Consulting-Specifying Engineer - August 2008 - (Page 32) M/E Roundtable from many market segments are looking to some power quality products. SPEARS: Proper grounding and handling of the neutral circuit continues to be a challenge in many installations, and this is a matter of vendors recognizing the need to provide clear, concise, communications regarding proper methods for installing and connecting power protection infrastructure. Harmonics (created either by the UPS or the connected load) often are an unexpected problem, because the impedance of the generator or utility source varies for different sites. In general, this issue is becoming much less common with the advent of improved UPS rectifier designs, and the use of power factor (PF) corrected IT equipment. Speaking of PF-corrected IT equipment, we are seeing increased incidences of “leading” or capacitive power factor loads, especially in newer data centers where the processing loads are relatively light. This often is a problem for UPS systems, as they can become unstable when faced with a leading load. Only a few vendors have successfully addressed this issue without requiring de-rating of their equipment. Generators, however, may respond to a leading load by shutting down unexpectedly. It has become more important to ask, early in the game: “Is there a chance that my data center will exhibit a leading power factor?” The answer may significantly affect the UPS and generator purchasing decision, and this is not a pleasant issue to deal with after equipment is already installed and operating. WALTER: In the developed world, the issues are generally transients, sags swells, and outages. Frequency variations from utilities are exceedingly rare, and when they do occur, they are generally an immediate precursor to a major power outage. The situation where frequency stability may be a problem is when the primary power source is an on-site cogeneration plant. In addition, modern IT and communications equipment is very tolerant of frequency variations and relatively tolerant of voltage variations compared to industrial control systems. CSE: Looking into the future, what t y p e s of fu t u r e a d va n c e m e nt s could improve power reliability for these facilities? RENER: I think that reliability issues are being joined by another connected issue—energy efficiency. Rising energy costs are forcing a new perspective on mission critical facilities. An unexpected benefit of implementing energy saving measures can be an increase in reliability. One example might be careful air flow modeling. Delivering air only where it is most needed affects reliability but also can help minimize oversizing air systems and save energy. Properly sized and loaded electrical and mechanical systems are more efficient and provide better performance. Alternative power sources such as geothermal also are being explored. Another significant challenge is providing cooling to rack-mounted equipment with very high power densities (i.e., more than 4 kW per rack). WALTER: One improvement might be distribution of IT power at 400 V/230 V to eliminate the two-step conversion from medium voltage to 480 V and from 480 to 120 V/208 V. More and more infrastructure power equipment is becoming “world class” that operates at 716 F at 400 V in some cases, or 415 V at 50 or 60 Hz as well as 480 V at 60 Hz. Converting directly from medium voltage to 400 V/230 V reduces the amount of equipment in the critical path while simultaneously reducing losses in the IT equipment power path and increasing the capacity of feeders going to the racks at a given current rating. CSE : What do engineers need to know about Article 708 : Critical Operations Power Systems, in the 2008 National Electrical Code and how it applies to mission critical power? COTTULI: The big take-away for the engineer on any new code of this sort is to understand the current position of the authority having jurisdiction where the facility is installed. Although the NEC has issued and approved this standard, the local authority having jurisdiction may ignore this code section or adopt it. In fact, only a few local authorities have even reviewed and issued a position, some have adopted the new section, and some have rejected it stating existing code and local interpretation cover the items to and adequate level. So call your local building inspector office. RENER: This section was added to address homeland security related issues. Practices such as wiring separation and protection that have been required for systems like life safety or fire protection now are applied to power systems that affect public safety and national security. There are other interesting requirements in this new section that have been considered good engineering practice in other types of mission critical facilities, like multiple levels of ground fault protection, surge protection, commissioning, and acceptance testing. All in all, it is an excellent new addition to the code and hopefully a source of ideas for other types of mission critical facilities, which are not covered by this new article. CSE: What role does commissioning and acceptance testing play in today’s mission critical facilities / power systems? KOSIK: The activities that take place during the life of the project that have to do with equipment/system performance testing and verification can make or break a project. It is the weakest link metaphor at work again. If the equipment is delivered to the site without proper factory acceptance testing, it is 32 Consulting-Specifying Engineer • AUGUST 2008
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