how the harmonics are generated in the electrical system?

Question

what is the main source for harmonics generation

Answer 1

Any time alternating current passes through any device other than a resistor, capacitor, or inductor, including transformers connected to only resistors, capacitors, and inductors, harmonics are generated.

Resistors, capacitors, inductors, and transformers connected to the same, are all linear devices. Any other type of device is non-linear. The most common non-linear device that AC would encounter is a diode -- especially a diode immediately following a transformer (or the diodes in those variable frequency drives mentioned above).

Any non-linear device will provide a non-linear load to the AC. When the value of the AC voltage reaches a certain point in the non-linear device, the slope (resistance) of the load changes, thus changing the amount of current necessary from the source.

At this point I could talk about reflections (as in transmission line theory), modulation (communication theory), or Fourier transforms, to explain exactly why the harmonics are generated, but it would take too much space for this little answer box.

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Answer 2

Poor one is on the right track, but stops short of explaining the details. And it's not just from VSD's, but also from anything with a DC load like PC's, copy machines, etc.

The main source of the harmonics are rectifiers and their filters. A common DC supply driven from AC mains typically has a bridge rectifier connected directly to the lines, which then is filtered by capacitors. The caps buffer the rectified AC, loading the AC when somewhere between RMS and peak value, and not loading the AC at all other times. The result is heavy AC current flow only at and near the peaks, which appears as odd harmonics, mostly 5th.

This can be alleviated with active power factor correction (PFC), which generally is extra circuitry that will load the AC at the times that the rectifier/filter wouldn't, and supplying power (thus partially unloading the AC) when the rectifier/filter is loading the AC. The result is a current waveform that more closely following the AC voltage, thus making the load appear more as a purely resistive load, which minimizes line losses all the way to the generator.

Answer 3

Harmonic power is a mathematical representation.

Harmonics are voltage/currents at a frequency some multiple of the fundamental (mains) frequency.

A big cause of harmonics is variable speed drives. 5th order being the main component. IEEE 519 regulates the allowable levels. There are a number of ways to reduce it. Its usually the water companies that have most concern as they connect to domestic power supplies with large speed controlled pumps.

Answer 4

WOW... and you want an answer to THAT on Yahoo Answers?... do you have ANY idea how complex that problem is?

Basically... and childishly simply... it has to do with the fact that AC reverses direction 50 or 60 times each second (depending on your country) and in long transmission lines, the waves can actually collide, causing a violent change in the characteristic impedance of the line and a swing through resonance and I seriously doubt you understand a word of this explanation.

Answer 5

Any frequency (waveform) that is not a perfect sine wave can cause harmonic frequencies.
For more reading go to http://bg.ecmweb.com/ar/electric_harmonics_made_simple/index.htm

Answer 6

you need at least 2 frequencies mix together to get the sum difference and the original freqs