Consulting-Specifying Engineer - March 2008 - (Page 20)

M/E Roundtable measure the current with an averaging ammeter and then measure it with a true root mean square meter. If the two numbers are relatively close, there are no significant harmonics present. If the two numbers are more than 15% or 20% apart, then there is a potential for current harmonics. The same technique may be used to determine whether voltage harmonics are present in a system. NASH: Sometimes the source of excessive harmonic content is obvious. For example, there may be only one large VFD present. Often, however, VFD loads are distributed throughout a facility and it is the combined effect of many non-linear loads that result in an unacceptable harmonic situation. In such cases it is difficult without a formal analysis to determine how to best address the problem. Passive and active filter vendors often will agree to provide a free harmonic analysis with recommended solutions. The only other approach that will achieve reliable results is to formally contract with an electrical consultant who specializes in harmonic analyses. Though more costly, this approach has the advantage that such consultants have broad exposure to many different solution techniques and are more likely than an individual vendor to provide a recommendation that optimizes both the technical and cost factors involved. CSE: Are active and passive filters necessary in some cases? What are some of the recent technological developments in the area of harmonic filters? PEERAN: Active filters have not had much success in the industry. Passive filters, however, find use in several situations, particularly in existing systems when new VFDs and nonlinear loads are added. The passive filter is simply a reactor-capacitor combination tuned to a particular harmonic frequency usually slightly lower than the expected harmonic frequency. The filter also offers power factor correction because it simply acts like a capacitor at power frequency. To prevent over-correction of power factor at low loads, it is better to use an individual passive filter for each VFD instead of a large centralized filter. LOVORN: The requirement for filters has changed over the past few years. Previously, if you had a VFD, which was normally a six-pulse drive, you had to have some type of filter to prevent Input #213 at csemag.com/quickResponse 20 Consulting-Specifying Engineer • MARCH 2008 http://www.uecorp.com http://www.uecorp.com http://csemag.com/quickResponse

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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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