% Total Harmonic Distortion 1

[koolbean]

A building that I monitor is exhibiting a %THD I(N), of 126%. Logic tells me this is not good. Any ideas as the what the %THD should be for the current neutral? Or what may be causing the high reading?


thanks,

 

[stevenal]

Looks like your loads are well balanced at the fundamental and the neutral current is mainly triplen harmonics. Unless the harmonics are overloading neutral conductors or transformers, I don't see a problem.

 

[tinfoil]

A value for THD on the neutral of a three-phase service is almost meaningless without other data to place it into context.

The THD is calculated using the magnitude of the fundamental frequency (60Hz in North America) in the denominator. If there is very little neutral current, then ANY measurable amount of harmonics will look bad, particularly the triplen harmonics (3rd, 6th, 9th, etc) because the phase contributions reinforce each other rather than cancelling each other.

What did you observe for the individual phase THD's? What are the three fundamental phase currents?

I have seen phase current THD's of around 3-4% on a nicely balanced 800 Amp service, with a neutral current THD of 1057%. It turns out that the neutral current was in the <1amp range, so the neutral THD was calculating out to a horrendous number. There was nothing wrong with the service.

 

[pqsl]

Measure V thd phase to neutral - should not be more than 5% (or less hospitals gaming)
Measure largest harmonic – should not be more than 3%
Measure phase I(rms) - should be not more than conductor rating.
Measure Phase I(thd)
Measure Neutral I(rms) – should be not more than neutral rating.
Neutral current can be higher than phase current
High neutral I(thd) is possible if 0 sequence harmonics are high.
You will most likely not find any even harmonics.
Also measure neutral to ground voltage. If at the panel is say 3V you can expected higher at receptacles. High V(n-g) is bad news and could indicate high level of triplen harmonics.

 

[hazer2004]

One of the buildings I am monitoring has a THD of 33.2%. This builidng is served from a 2000 KVA transformer at 48 Volts and the amp reading at the time was 697. There are presently 2 ABB VSD for 2 150 HP AHU Supply fans and the dirves run continuously at 44 HZ. We shut the 2 fans down and the THD dropped to 7.1% with and the amperage was 289. The owner is blaming the drives for the motor problems and we cannot run the fans in bypass to run the harmonics test due to the increased air flow to the space. Any comments/advise would be appreciated

 

[peebee]

hazer -- yes, it sure sounds like the VFD's are causing the harmonics problems. It seems almost so obvious that I'm not quite sure why you need to test what the harmonics look like when the VFD's are bypassed. . . .

But maybe you could perform your test using a load bank connected to the VFD's instead of the fan motors?

 

[gigahertz]

% current THD can be very misleading. It is percent of the fundamental current. If the fundamental current is 100 amps on a wire rated for 100 amps, 126% THD is bad. If the fundamental current is 1 amp on the same wire, 126% THD is completely negligible.

Generally, current will almost always be distorted and exhibit high %THD because almost all modern loads are not pure resistors.