Measurement on a circuit with several cables per phase
Measurement on a circuit with several cables per phase
(OP)
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
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






RE: Measurement on a circuit with several cables per phase
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
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: Measurement on a circuit with several cables per phase
I would expect the active current to divide equally and the reactive current to show more variation.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Measurement on a circuit with several cables per phase
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
RE: Measurement on a circuit with several cables per phase
At these low impedance levels, even small differences make a big difference, so to speak.
Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: Measurement on a circuit with several cables per phase
RE: Measurement on a circuit with several cables per phase
I believe you can summ the outputs and measure cables measure cables in multiple large bundles.
RE: Measurement on a circuit with several cables per phase
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: different currents on the parallel wires of a phase (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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RE: Measurement on a circuit with several cables per phase
"cause a noticeable difference in the self-inductance of that conductor"
=====================================
Eng-tips forums: The best place on the web for engineering discussions.
RE: Measurement on a circuit with several cables per phase
I support BJCs suggestion.
Any tips on field fabricating a Rogowski coil folks.
Bill
--------------------
"Why not the best?"
Jimmy Carter