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Current sharing in parallel circuits 5

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lennythegent

Electrical
Feb 22, 2017
4
hi I'm new so go easy!

we have installation of 9 (3 x r-y-b trefoil groups) and current sharing between phases is imbalanced greater than 10%. Could the physical arrangement/layout of cables be cause (or contribution) to this imbalance.

The lay is trefoil as mentioned, but the route is not consistent, in some places all three trefoil bundles are touching, in other parts equally spaced, and other cases where the cable run has two trefoil groups touching and one group is spaced.

i've browsed net a bit and found couple of discussion on this site and also a found this commentary that mentions with 9 cables there is no perfect arrangement and 5% imbalance could be expected (even if spaced)
the fact our installation doesn't have consistent spacing could this be cause for imbalance.

i've also read iec 60287 has more info but I don't have access to this at moment and would like to probe the thinking first before purchase.

I realize there could be other reasons for imbalance but I would like discussion to concentrate on this particular question if possible, thanks
 
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In each section one of the vertical struts is aluminum, the others are steel. This has worked well for us for 35+ years.
 
Good catch Mike.
Good solution wroggent. The aluminum strut is not evident in the picture.

Bill
--------------------
"Why not the best?"
Jimmy Carter
 
lennythegent (Electrical),

The photos indicates that the cable installation is not in trefoil. Just provide the length, voltage rating, size, designed rated current, whether the cable shield grounded at both ends or only at one end, whether the cable is transposed at equal route lengths etc.

Without these inputs it is difficult to assess the situation.
 
Thanks for responses guys.

Cable length ~100ft
Conductor size 500kcmils
Inner screen plus outer jacket armour
Grounded at transformer (supply end)
Not transposed in cable run, just the inconsistency in lay as pictured
Cables not counter phase rotated in their groups, all are a,b,c a,b,c a,b,c in order as illustrated in my earlier sketch with colour coding of cables. Counter phase rotation seems to be promoted by some publications online and not by others? From what i read there should be rotation, i.e. a,b,c c,b,a a,b,c
Volts 1200v to vfd input, single load

Krisys: photos are from another contributor not the installation i raised original question about When you say photos are not trefoil are you referring to the embedded photos by others or the sketch i made? Why do you consider the arrangement not trefoil? There are 3 sets of trefoil groups are there not?
 
lennythegent (Electrical),

O.K. Regarding the sketch, Noted.

Hope there is no loose connection or difference in the connection resistance. Just to narrow down the possibilities, you may try to measure the contact resistance with ductor test.

Hope all the cables are having same conductor cross section area of 500kcmils.

What about the length of the cable? Each have same length or what is the variations in the cable length?

Sometimes you know, when you load the cable, the current sharing is getting re-adjusted (i.e; current equalizing effect) due to the conductor temperature.
The hotter conductors tend to conduct less current as compared to the cooler conductor. This would balance out to some extent.

But if the unbalance is due to the impedance alone, equalizing effect may not help much. It shall be further studied.
 
In addition to the cable configuration, there is any impact of variable frequency drive (VFD) distort the input current on the feeders?

Appear that 10% current unbalance is within an acceptable range. Please verify this with the VDF manufacturer.


 
following some measurement we noted >10% current imbalance what seemed a large second harmonic (> 20 %) in the supply current. firstly I was interested to know about cable lay management to know if it could lead to greater than 10% imbalance - it would seem from discussion above and papers attached under certain circumstances it is possible that bad cable lay could be responsible for this imbalance (also the many other more plausible causes are noted).

i was then going to ask if this level of current imbalance could be responsible for the large 2nd harmonic and if so what impact could this have. I was thinking this should be raised in a separate thread though to save this discussion drifting off into a different tangent? OEM insists drive is working normally, so we then are looking at the installation which was by 3rd party.

Possible we will look to refine the cable lay, so will let you know if this improves (or worsens) current sharing!
 
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