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Current sharing in parallel circuits
5

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




RE: Current sharing in parallel circuits

I don't know about cables. I have seen the currents on a ring bus split very weirdly. On bus work or maybe even short cables, the connection impedance might play a bigger role in creating imbalance than the inductance for that length of cable or bus.

RE: Current sharing in parallel circuits

I am not sure if purchasing the standard will help you answer your question.
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

2
In a very low impedance loop circuit exceedingly small differences in impedance one way vs. the other way around the loop can result in significantly large imbalances in current splits.

RE: Current sharing in parallel circuits

2
If the cables are touching and the configuration is r-y-b b-y-r b-r-y then unbalance could be 11.8% See:
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

(OP)
thanks for response, especially paper provided by 7anoter4. As usual theory never matches practice and our install differs slightly from the cases presented in that particular paper.

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

It is not so hard to obtain this. However you have to pay 33$ !
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

Are there any commercial software packages available for modeling these types of systems?

RE: Current sharing in parallel circuits

There are several software available such as AmpCalc, ETAP, CYME, etc.

RE: Current sharing in parallel circuits

I have ETAP. It most definitely is not capable of modeling and determining current sharing by individual cables in a cable racking system like this:

RE: Current sharing in parallel circuits

wroggent,

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

scottyUK,

Are you talking about this part?

RE: Current sharing in parallel circuits

To support further calculation and understand the cause of current imbalance, we suggest to measure with a clamp amp meter the current on each cable at the beginning and the end of the tray to determine the location and cause of the current imbalance.
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

Yeah, that's the one - it has attracted my curiosity because it is quite different to what I'm familiar with on LV gear. We normally have a cable termination box where the armour (if any) ends at a gland and the conductors terminate to a palm connector within the box. The palm is sometimes a bushing through the wall of the main tank, other times it may be part of a disconnection link. We do use vertical bushings on the tank top for connection to bare bars, but that's typically at higher voltages.

RE: Current sharing in parallel circuits

In the very bottom of the picture is a Richards 313 network protector. It is a throated connection to the transformer to the right. This is a 5 x 2MVA 480V spot network system. On top of the network protector are Bussmann 5000 amp KPRC fuses. On top of the fuses are HOMAC flood seal connectors (175 series I believe). The cables are just ordinary xhhw-2 600V aluminum 750MCM - no shield or armor.



RE: Current sharing in parallel circuits

Thanks. Now I've found those items on the web I can see how it goes together. I hadn't realised what the network protector was, and the spreader plates above aren't something we see in that form. The whole thing wrapped in tape doesn't give much away. My little bit of learning for today. smile Time for a beer!

RE: Current sharing in parallel circuits

Beside the picture, this is what we know about your system, questions and suggestion:
• 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

cuky2000,

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

No particular expertise claimed here, but I see something that reminds me of a story.

<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

In each section one of the vertical struts is aluminum, the others are steel. This has worked well for us for 35+ years.

RE: Current sharing in parallel circuits

Good catch Mike.
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

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.

RE: Current sharing in parallel circuits

(OP)
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?

RE: Current sharing in parallel circuits

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.

RE: Current sharing in parallel circuits

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.


RE: Current sharing in parallel circuits

(OP)
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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