Consulting-Specifying Engineer - March 2008 - (Page 22)

M/E Roundtable When the system is fed by the standby generator, harmonic distortion would be greater because of the larger impedance of the generator as compared to that of the utility transformer. Therefore, the harmonic filter is a little less effective with the generator operation. Irrespective of the utility operation or generator operation, it is best to locate the filter close to the harmonic source—i.e., at the VFD. LOVORN: We believe that the filter should be located as close as possible to the VFD, even in the same enclosure. This prevents voltage harmonics from leaving the immediate vicinity of the VFD, thus not impacting the generator operation. Most generators are now permanent magnet generators that derive their voltage regulation from the magneto on the generator shaft, which has no electrical connection to the generator output, thereby lessening the impact of the VFD on the generator output. CARR: Placing the filter as close to the VFD as possible also minimizes the wire length between the two. This wire should be shielded VFD-type cable to prevent electromagnetic interference (EMI). Failure to adequately contain high-frequency EMI at the source allows it to be radiated and picked up by Ask the experts: motors, drives, and harmonics CSE gives its readers and Web visitors the opportunity to pose questions directly to the panelists a t www .csemag.com/asktheexperts. Below is a question specifically about harmonics. “ We have 750 kVA generators in our press. Last week this generator malfunctioned. Meanwhile we arranged a generator of 1,200 kVA. Press operates perfectly on a 1,200 kVA generator and power utility company. We measured harmonics at load side. When load is connected to generator (750 kVA), 8.5% total harmonic distortion (THD) observed. Magnitude of third harmonic is 8.2%. When load is connected to main distribution system, THD became 2.3% with third harmonic 0.108% and fifth harmonic 2.077%. My questions: Why does the press operate perfectly at 1,200 kVA genset and main distribution system? Why is the third harmonic more when load is connected to 750 kVA generator? How should I mitigate the harmonics?” —Atif Haider pritchard brown PROTECTING TECHNOLOGY AND THE WORLD. Pritchard Brown, LLC the Industry Leader in Technology, Service, Quality and State-of-the-Art Design for Enclosures and System Integration. • Outstanding reputation for quality and service • Total packaging and testing services by trained professionals • Strategically located to serve markets in all of North and South America • U.L. classified enclosures and U.L. listed base tanks, extensive options and total flexibility • Professional sales and engineering staff • High performance, maintenancefree materials • Building shelters and trust for more than 50 years Building shelters—and trust—for more than 50 years. Baltimore, MD 800.231.2258 Fax 410.483.5695 info@pritchardbrown.com www.pritchardbrown.com KEN LOVORN: The harmonics are higher when connected to generators because the utility has a higher available fault duty. Based on the information available, it is doubtful that the failure of the 750 kVA generator was caused by the harmonics. If the 750 kVA generator is more than 10 years old, then it is probably sensitive to harmonically rich loads. If the 1,200 kW generator is new it could be a permanent magnetic generator (PMG) as noted. But even if it’s not a PMG, by nearly doubling the size of the generator the harmonics would have much less effect than on the 750 kVA. As far as mitigating the harmonics, the metered levels of harmonics are not of magnitude that should cause problems with either the generator or any other equipment. Most electronic equipment is designed to accommodate THDs up to 10%. If harmonic rich loads are added, you should install harmonic filters sized for each of the new loads. SYED PEERAN: Because the 750 kVA generator has greater impedance, higher THD is expected. The harmonics may have caused malfunction of the press controls. Most of the voltage distortion (8.5% THD) is the due to the third harmonic (8.2%) when 750 kVA is online. With the utility power, most of the distortion (2.3%) is due to the fifth harmonic (2.03%). This suggests that the third harmonic is created by the generator. If the generator stator winding is not 2/3 pitch winding, the generator creates considerable third harmonic voltage. You can check this by measuring the harmonics of the generator voltage on no load. There is no easy way of mitigating the third harmonic. The generator can be rewound or a zigzag grounding transformer at the terminals of the generator can bypass the third harmonic currents from the load. Input #215 at csemag.com/quickResponse 22 Consulting-Specifying Engineer • MARCH 2008 http://www.csemag.com/asktheexperts http://www.csemag.com/asktheexperts http://www.pritchardbrown.com http://www.pritchardbrown.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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