That's some big load at 35 kV. 20 - 30 MW with 750 CU.

Sometimes the conductor size underground is due to a derating from the 311 tables

 

[philly]

That's some big load at 35 kV. 20 - 30 MW with 750 CU.

Yes its 30MW of load. Utility substation application.

 

[David Castor]

Pulling 3/c 35 kV 750 kcmil CU will be a PITA, but I have to think it will be less expensive overall. Most utilities I've worked with would run 3 conduits plus a spare and pull in single conductor in each conduit - assuming it's all plastic conduit. If you do that, you could look at 1000 kcmil AL if it gives you enough ampacity.

 

[Julius Right]

First, the shield grounding policy is important. If we follow IEEE 835/1994 for buried conduits in 90 RHO earth and the shield grounded only one end-or well transposed as per ANSI/IEEE Std 575-1 988-then 750 kcml [MCM] copper conductor, 90oC insulated [XLPE ] 667 A is the ampacity but only 500 A for aluminum conductors [however, no indication for open shield but, only for 1/18 both ends, grounding]. 1000 MCM aluminum with open shield -grounded one end - the ampacity
is 622 A-in the same conditions.

 

[philly]

Pulling 3/c 35 kV 750 kcmil CU will be a PITA, but I have to think it will be less expensive overall. Most utilities I've worked with would run 3 conduits plus a spare and pull in single conductor in each conduit - assuming it's all plastic conduit. If you do that, you could look at 1000 kcmil AL if it gives you enough ampacity.

I've seen that before as well but most likely the transmission side of things. Is that common practice from what you've seen at MV distribution level inside of substations.

 

[David Castor]

Common for distribution substations. Single phase per conduit is felt to be more reliable compared to a common conduit where a single ground fault could take out all three phases. And it's what they are used to due to single-phase distribution circuits.