Consulting-Specifying Engineer - February 2009 - (Page 16)

M/E/P Roundtable of this method. The 18-pulse system uses three sets of 6-pulse bridges that are fed from three different sources of power. Each of the bridges is phase-shifted by 20 degrees apart. Fifth, seventh, 11th, and 13th harmonics will be cancelled with the use of this system. Peeran: At the design stage, the engineer should evaluate potential harmonic problems if the installation includes a large number of VFDs. After the equipment is selected, a preliminary harmonic analysis would bring out the severity of the problem. Design modifications may be required to address the problems. Individual and common harmonic filters at the LV switchboard, detuning power factor correcting capacitors, specifying 18-pulse drives, and harmonic-free drives are the options that need to be evaluated during the design of the system. Swamy: The following points regarding harmonic mitigation should always be kept in mind: • Capacitor-based filters should be avoided, especially if standby generators are involved. • Harmonic currents are not always a problem. Specifying harmonic filters on VFDs that are rated at 1 hp or 3 hp is counterproductive because they are more expensive, occupy more space, and consume precious natural resources to manufacture. • If the content of nonlinear load to linear load in a given electrical environment is greater than 50%, then it may be necessary to think about harmonic mitigation techniques. The only exception to this rule of thumb would be critical nonlinear loads supplied by standby generators. • It should be kept in mind that the motivating factor behind the reason for reducing harmonics is system efficiency. If external filters are used in such a way that the overall current drawn is higher than the current drawn by a load with a power factor of 0.95, then the filter is useless. • Basic technique of reducing harmonics goes a long way and should be implemented first. This would include adding a 3% input inductor and/or adding a dc link inductor. Walcott: For new installations, it is best to apply harmonic filtering of each VFD. This is already a statutory requirement whenever the utility is governmentowned. The trend is toward national or at least state legislation, mandating compliance with IEEE 519. rn If your Wa PVC-coated Galvanized Rigid Conduit doesn’t carry this ETL-Verification label you are risking the high cost and catastrophic potential of product failure. Sound alarmist? It isn’t – and here’s why. Fact One: All brands of PVC-coated galvanized rigid conduit manufactured in the U.S. meet exactly the same safety standards established in UL 6. However g! in Fact Two: Not all brands are authorized to carry the ETL PVC-001 label predicting product service life. Why? Because not all brands of PVC-coated galvanized rigid conduit have passed the stringent requirements of ASTM D870 and ASTM D2247, the two accepted tests designed to predict service life of a coating under the two most common conditions that affect the adhesive bonds of protective coatings: heat and humidity. If product performance and service life are important to you look for the ETL-Verification label or add ETL-Verification to your spec! If your PVC-coated galvanized rigid conduit doesn’t carry an ETL-Verification label you risk premature product failure and the consequences of costs related to downtime as well as safety, legal and environmental issues. For additional details about the importance of specifying ETL PVC-001, logon to: www.plastibond.com or contact us • Phone: 903.843.5591 • E-mail: sbickerstaff@robroy.com Input #211 at csemag.com/quickResponse 16 Consulting-Specifying Engineer • FEBRUARY 2009 http://www.plastibond.com http://www.plastibond.com http://www.csemag.com/quickResponse

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Consulting-Specifying Engineer - February 2009 Contents Viewpoint News M/E Roundtable Codes & Standards Electrical Modernization: A Tale of Two Hospitals Retrofitting Office Lighting Controls Case Study New Products Equipment Lifecycles Advertiser Index Green Space

Consulting-Specifying Engineer - February 2009

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