Parallel Cable imbalance load 2

[vithanidevesh]

Hi
I have following system installed
Govt supplied transformer is incoming supply
From transformer - there are 3 energy meters installed. I have laid 3 cables 3.5 core 150 sq mm aluminum cable (length around 5 meter each) and all of them are connected to one Busbar chamber from there load is distributed

Now when I check current, following is my readings

Cable 1 - 103/133/160
Cable 2 - 144/135/104
Cable 3 - 125/103/120

Main issue is due to this, my maximum demand is crossing assigned limit and i am getting one meter reading higher than other 2

How to solve this and make even distribution ?

 

[LittleInch]

Are there any breakers in this system? Could you check each cable in turn when you have reduced loads?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[waross]

Do you have any spare CTs?
If so, try a CT on one of the high current cables and see if it diverts current to the other cables.
A CT MUST NEVER BE ENERGIZED WITH THE SECONDARY OPEN.
START WITH THE SECONDARY SHORTED.
If it is working but you want more effect, connect a resistor across the CT secondary.
WARNING

The cable must be de-energized when installing the resistor.
Do not remove the resistor while current is flowing.​

No CTs available. Pete's post is an inspiration.
Try running one of the high current conductors through a short piece of steel conduit.
No need to ground the conduit at both ends. It may be grounded in one place if desired.
Grounding at both ends: When a ground cable is run through even a short conduit (As is often done for physical protection where a cable passes through a floor) the effect of magnetic encirclement will seriously restrict fault currents.
The cable is connected at each end to the end of the conduit. Thus the conduit becomes a parallel conductor.
While the cable is grounded at each end, the grounding is incidental to the conduit being connected as a parallel current path.
Obviously, this cannot be done with phase conductors. (You don't want sections of conduit at line voltage and grounding would have no effect on the high impedance caused by the magnetic encirclement.)
Physical protection of phase conductors; All three conductors must pass through the same conduit, or, a steel protection may be fabricate such that all three phases pass through the same opening.
or
Use aluminum conduit.
In this instance we want the effect of magnetic encirclement to help balance the currents so this is the exception to the "All three Phases in the Same Conduit" rule.
To anyone filing this information away for possible use in the future, I would be cautious using this method for higher currents.
At 200 Amps or more, too much heat may be generated to be safe. Use CTs for higher currents.

In regards to the photos.
the yellow cables in the bottom picture.
Two cables are close together and one is some distance away.
That will make a difference.


--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[7anoter4]

In my opinion, the cable conductors are not the same -it is not same metal-the aluminium has a resistivity between 1000/35.4 and 1000/37.7 [2.82*E-8 and 2.65*E-8 ohm.m] .See:

https://hypertextbook.com/facts/2004/ValPolyakov.shtml

and not the same cross-section area- even if it remains in the limits of the standards.
I calculated the maximum and the minimum of the resistance for a 2.82 E-8 and 2.65E-8 ohm.m and the cross-section area as -5% and +5% of 150 mm^2.
The reactance , the skin and proximity effects are negligible [however I calculated it] .
The total currents per phase are close, but I had to change a little the arguments in order to reduce the neutral current [the remaining still is 0.137%] since ,actually, it does not exist.
Using IEC 60228 for minimum and maximum conductor diameter and maximum resistance at 20oC d.c. and IEC 60502-1 for insulation thickness for 3*150+70 aluminium conductor and PVC insulation and jacket, no armour I considered this:
the conductor diameter may be 13.9 or 15 mm and the insulated core 17.5-19.4 mm; the reactance per 5 m per cable is 0.000373 and 0.0003815 ohm and resistance at 70oC including skin and proximity effect -for aluminium resistivity of
2.82 E-8 and 2.65E-8 ohm.m is 0.21046 and 0.24773 ohm.
Considering Zmax and Zmin as 1/3 of these value per single core [as 3 in parallel] the voltage drops per phase are 26.1 and 30.7 for R;26 and 30.6 for Y and 26.9 and 31.7 V for phase B.
Dividing these voltage drops by Zmax and Zmin I found for IR maximum 146 A and 105 A as minimum ;for IY 105 min and 145.5 A max and for IB 108.7 and 150.7 A keeping the length 5m constant.
That means if the conductors are not from the same core even, they are the same length the current could be very different.
In my opinion, one has to measure the core resistance for each cable in order to see the differences.

 

[waross]

In my opinion, the cable conductors are not the same -it is not same metal-the aluminium has a resistivity between 1000/35.4 and 1000/37.7 [2.82*E-8 and 2.65*E-8 ohm.m] .See:

https://hypertextbook.com/facts/2004/ValPolyakov.s...

and not the same cross-section area- even if it remains in the limits of the standards.

I have laid 3 cables 3.5 core 150 sq mm aluminum cable

It is very unlikely that a manufacturer will use different material and different cross sections in the same cable.
There are a few exceptions that don't apply here. eg; A 3 core cable will have 3 equal conductors and may have a fourth conductor of different cross section and possibly a different material.
Resistance, impedance with a high reactive component is more probable. This must be determined in situ.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[7anoter4]

How the neutrals of 70 mm^2 in each cable are connected ?

 

[LittleInch]

Any pictures of the connections / bus bars

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[7anoter4]

In fact, the cables are not completely parallel because from the common point on the transformer it goes through the measuring device and only after the device does the cable begin. So the meters are in series with the cables. The entire current course includes- in series-the measurement device also. As the cable reactance phase-to-phase is negligible but each cable presents a homopolar component -one different from the other in amplitude and argument, I think the measurement devices introduce a load in each cable.

 

[waross]

While it may be difficult to identify and possibly more difficult to mitigate the cause of lower impedance in one cable of each set,
It may be more productive and more doable to increase the impedance of the lower impedance cables.
This was once done to improve the load sharing of parallel transformers of different impedances.
A window CT with the secondary winding removed may be used to increase the impedance of a cable.
A window CT with the secondary winding shorted will be more effective to increase the impedance of a cable.
A window CT with the secondary winding shunted with a suitable resistor will be more effective to increase the impedance of a cable.
Passing the cable through the window more than once will be more effective.
A couple of short lengths of iron pipe over the cables will test the concept, but the pipes will get hot.
In this area, there is a trend for arenas to change out discharge lighting for LED lighting.
There are hundreds of removed ballasts around the country.
(I have about 80 or 90 piled in the grass behind the shop. If this problem was local, I would strip the windings out of some of these and use them as wireless reactors.)

--------------------
Ohm's law
Not just a good idea;
It's the LAW!