Current sharing in parallel circuits
Current sharing in parallel circuits
(OP)
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) https://books.google.com.au/books?id=qLekBAZUJloC&...
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
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) https://books.google.com.au/books?id=qLekBAZUJloC&...
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






RE: Current sharing in parallel circuits
RE: Current sharing in parallel circuits
Without know the details such as a few representative cable section arrangement , length, type of cable, voltage, etc.
The first impression is that a trefoil configuration with the parallel cable tie at the sending and receiving ends may not be the cause of 5% of voltage imbalance.
We suggest to measure both ends voltage of the feeder cable to determine if this is the case of imbalance.
RE: Current sharing in parallel circuits
RE: Current sharing in parallel circuits
A CABLE CONFIGURATION TECHNIQUE FOR THE BALANCE OF CURRENT DISTRIBUTION IN PARALLEL CABLES
http://jmst.ntou.edu.tw/marine/18-2/290-297.pdf
IS_1 configuration.
Table 2. Results for a 3φ3W 9-cable system with structure
RE: Current sharing in parallel circuits
various cases of the cable installation along the route are shown in the attached. part of problem is that the cable rack in the route differs in width so the 2d spacing that was meant to be achieved was not possible due to cable rack
7anoter4, would you know if the reference paper No.3 by Du, Y. and Burnett, J is worth purchasing?
i'd also be interested to know if there are rules/standards regarding maximum length that cables are permitted to run out of the trefoil pattern, i.e. for purposes of aligning cables to their phase terminations at either end. At either end of run we have an arrangement that resembles that of configuration IS-1 "-" in table 2 of mr.San-Yi Lee's paper, that shows 76% imbalance!! there is approx. 7-8ft at either end of cable that is in this arrangement. the majority of run is trefoil however.
https://res.cloudinary.com/engineering-com/image/upload/v1487882856/tips/cable_run_lfuygr.pdf
RE: Current sharing in parallel circuits
RE: Current sharing in parallel circuits
Y. Du and J.Burnett Current distribution in single-core cables connected in parallel
http://ieeexplore.ieee.org/abstract/document/95967...
I think you could consider every segment unbalance as percent of entire length.
Let's say the total length is 1000' and you have 300' 12% unbalance 100' 76% and 600' 4% then total unbalance will be:
0.3*12%+0.1*76%+0.6*4%=13.6%
Of course, it is a conservative solution. Actually one segment unbalance could mitigate an other segment unbalance.
However, you may neglect unbalance if the cables are 1-2 diameters apart.
RE: Current sharing in parallel circuits
RE: Current sharing in parallel circuits
RE: Current sharing in parallel circuits
RE: Current sharing in parallel circuits
Have you got a close-up or a drawing of whatever that transition arrangement is where the cables joint the transformer? it's very different to anything we use over here.
RE: Current sharing in parallel circuits
Are you talking about this part?
RE: Current sharing in parallel circuits
If there is significant current induction by other feeder, particularly in the transition areas, check if there is a possibility to shield this region to minimize the imbalance effect.
RE: Current sharing in parallel circuits
RE: Current sharing in parallel circuits
RE: Current sharing in parallel circuits
RE: Current sharing in parallel circuits
• The installation consists of 9 -3 x r-y-b trefoil groups ?
• The current sharing between phases is imbalanced greater than 10%.
• This is a 5 x 2MVA, 480V spot network with a Richards 313 network protector & 5000 amp KPRC fuses. (primary side ? )
• What is the voltage and ampacity on the secondary side?. There is any single phase connected load that is causing current imbalance?
• Appears that there are primary and secondary feeders running in parallel. Please confirm
• Consult with ETAP to model the above ground cablesystem per IEC 60364 by section as was suggested. Measuring ampacity is another option in lieu of calculation.
RE: Current sharing in parallel circuits
Thank you for the suggestion. I am not the OP; I am not aware of any issues at the site I posted pictures of. I don't have the IEC 60364 module nor have I ever worked with IEC standards but I'll look into that.
RE: Current sharing in parallel circuits
<story>
Some of my friends were assembling a three phase generator, and didn't have a single large or trefoil bushing to get the connecting cables through the steel wall of the termination box, so they used three individual bushings, one per phase, in three distinct holes. When they fired up the generator for testing, the wall of the terminal box in between the three bushings got very hot very quickly.
You can probably figure out why.
</story>
WRT to the wires suspended from the overhead, I see two arrays of wires suspended above two metal channels, which are themselves suspended by similar verticals, presumably connected to metal structures above.
I.e., half the wires are going through one aperture surrounded by steel, and half the wires are going through a different aperture surrounded by steel. ... sort of as happened to my friends' generator.
Whereas, if the upper horizontal channel were installed upside down, so that the wires were hanging below it instead of above it, all the wires would be going through the same aperture surrounded by steel.
Mike Halloran
Pembroke Pines, FL, USA
RE: Current sharing in parallel circuits
RE: Current sharing in parallel circuits
Good solution wroggent. The aluminum strut is not evident in the picture.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Current sharing in parallel circuits
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.
RE: Current sharing in parallel circuits
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?
RE: Current sharing in parallel circuits
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.
RE: Current sharing in parallel circuits
Appear that 10% current unbalance is within an acceptable range. Please verify this with the VDF manufacturer.
RE: Current sharing in parallel circuits
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!