Consulting-Specifying Engineer - February 2009 - (Page 15)

M/E/P Roundtable On the other hand, multipulse techniques, 12- and 18-pulse, are becoming popular. The new techniques do not interfere with the existing power system either from higher conducted electromagnetic interfaces, when active techniques are used, or from possible resonance, when capacitor-based filters are used. Peeran: Methods of mitigating harmonic problems after they are identified include installation of active or passive harmonic filters and line reactors, and replacement of older 6-pulse VFDs with modern 12and 18-pulse VFDs. Walcott: For existing harmonics problems, various harmonic mitigation techniques may be employed. For triplen harmonics, a simple zigzag transformer will reduce the neutral current to zero even if the load is imbalanced. Triplen harmonics may be reduced for three-phase lines only if the harmonic currents are balanced. Active harmonic and power factor correction filters will eliminate harmonics by cancellation of harmonic currents. These filters use active electronic drive technology to inject cancellation current to overcome harmonic current distortion and poor power factor. Passive filters are available that guarantee 5% THID VFD compliance to IEEE 519 standards at a much lower price. Typical three-phase reactors reduce three-phase harmonics to about 40% THID. CSE: What design solutions would you recommend for new systems to prevent VFD-related harmonic problems? Lane: There are a number of procedures you can use to reduce harmonic distortion in the electrical distribution system. These methods include diverting harmonics to an alternate path, increasing effective source impedance, and harmonic cancellation. With the diversion of harmonics, capacitors and reactors are used in combination to structure a “tank circuit.” The tank circuit provides for a lower impedance path for harmonic frequencies than the serving power source. A series connected line reactor at the source side of a 6-pulse rectifier on a VFD will reduce source impedance. A 12- or 18-pulse rectifier can be used in a VFD to significantly reduce the level of harmonic distortion. A 12-pulse rectifier uses two 6-pulse bridges that are fed from two different sources. These sources are phase shifted by 30 degrees. Fifth and seventh harmonics will be cancelled with the use VFD-Induced Bearing Currents Kill Motors! Save energy with a VFD, Save your motor with SGR BEARING PROTECTION RING Capable of saving 30% or more in energy costs, VFDs can help you create “green” systems, but without an AEGIS SGR™ to protect motor bearings, you will not have a sustainable truly green solution. Safely channels harmful currents away from bearings to ground Easy field or OEM installation — in just minutes Maintenance-free, contaminationproof, lasts for life of motor Standard sizes for any motor To download a free technical paper on bearing protection technologies, visit our website: Split mounting kit simpliﬁes ﬁeld installation on NEMA or IEC motors including those with shaft shoulders, slingers, or other end bell protrusions 1-866-738-1857 | sales @est-aegis.com www.est-aegis.com/cse2 Input #210 at csemag m/quickResponse Consulting-Specifying Engineer • FEBRUARY 2009 15 http://www.est-aegis.com/ http://www.est-aegis.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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