Can someone show me pictures of electrical sine waves that have been affected by harmonics? THDs of around 3%?

Question

If you could tell me what harmonics caused the distortion and the actual THD value that'd be nice. I tried having a look on the net to no avail. I would like to know what the sine wave at the socket in my house would look like too, I would not expect it to be perfect but I don't know how far from perfect to expect.
THANKS!!

Answer 1

I've seen quite a few scope displays of distorted sinewaves over the years, and I could not visibly discern a wave as distorted unless it had at least something like15 or more percent THD. 3% THD, assuming you're talking about typical values of 3rd-5th-etc harmonic distortion, would be difficult to see even if you had a pure reference wave superimposed onscreen. And that's about your only option I can think of if you don't have access to a distortion analyzer or power quality analyzer.

I think the answerer below is thinking of THD in the current waveform rather than the voltage, which of course will appear distorted by the load unless it is a linear virtually-resistive-only load. This ideally has nothing to do with the voltage waveform.

What causes distortion is heavy non-linear loads such as large power supplies. They typically have input rectifiers that only turn on when the filter cap voltage drops below the instananeous AC voltage, minus a diode drop or 2. This tends to cause odd harmonic distortion.

Don't remove grounding caps from power supplies, they are there to reduce EMI. Don't ground the neutral ANYWHERE, that should only be done on the pole by the elec company and nowhere else.

Answer 2

I cant show u what it looks like,but I know. Do u have several components that have AC to DC power systems. The diodes in the power supplies conduct about 30% of each half cycle. This will produce a square wave in the neutral . If the neutral and ground are connected at the power panel it will put this harmonic distortion in all power supplies. U need to keep the neutral and ground separate except at the transformer. U can also check for AC current in the ground system. Some old supplies had a small cap from the hot side to ground ,take them out. MOV of 24 v. in the power box to ground.

Answer 3

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Answer 4

Check out power quality monitoring equipment manufacturers, like Dranetz. They have a wealth of info on their site, including pictures and diagrams of harmonic distortion.

http://www.dranetz-bmi.com//pdf/waves.pdf

or

http://www.dranetz-bmi.com//pdf/harmonics.pdf