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Harmonic in Electrical System

The presence of harmonics in an electrical system will result in a reduction in efficiency of the equipment exposed to the harmonics. The reduction in efficiency is due to the fact that the harmonics produce heating effects and can flow counter to the normal power flow direction. The equipment which produces this is generally found to be that which utilizes some form of switched mode power supplies, generally comprised of SCR's or similar devices. One good method of providing positive isolation is through a delta/wye isolation transformer. You can get a general idea of the level of harmonics by measuring the current with an amp meter and then again with an amp meter utilizing true RMS measurements. The difference in the readings is a reflection of the level of harmonics. There are also a good number of devices which can directly measure the harmonic content directly and to a high degree of accuracy.

An averaging meter will not pick the harmonic current as it is calibrated to a sine wave where a true RMS meter will pick the harmonic content. The difference between the readings when compared will provide you a ratio that is a reasonable representation of the percentage of harmonics present. This will not tell you the specific nature of the harmonics, but will inform you of the percentage of harmonics present. If you have an amp meter with a selectable true RMS function then just take two measurements, one with and one without, then compare, if the averaging meter says 80 amps and the true rms meter says 100 you can imply that the waveform contains 20% harmonic content.

All equipment with built-in switching devices or with internal loads with non-linear voltage/current characteristics, Induction motors with irregular magnetizing current associated with saturation of the iron have harmonic source; as indicated they have inductance nature, to reduce it should change to capacitance, this is why using power factor correction help to reduce harmonics.

In this time some power factor corrections are included active filters, so in some cases we do not need to have harmonic filter for each equipment. Active filters analyze the harmonics drawn by the load and then inject the same harmonic current to the load with the appropriate phase. As a result, the harmonic currents are totally neutralized at the point considered. This means they no longer flow upstream and are no longer supplied by the source. A main advantage of active conditioners is that they continue to guarantee efficient harmonic compensation even when changes are made to the installation.

In design stage can calculate how much harmonic can reduce by this device, and which one needs independent harmonic filter, especially in high number of variable speed drive in service.

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