Consulting-Specifying Engineer - January 2009 - (Page 40) plied by the backup emergency generator system via automatic transfer switches. For facilities with critical loads, the transformers should be sized such that each transformer peak loading is no more than 50% of its maximum rating to provide backup for one another. This is important if one transformer fails or needs maintenance. Using the parallel source redundant configuration in the design of an emergency power system has several advantages, including increased reliability, flexibility, and ease of uninterruptible maintenance. The emergency loads are supplied by multiple power supply sources, multiple emergency generating units, and two separate supply circuits, which are connected to a multi-sourced switchgear bus. When properly designed and implemented, the loss of any single power supply source does not impact the continuous operation of the emergency loads. Multiple generating units provide an uninterruptible maintenance capability. A generator unit can fail to start or be out of service for maintenance or repair, while other generator units, synchronized on the same bus, provide the needed backup power should an outage occur. Main service switchgear and the emergency switchgear can be scheduled for routine maintenance without disruption to the entire facility loads, and maintaining operation of the critical emergency loads. For improved reliability, the automatic transfer switches serving critical low-voltage emergency loads should have a maintenance bypass. Because of these advantages, the parallel source redundant configuration has become one of the best choices for meeting today’s need for a reliable emergency power system. However, the reliability of the emergency power system also depends on a clear understanding of control strategies for system emergency response and recovery, system sequence of operation, and a successful implementation of the control strategies. The designer and the emergency power system operator need a clear understanding of how the emergency power system and the critical loads will respond during an emergency, upon return to normal conditions, and during maintenance and testing. Questions designers and operators might ask include: • How should the system react upon the loss of the normal power supply source? • What actions are automatic and what actions are manual, requiring operator interaction? • What is the required emergency system response time? Momentary power interruption may be acceptable for some emergency loads, but not others. • What is the minimum emergency power capacity requirement to serve the emergency loads, including critical motor starting? • How are the emergency loads segmented and connected to the power system? • Are load shedding and sequencing of emergency load pickup required to maintain continuous system operation without overloading the emergency generators? Input #220 at csemag.com/quickResponse 40 Consulting-Specifying Engineer • JANUARY 2009 http://www.emon.com/cse109.asp http://www.emon.com/cse109.asp http://www.csemag.com/quickResponse
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