AEMC 721 User Manual page 18

Clamp-on harmonic meter

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Most single-phase office equipment draws nonlinear current. While fluo-

rescent lighting with electronic ballasts and many types of office equip-

ment contribute to creating harmonics, personal computer power supplies

are the largest contributor of harmonics within the office environment.

Although THD levels will be lower than in an industrial setting, the suscep-

tibility of office equipment to variations in power quality is extremely high.

In the industrial environment, there can be many three-phase, nonlinear

loads drawing high levels of load current. The most prevalent harmonic

frequencies are the odd integer multiples of the 60 Hz frequency. The third

harmonic (180 Hz) is always the most prevalent and troublesome.

In general, even harmonics cancel out and are negligible. The largest

single current draw for ballasts is the third harmonic. Triplens (odd

multiples of the third harmonic) are typically the dominant harmonics and

are most common in single-phase, nonlinear loads. The fifth and seventh

harmonics are dominant generally in motor drives.

Large commercial buildings have many different sizes and types of loads.

In most installations the power is distributed with 208/120 volt transform-

ers in a Delta-Wye configuration. When multiple loads are supplied, each

generates triplen harmonic currents on the neutral conductor which are

sent on to the transformer secondary and reflected into the delta primary.

These currents circulate within the delta primary causing overheating and

shortened service life.

Harmonic Meter Model 721

Computer Current Waveform

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Delta Primary, Circulating Current

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