26-06-2012, 05:54 PM
SEMINAR ON HARMONICS
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A pure sinusoidal voltage is a conceptual quantity produced by an ideal AC generator built with finely distributed stator and field windings that operate in a uniform magnetic field. Since neither the winding distribution nor the magnetic field are uniform in a working AC machine, voltage waveform distortions are created, and the voltage-time relationship deviates from the pure sine function. The distortion at the point of generation is very small (about 1% to 2%), but nonetheless it exists. Because this is a deviation from a pure sine wave, the deviation is in the form of a periodic function, and by definition, the voltage distortion contains harmonics.
Total Harmonic Distortion (THD)
Harmonics work together in distorting the fundamental waveform. The total harmonic distortion, or THD, of a signal is a measurement of the harmonicdistortion present and is defined as the ratio of the sum of the powers of all harmonic components to the power of the fundamental frequency. There is a THD for both voltage and current. I will talk about current THD since it pertains to this discussion better than voltage THD. The THD of a waveform is calculated by taking the square root of the addition of the squares of the harmonic currents, and dividing them by the fundamental current. As a formula.
General
Three Phase Applications
Harmonics get more complicatedin three phase applications. Herenot only do you have to deal withphase conductors, but also theneutral conductor, triplen (oddmultiples of 3) harmonics, andsequence harmonics.The triplen harmonics (3rd, 9th,15th, etc.) are the major cause ofheat because they add together inthe neutral conductor. Themagnitude of the harmonic currentproduced by the triplens canapproach twice the phase current.This causes the neutral conductorto overheat because neutralconductors were historicallydesigned with the same capacity as the phase conductors.A situation that producesabnormal amounts of heat inmotors is the combination of positive and negative sequencedharmonics. The positivesequenced harmonics are thefundamental, 7th, 13th, 19th, etc.They tend to apply an additionalforward force in the direction ofthe motor rotation. The negativesequenced harmonics are the 5th,11th, 17th, etc. They present aforce that opposes the motorrotation and tries to make themotor rotate in the oppositedirection. The force of theseharmonics acting upon each othercreates heat which leads topremature failure.Transformers configured with adelta - wye connection help toreduce the effects of harmonics.The triplen harmonics are trappedand circulate in the delta primaryof the transformer. Since mostloads produce high levels of the3rd harmonic (one of the triplens),the harmonic content reflectedback to the source is reduced.The circulating harmonics in theprimary of the transformer createsheat because of their higherfrequencies. For this reason, atransformer that can handle theexcess heat is needed. Thistransformer is called a K-rated.