Cross-Bonding Specifics 1

[FEinTX]

I'm hoping there's a cross-bonding expert out there...

We have a 34.5 kV underground circuit 9600' long. Our EPC is transposing the cables and cross-bonding the neutrals every 1600' (6 total sections), except at the mid-point, they neither transpose or crossbond (a straight-thru splice).

They have:

SUB----X----X----S----X----X----JB
| |
gnd gnd (at begin and end)

SUB = substation
X = cross-bonded neutral and transposed cable
S = straight-thru splice
JB = junction box

I think they are mis-reading the drawings (which only show a 3 section example) and this middle splice should be cross-bonded and transposed as well.

Please tell me what you think, and I need help soon--they will be doing this work in the next 2 days...

Thanks in advance!

FEinTX

 

[matwong]

Dear FEinTX,
Based on my understanding on IEEE575, I would agree with you. It will fall under Clause 5.6.5: Continuous Cross Bonding where you have to transpose cable & X-bonding the sheath instead of applying st-thru splice. However, under Clause 5.6.3: Sectionalized Cross Bonding, you may just apply the local ground to the sheath but not mere st-thru joint.

I wonder what is the size of the cable and whether it's laid in flat formation? According to Electric Cable Handbook by Ir D. McAllister, distribution cable of less than 630mm2 in size might not be neccessary to use this bonding system and is more economical to derate the cable.

 

[FEinTX]

Matwong,

Thanks for the post.

The cable size is 1250 kcmil laid in a flat configuration (7.5" center-to-center). This is the largest cable used at the site, and I don't know if this cable is available in a larger size.

-FEinTX

 

[ausphil]

Conceptually there is nothing wrong with this setup. The overall aim is to try to get each core in each physical position for the same distance over the whole cable length, and each sheath coupled with each core for the same distance over the whole length (wordy but I think it is clear)

The only thing you need to determine is whether you solidly earth the straight though joint (to make it sectionalised cross bonding) or whether you connect SVLs at the joint to make it into a continuously cross bonded system (as matwong suggested).

If you do a traditional transpose at each joint, then your core arrangement will end up as (diagramatically for A phase) (the numbers denote phase laying positions):

|----------\ /----------\
1 \----------\ / 1 \----------\
2 \----------/ 2 \----------|
3 3



whereas with the connections you have described, your core arrangement is (again diagramatically for A phase):

|----------\ /----------\
1 \----------\ / 1 \----------\
2 \----------|----------/ 2
3 3

Now looking at the sheath connections for the first arrangement, they will be (the letter denotes which phase conductor it resides with for that section, E=earth point, X=crossbond point):

A B C A B C
|----------|----------|----------|----------|----------|----------|
E X X E X X E


whereas for the second arrngement, they will be:

A B C C A B
|----------|----------|----------|----------|----------|----------|
E X X E X X E


Which looks different diagrammatically, but electrically is identical (assuming that your lengths are the same)

 

[davidbeach]

whereas with the connections you have described, your core arrangement is (again diagramatically for A phase):

|----------\ /----------\
1 \----------\ / 1 \----------\
2 \----------|----------/ 2
3 3

Interesting that you would show it that way. I pictured it differently in my mind:
|----------\ /----------|
1 \----------\ /-----------/ 1
2 \----------|----------/ 2
3 3

 

[matwong]

The sheath voltage on the section (3 + 3)will shoot up if neither x-bonding or grounding is being applied between the major section.
Extraction from Electric Cable Handbook for reference :

http://i11.tinypic.com/34t69g2.jpg

 

[cuky2000]

For illustration purposes, check in the enclose link the effect of three bonding method in the shield induce voltage profile resulting and the impact on the current carrying capability of the cable.

http://cuky2000.250free.com/Shielding2.jpg

The next link shows a common practice in the US when grounding the cable shield at one end. The enclose curve shows a typical the maximum length as a function of the cable size to limit the induced shield voltage to 25 volts.

http://cuky2000.250free.com/Cable_Shielding.jpg