Consulting-Specifying Engineer - March 2008 - (Page 16)

M/E Roundtable PARTICIPANTS VFDs, harmonics drive power quality debate This month’s panel of experts discusses how to mitigate harmonic effects, passive and active filters, and technological advances with these systems and VFDs. BY PATRICK LYNCH, Editorial Intern CSE: In the design phase, what ensures that variable frequency drives (VFDs) don’t cause harmonic distortion? JIM NASH: The first step is to analyze what portion of the total connected load VFDs represent. Most commercial and industrial installations tolerate up to 20% drive loading before additional hardware needs to be considered. This assumes that the drives to be installed have at least 3% ac line reactance or dc link reactance as a part of their standard designs. If the drives don’t include internal reactance, then the system tolerance level drops to about 10%. Likewise, special locations, such as airports and hospitals, have lower harmonic tolerance levels where 10% loading may represent the permissible threshold even when drive designs do include reactance as standard. If it is known that these levels will be exceeded, then a formal harmonic analysis should be made. The drive vendor may be able to assist in providing a first order analysis that includes harmonic mitigation recommendations. If considering this approach, be prepared to provide a network one-line diagram that includes both expected drive loads and other loads. Keep in mind that accurately establishing the ratio between nonlinear loads—such as VFDs—and linear loads (across-the-line motors, incandescent lighting, etc.) is vitally important to accurately predicting harmonic content. Establishing the kVA and impedance of the utility transformer (or generator) also is necessary. Finally, the short circuit capacity of the utility feed at the primary of the utility transformer should be obtained from the utility. The appropriate harmonic solution will vary based on various installation factors. If the level of excess harmonic content is relatively small, the addition of simple passive filters will probably be most cost-effective. If the level of excess harmonic content is high and many drives are involved, installation of an active filter will likely be most appropriate. Finally, if the level of harmonic content is high and only a small number of large drives are involved, then installing drives with active front-ends probably is the best solution. SYED PEERAN: Much depends on the distribution system. Smaller drives, typically less than 50-hp rating, can be six-pulse with passive filters. Larger drives can be the harmonic-free type or the 18-pulse or higher pulse drives. Still larger drives can be the active front-end type drives. One must select the most cost-effective combination. Satisfactory harmonic performance must be verified by a comprehensive harmonic analysis. KEN CARR: If the VFD is installed in an existing facility, there are several options to minimize harmonic distortion, in increasing order of benefit/ cost: An ac line reactor: Specifying a 3% ac line reactor at the input to the drive usually will reduce the total harmonic current distortion (THID) to approximately half of what it would be otherwise. Harmonic filter: Consider a passive, resonant-based harmonic filter that effectively traps the lower current harmonics, such as the fifth, seventh, 11th, etc. Isolation transformer: Installing an isolation transformer to supply the VFD will reduce the primary-side current harmonics sufficiently to meet IEEE 519 at the point of common coupling. Active filter: Installing the right active filter at the input to the drive can reduce the THID to as low as 3%. Installing VFD in a new facility, there are Ken Carr Team Leader Baldor Electric Fort Smith, Ark. Ken Lovorn, PE President, Chief Engineer Lovorn Engineering Pittsburgh Jim Nash Principal Engineer ABB Inc. New Berlin, Wis. Syed Peeran, PE Senior Engineer Camp Dresser & McKee Inc. Cambridge, Mass. 16 Consulting-Specifying Engineer • MARCH 2008 16

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Consulting-Specifying Engineer - March 2008 Contents Viewpoint Letters News M/E Roundtable Grade A HVAC When UPS and Gensets Have Issues Ignite Opportunities To Mentor Fire Protection Engineers Cooling People, Not Buildings Codes & Standards Case Study New Products Equipment Lifecycles Advertiser Index Green Space Pure Power Supplement Edison Institute Honors Recovery Efforts Canada Launches Clean Energy Software Climate Change Drives Increased Investment Critical Power for R&D Heating Calculations for Electrical Duct Bank Submetering for Energy Profiling New Products

Consulting-Specifying Engineer - March 2008

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