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Author Topic: current addition on neutral  (Read 2010 times)

Keith Broughton

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current addition on neutral
« on: November 06, 2014, 09:33:50 am »

power is delivered on a 3 phase distro. Load is tested on 3 120 volt recepticals , 1 on each "leg".

scenario 1: 1kw par plugged into outlet 1, current on neutral and hot the same.
                 1 kw par plugged into outlet 2, current on neutral drops
                 1 kw par plugged into outlet 3, current on neutral 0
All as expected with a resistive load and I understand why.

scenario 2: the same process with 3 identical video projectors causes current on neutral to INCREASE as each load is added.

Please explain what is happening here. Is it something to do with the type of power supply the projectors are using?
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Mike Sokol

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Re: current addition on neutral
« Reply #1 on: November 06, 2014, 10:21:45 am »

power is delivered on a 3 phase distro. Load is tested on 3 120 volt recepticals , 1 on each "leg".

scenario 1: 1kw par plugged into outlet 1, current on neutral and hot the same.
                 1 kw par plugged into outlet 2, current on neutral drops
                 1 kw par plugged into outlet 3, current on neutral 0
All as expected with a resistive load and I understand why.

scenario 2: the same process with 3 identical video projectors causes current on neutral to INCREASE as each load is added.

Please explain what is happening here. Is it something to do with the type of power supply the projectors are using?

A few quick questions comes to mind. Are you measuring this current with a True RMS clamp meter or what? I've not thought this through completely, but these are likely switching power supplies. Trying to measure the current draw of a switching supply with a standard clamp meter (or even voltage with a standard DMM) will likely result in a wacky measurement. Also, switching supplies on a 3-phase circuit can introduce "Triplen" currents which are real odd-order harmonics of 60 Hz that can cause overheating of the neutral conductor. Please detail how and what you're using to measuring this.
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Guy Holt

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Re: current addition on neutral
« Reply #2 on: November 06, 2014, 02:16:37 pm »

Please explain what is happening here. Is it something to do with the type of power supply the projectors are using?

I second Mikeís conclusion that it probably has to do with the projectors using non-power factor corrected switch mode power supplies (SMPSs). As you can see from the power point slide below (from a workshop I offer on power quality to the members of IATSE Local 481) SMPSs draw a pulsed current that is rich in harmonics (upper left FFT is for a single SMPS). In three phase systems, something interesting happens when the current drawn by SMPSs on each phase leg is dumped back into the common neutral return one at a time (from left to right in the power quality meter readings in the power point slide below.)


The Fundamentals (A1,B1,C1 ) cancel each other out. The positive sequence harmonics (4th,7th, etc.) cancel each other out. The negative sequence harmonics (2nd, 5th etc.) cancel each other out. The zero sequence harmonics (3rd, 9th, etc.) do not cancel each other out*. Instead they add. Why? If, for a moment, we consider only the 3rd harmonic (180 Hz) of each phase as they return on the neutral, you will notice in the illustration in the lower left corner of the power point slide above that in each positive half-cycle of any of the fundamental waveforms, you will find exactly two positive half-cycles and one negative half-cycle of 3rd harmonic. The net result, as illustrated here, is that the 3rd-harmonic waveforms of three 120 degree phase-shifted fundamental-frequency waveforms are actually in phase with each other and so stack on one another rather than cancel out as the fundamentals, positive, and negative sequence harmonics do. The phase shift figure of 120 degree generally assumed in three-phase AC systems applies only to the fundamental frequencies, not to their harmonic multiples!

A closer look at the harmonic currents making up the neutral return once each phase leg is loaded with a SMPS reveals that, though made up primarily of the third harmonic from each phase stacking one on another, the high neutral current also consists of the, 9th, and 15th harmonics from each phase stacking one on another as well. Due to their significance in three-phase power systems, the 3rd harmonic and its multiples have their own special name: triplen harmonics.  All triplen harmonics add with each other in the neutral conductor of a 4-wire Y-connected loads. In power systems containing substantial nonlinear loading, the triplen harmonic currents may be of great enough magnitude to cause neutral conductors to overheat.

It doesnít require a True RMS meter to determine whether high neutral current is the result of harmonics. If you take a frequency reading of the current traveling on the neutral and it is 180Hz rather than 60Hz you know you have a harmonic issue. Why would we read 180Hz on the neutral?  If instead of the FFT of the neutral, we look at the current waveform of the neutral as SMPS are turned on one phase leg at a time (left to right in the power point slide below) we see clearly where the 180Hz current on the neutral comes from.


When the distorted current drawn by a nonlinear load on one phase leg is returned to the neutral conductor it is at 60 Hz (upper left reading.)  As we add a second  leg (the center reading), the frequency (cycles/second) increases from 60, to 120. And, when we add the third leg (the right reading) the frequency increases to  180hz. That is how we end up with 180hz current on the neutral conductor of our distribution system.

This elevated neutral current is just one of the artifacts of using non-power factor corrected equipment that use SMPS (see this thread I started for other artifacts: http://forums.prosoundweb.com/index.php/topic,151686.0.html.) These same power quality issues have vexed film electricians for years, which is why most movie lights are now power factor corrected.  If you havenít already, you might want to read an article I wrote for our company newsletter that explains the electrical engineering principles behind these issues and how to resolve them. The newsletter article is available at http://www.screenlightandgrip.com/html/emailnewsletter_generators.html.

Guy Holt, Gaffer,
ScreenLight & Grip,
www.screenlightandgrip.com

* If you are not familiar with the terms positive, negative, and zero sequence harmonics see my  LED Talk - Paralleling Two Generators  at http://forums.prosoundweb.com/index.php/topic,150582.0.html
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Keith Broughton

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Re: current addition on neutral
« Reply #3 on: November 06, 2014, 06:54:05 pm »

This happened quite some time ago so re-measuring isn't an option...for the moment.
I was using the ammeter built into the distro which I would figure is an inductive device.
It was Guys thread on non power factor corrected equipment that triggered this from my (old and feeble) memory.
As I recall, they were switching supplies.
I remember being surprised and baffled as to the cause.
All I could think was the switching supplies did not present the type of load that could balance out over the phases.
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ProSoundWeb Community

Re: current addition on neutral
¬ę Reply #3 on: November 06, 2014, 06:54:05 pm ¬Ľ


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