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A few manufacturers make ultralow-harmonic VFD technologies. An ultralow-harmonic VFD has six insulated-gate bipolar transistors (IGBTs), rather than passive diode-bridge components, in its convertor section (Figure 11). These IGBTs control the harmonic current drawn by a VFD. With no harmonic current drawn, no cancelation is required. Ultralow-harmonic technology typically reduces input harmonic currents to 4 percent or less at a VFD input (Table 2).

In one test, a 3-percent voltage imbalance on the input of an 18-pulse transformer/drive caused a 1.5-percent-per-unit increase in current distortion. Thus, if the computer harmonic-analysis estimate had been 4 percent, actual THDI would have been 5.5 percent.

With an ultralow-harmonic or active-filter system, a 3-percent voltage imbalance increases harmonic-current distortion by less than 0.5 percent per unit.

CONCLUSION

A harmonic analysis should be performed before a design is finalized. The analysis should be conducted at the PCC to determine current distortion at the main utility service entrance to a building. Hardware-based specifications dictating that any drives over a certain horsepower shall be a certain technology should not be utilized.

REFERENCE

1) Mansoor, A., Phipps, K., & Ferro, R. (1996). System compatibility research: Five horsepower pwm adjustable-speed drives. Knoxville, TN: Power Electronics Applications Center.

For past HPAC Engineering feature articles, visit www.hpac.com.

The manager of HVAC applications for ABB Inc. Power & Control Sales, Michael R. Olson has extensive experience in the HVAC, water/wastewater-treatment, and chemical industries. He has written numerous trade-journal articles discussing the application of adjustable-speed drives and been a contributing editor to several books on the subject. He has a bachelor's degree in electrical engineering from the University of Illinois and a master's degree in engineering management from the Milwaukee School of Engineering. He is a member of the American Society of Heating, Refrigerating and Air-Conditioning Engineers and BACnet International. He can be contacted at mike.olson@us.abb.com.