What are the worst type of harmonics to have on an electrical distribution system?
Answer:
I assume that by "distribution", you refer to your own local building power distribution...
The answer: All of them -- pick your favorite number, with the lower odd numbers (e.g., 3rd) being the most harmful given that they're typically at much greater amplitudes.
They can cause overheating of supply transfomers. Bad.
They can cause high neutral loads in 3 phase systems. Bad.
They can cause high neutral/earth potentials. Bad.
They can cause high power factor bills from your utility. Bad.
Power systems aren't my specialty, but I wouldn't think that any one harmonic would be any better/worse than any other.
Power distribution systems use low pass filters for the output so why be concerned with harmonics?
This is not a question that can be answered in few words. Harmonics in distribution systems can cause distortion of the voltage and current waveforms. Transformers have windings which are connected either in Star or delta. The effect of the even harmonics eg 2 4 6 8 etc is negligible. The effect of odd harmonics is much greater especially in delta connected system where the odd harmonics cause currents to circulate in the closed windings causing unwanted heat to be generated and hence expensive trying to keep the machines cool. Please note that the addition of odd harmonics change a pure sinewave into a square wave.
On a three phase system, the third harmonic is worst. That's because the three phases are protected by fuses, but the neutral isn't. So the amperage of the triplens add together (3rd, ninth, etc) on the neutral wire. Since the third is usually the largest amplitude, it has the greatest total current.
In most new systems, the neutral wire size is increased to allow for this potential problem. In the eighties and nineties, there were frequent cases of furniture cubicles catching on fire from the third harmonics created by early switch-mode power supplies in PCs and other electronic equipment.
Note that the third is also the easiest to get rid of, by going through a wye-delta transformer. The triplens get filtered out.
fifth harmonic affects battery hens. Thyristors are used to rectiofy the problem
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