When is cross-bonding needed? 1

[FEinTX]

I'm a degreed EE, but my background is electronics and I could use some help.

BACKGROUND: I'm working at a wind farm site w/ a 34.5 KV collection system. The individual circuits feed into a 34.5 - 345 KV substation. The circuits are typically between 25,000 and 50,000 feet in length, and each has 12-13 2-MW turbines. Cable sizes vary from 3/0 at end of a string to 1250 kcmil on the long runs to the substation and all cable is laid in a trefoil configuration.

ISSUE: Our drawings have the following note on the 1250 kcmil runs to the substation which are ~10,000 feet in length: "Bond cable screens together and to ground at quarter, midway, & three-quarter points along the length of the cable." (This means "cross-bonding", right?) Our developer is saying this is an old note that applied to the orignal plans which did not have the cable in trefoil, but rather, in a flat configuration. They claim the trefoil layout eliminates the need for any cross-bonding.

QUESTIONS:
(1) Am I being fed a line, or is it true that laying cable in trefoil can eliminate the need to do cross-bonding?

(2) What exactly is the purpose of this cross-bonding in this application? I'm guessing it has to do w/ minimizing losses and possibly protection in the event of arcing/lightning, but I'd sure like an educated answer here.

On a related note,
(3) Another issue is the bare ground wire. The drawings call for this to be at the bottom of a 4' trench w/ 6" of grated fill between it and the MV cables. In practice, this is sometimes difficult to achieve. I've seen cases where the ground has been pulled up for some reason and is virtually against the 3 MV cables. Is this something to be concerned about?

Thanks for any help you can offer!

KRCinTX

 

[benlanz]

KRCinTX,

We have been working with the wind industry for several years. The questions you have asked are the same questions we receive all the time. The choices you make will have a direct impact on the reliability and serviceability of your collector system.

Question 1:
Are your cables 10k to 50k ft long without the capability of sectionalizing? (e.g. junction boxes every mile?)

Question 2:
I assume you are using the typical concentric neutral design. What is the percent neutral of your cable? (1/3, 1/6)

Question3:
Would it be beneficial for you to talk to some wind farm owners who learned the answers to your questions the hard way?







Benjamin Lanz
Vice Chair of IEEE 400
Sr. Application Engineer
IMCORP- Power Cable Reliability Consultants