Measurement on a circuit with several cables per phase 1

[PMACP]

Hi,

In my electrical measurements I have been using a power analyser that do not allows me to embrace the number of cables per phase I have (6).

Consequently, I had to take measurements only at one cable per phase. Therefore, the measured values (current, active power, apparent power and reactive power) are referred to the measured cable of each phase.

Now, I want to convert these values to actual values, ie the sum of the six cables.

If the current of each phase is distributed uniformly by the six cables that exists per phase, the operation would be easy (just multiply by 6). However, the cables have different lengths (apparently) and therefore different impedances and different values of current. I cheked that and you can see that on the attach I present.

To solve the situation I decided to set a correction factor capable of allowing me to measure the current in a single cable and then use that factor in order to determine the total corrent and powers on each phase.

The results I obtained are within the expected values for the case of total current and apparent power in each phase. As well, the values obtained for active power are near the ones I expected. However, the obtained values for the reactive power appears strange.

Some idea?

Thanks

 

[Skogsgurra]

It is very common to do just what you did. If the cables are more than around ten metres, the relative difference in length usually is not a problem. The cables share current uniformly. There are two factors that influence current sharing and that is if you have badly tightened cennections and if there is a big temperature difference between the cables. The latter is often the case if the cables are bundled together. The centrally located cables will usually be a lot hotter than those on the outside.

Measuring active and reactive power in one cable out of six parallel and then multiply with six is perfectly valid. What is your concern?

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[waross]

When cables are paralleled and the connections are good, the resistance per cable will be very close. The reactance of the individual conductors varies depending on the interaction between the cables.
I would expect the active current to divide equally and the reactive current to show more variation.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[PMACP]

Hi Skogsgurra:

Thank you for your answer.

Well, my concern is about the results I obtained. To do a better ilustration of the problem, I attached a small excel example (with real data). Note that this data refers to the connection between a bus bar and a transformer and it has 4/5 meters. Some cables of each phase may have 4 meters and others may have 5 meters. This means that it may have significant diferences on the impedances of the cables that belong to each phase (20%!).

Note, in the sheet of the excel named "Data Collected", that the measured values reveal important diferences between the currents on each cable I select to do the measures.

Also we can see that the active, reactive and aparent powers have important diferences.

On the second sheet, I made the correction by using the procedure defined on my previous post. Note that the active and aparent power become more balanced after the correction (as I expected). Also, the total current on each phase become more balanced. However, the recative power has significant unbalances? Why is that?

PMAC

 

[Skogsgurra]

OK, at 4 - 5 metres, the impedance of the cables differ significantly. I guess that we are dealing with sub-milliohms in each cable and since there is also a reactive part in the impedance, I can imagine that the currents in cables that encompass a larger area (larger area = more inductivity) have more inductive lag than currents in the cables that have less inductivity.

At these low impedance levels, even small differences make a big difference, so to speak.

Gunnar Englund

http://www.gke.org

--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[LionelHutz]

Putting a current clamp with an iron core around one conductor would add a slight inductance. Could this be affecting your measurements?

 

[BJC]

I have done 6 cables using a Rogowski Coil. The ones I used were made by Dent Instruments. They were 36" long and would reach around 6 cables in every case I needed.
I believe you can summ the outputs and measure cables measure cables in multiple large bundles.

 

[electricpete]

For very small diameter cables, resistive effects dominate the impedance and current sharing. As we increase cable diameter, the resistance goes down and inductive remains approx constant and so inductive effects can become more prominent.

These inductive include cable spacing relative to other conductors of same phase, other conductors of other phases. For unshielded low voltage solid conductor calculation, I calculated more than factor of 2 difference in current between conductors here thread237-238383 (I have to admit those results surprise me, but I am pretty sure thay are correct). I think cables with shields grounded both ends as used on higher voltage would tend to lessen the interaction effects and unbalance.

Good point by Lionel, putting an iron core around one of several conductors could cause a noticeable difference in the self-inductance of that phase. I hadn't thought of that before myself.

Very good practical measurement suggestion by BJC.

In absence of measurement all phases or good understanding of the expected behavior of your configuration, I think the assumption should be there is significant unbalance among phases unless proven otherwise. There are some ways of estimating unbalance discussed in the other thread, but it quickly gets pretty complicated and results should also be viewed with caution until verified. I did not understand what was the purpose of your calculation.


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[electricpete]

Correction in bold:
"cause a noticeable difference in the self-inductance of that conductor"

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[waross]

I was assuming that a Rogowsgi coil or large diameter clamp meter was not available.
I support BJCs suggestion.
Any tips on field fabricating a Rogowski coil folks.

Bill
--------------------
"Why not the best?"
Jimmy Carter