AEMC 721 User Manual page 20

Clamp-on harmonic meter

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Power factor correction capacitor failure in many cases can be directly

attributed to harmonic content. Capacitors appear as extremely low imped-

ance values and are more susceptible to harmonics. Inductive reactance

varies directly with frequency (XL=2pfl). Parallel resonance between the

capacitor bank and the source impedance can cause system resonance

resulting in higher than normal currents and voltages. High harmonic cur-

rents have been known to overheat correction capacitors, causing prema-

ture failure and sometimes resulting in explosion.

Power Factor Capacitors at Resonant Frequency

Most harmonic problems result when the resonant frequency is close to

the 5th or 7th harmonic. These happen to be the largest harmonic ampli-

tude numbers that adjustable speed drives create. When this situation

arises, capacitor banks should be resized to shift the resonant point to

another frequency.

Another useful parameter is the Distortion Factor, or %DF. %DF is the

Total Harmonic Distortion referenced to the total RMS signal.

The Distortion Factor is expressed as a percentage, and may not be

greater than 100%. We provide this term because of the market need and

the requirement of this value under the international standard IEC-555.

Mathematically, %DF is the ratio of the root-mean-square(RMS) of the

harmonic content to the root-mean-square (RMS) value of the total signal,

and expressed as a percentage.

Harmonic Meter Model 721

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