Consulting-Specifying Engineer - March 2008 - (Page 38)

distortion which is immediately seen by the UPS monitoring circuitry. 5. The UPS deems VTHD to be at unacceptable levels prompting it to automatically switch back to battery power. 6. Once back on battery power, the volt- The occurrences of loads being dumped as a result of this phenomenon have decreased over the years due to improvement in UPS front-end designs, lower generator reactances, and a general awareness that non-linear loading on generators needs to be limited. The occurrences of loads being dumped as a result of harmonics has decreased due to improvements in UPS front-end design. age from the generator looks good again, and returns to Step 3. And so, an unending cycle ensues. Of course, it’s not completely unending, as the batteries eventually run out and the UPS goes to static bypass (assuming it has one). Either way, the critical load is left without protection and is likely to be “dumped.” None of this should result in your lowering your guard against the possibility of this harmonic issue occurring in a system. Here are some things to consider in any design involving a UPS downstream of a generator: 1. One manufacturer, in its engine and generator sizing publication, recommends that SCR loads be kept to no more than 66% of their generators prime power ratings. This is doubtless a safe threshold designed to keep the designer out of trouble regardless of the UPS used. 2. If circumstances require that you exceed the above threshold, consider a UPS with an IGBT-designed converter in lieu of an SCR type. The faster switching speeds of IGBTs afford a design with significantly lower reflected current harmonics. One manufacturer whose UPS front ends are predominantly of the IGBT variety boasts an ability to match the kVA rating of its UPS to the kW rating of a generator feeding them for many of its models. Be aware of the potential problem and work closely with your UPS and generator vendors to assure compatibility under a worst case loading scenario. Shields has nearly 20 years of electrical engineering experience on residential, commercial and, municipal projects. Get your blog on Hear what industry leaders, CSE editors and your peers think about critical issues facing the engineering industry on csemag.com. Visit the NEW csemag.com. The ﬁrst destination for consulting engineers. www.csemag.com cse0705jh_classHlf.indd 1 5/2/2007 4:08:43 PM 38 Consulting-Specifying Engineer • MARCH 2008 http://csemag.com http://csemag.com http://csemag.com http://www.csemag.com

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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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