THD what?
Voltage or current?
Thanks Rob!
That was an interesting paper. I knew quite a bit of what was presented in there from my CEU classes. In fact, the PE that taught one of my classes went on about needing to make sure your neutrals were sized correctly when dealing with computer centers. But, he didn't lay it out as well as that paper did.
So, in a typical residential setting, this particular feature will more than likely not be of any help or significance for me. Right?
I don't have any customers that have VFDs that I can think of off the top of my head.
What would be a "bad" THD. It measures it in %'s.
3 phase motors don't like 5th harmonic.I have only seen harmonics cause issues in industrial settings, and most of these issues were related to capacitor banks failing due to resonance. I have not had any experience in data centers, though.
In a residential setting, harmonics have not caused any issues in my experience. In fact, IEEE is going to raise the harmonic limits in IEEE 519 for the next update of the document, because there are very few problems associated with harmonics.
I can't give you a bad THD percentage, because harmonics are everywhere, but there really isn't much equipment that is sensitive to them. It really is situation dependent.
Thanks Rob!
That was an interesting paper. I knew quite a bit of what was presented in there from my CEU classes. In fact, the PE that taught one of my classes went on about needing to make sure your neutrals were sized correctly when dealing with computer centers. But, he didn't lay it out as well as that paper did.
So, in a typical residential setting, this particular feature will more than likely not be of any help or significance for me. Right?
I don't have any customers that have VFDs that I can think of off the top of my head.
What would be a "bad" THD. It measures it in %'s.
3 phase motors don't like 5th harmonic.
CFLs and switching power supplies cause odd order harmonics. 5th harmonic causes three phase motor to pedal "backward" which cause reduction in efficiency and sometimes failures. Computers are usually the worst offenders since the VA draw is significant. Externally ballasted fluorescent lights are not really an issue, because they're required to have front end that limits harmonics.
Power supplies used in data center equipment or the double conversion UPS' front end portion these days have active PFC like the type used in fluorescent ballasts, so they're generally not a bad offender when it comes to causing harmonics.
Here, harmonics from an office building with a lot of computers served on the same PoCo transformer as the machine shop caused damage to motors.
Also, switching power supplies pull power from the wave peak and everything downstream essentially operates from 170v DC. Put enough of these loads and the waveform will become trapezoid and peak voltage will drop enough to cause similar loads to not operate properly. This is called "flat topping"
http://www.fluke.com/Fluke/usen/solutions/pq/Case-Study-Machine-Shop-Motor-Failures.htm
5th harmonic is like an additional load, because its more or less like applying brakes.A fully loaded motor can only take 1% of voltage unbalance before derating is needed.
Take a look at the graph on the left under HARMONIC ANALYSIS & WAVEFORM CAPTURE header. About half way down the page:I am curious what loads in an office building produce 5th harmonic, because computers and flourescent lighting are mostly 3rd and very little 5th.
5th harmonic is like an additional load, because its more or less like applying brakes.
Take a look at the graph on the left under HARMONIC ANALYSIS & WAVEFORM CAPTURE header. About half way down the page:
http://www.mymeterstore.com/p7789/elog2009.php
Not endorsing them, but that's what I found using Google. Note the quite significant amount of 5th harmonic.
THD current is usually 100-150% (relative to fundamental) on uncorrected rectifier bulk storage loads.I stand corrected on the 5th harmonic for the computers (and probably lighting as well). The attached picture shows the waveform from my laptop. The 5th is almost as much as the 3rd in that waveform. I guess I am used to monitoring close to the main where voltage harmonics typically are not all that high due to the harmonic loads being a small part of the total load.
Doesn't look like it...I understand how the negative sequence harmonics act like additional load. My point was that the voltage unbalance also could have caused heating of the motor windings, but that was not mentioned in the article.
Thought so. That shape of distortion is a signature of significant amount of IT loads. It would be interesting to see the current THD on the 208Y/120 transformer serving that space.The measurement was taken at my desk at work (office building)