Consulting-Specifying Engineer - November 2007 - (Page 22)

One can use a tie breaker in parallel switchgear designs. As shown in Figure 2, the tie breaker in the open position will allow for the “A” To further clarify the performance of a generator in a fault scenario, the engineer must become familiar with the generator decrement curve. side generator to come up to speed and connect to the bus at the same time as the a “B” side generator. This effectively doubles the priority 1 load capabilities of the system because each side can be on in 10 s. Once they are on, the generators can synchronize and the tie breaker closes, creating a single bus, or the tie can remain open if design requires it. Typically the emergency distribution system is not the primary contributor of fault current, but when multiple generators are paralleled together, the fault can exceed that of the utility service. It is important that design engineers evaluate the fault both from a normal service and an emergency service standpoint. The example shown in Figure 2 is taken from a project where the emergency side generator fault (including the estimated fault contribution from the two future generators) is approximately 90,000 amps, significantly more than what is available at the utility service entrance. To further clarify the performance of a generator in a fault scenario the engineer must become familiar with the generator decrement curve. The decrement curve represents the fault output of a generator and is plotted as a function of various generator characteristics including subtransient reactance–Xd” (one to five cycles), transient reactance–Xd’ (five to 200 cycles), synchronous reactance Xd (above 200 cycles), subtransient time constant (Td”), transient time constant (Td’), armature short circuit time constant (Ta). The reactances are typically per unit values with the generator kVA and full load amps as the base. These values should be provided by the generator manufacturer, along with the a plot of the generator decrement curve. This document also should include the full load amperage of the generator, and the instantaneous fault output. Now that we have these values it is easy to determine what the instantaneous fault of the generator is. The equation for the instantaneous generator fault is Isc = Ea / Xd”, where Ea is the internal generated voltage identified a 1.0/ unit, simplifying the formula to Isc = 1 / Xd”. We should we evaluate instantaneous available fault current within our calculations because Generator Connection Cabinet Portable Generator Connection Made Safe and Easy! Don’t make backup power an afterthought The Berthold Electric Generator Connection Cabinet provides a quick and convenient station to safely connect a portable generator to a facility’s electrical power distribution system. User voltage– up to 600V 400A-3200A sizes Outdoor installation Save time & labor when connecting a rental generator. Share one generator among multiple facilities. Great for load banks, too. Applications: Telecommunications, hospitals, schools, industrial, commercial, and more. Berthold Electric Co. www.GenConCab.com 800-657-6650 Install a Berthold Electric Generator Connection Cabinet! The didn’t-plan-ahead way: Exposed energized parts and cables on the floor are safety and security hazards. Connection time: 8+ hours. www.GenConCab.com Input #213 at csemag.com/quickResponse The Berthold Electric way: All cables outdoors, no disruption to equipment or facility. Connection time: <1 hour. 22 Consulting-Specifying Engineer • NOVEMBER 2007 http://www.GenConCab.com http://www.GenConCab.com http://www.GenConCab.com http://csemag.com/quickResponse

Table of Contents Feed for the Digital Edition of Consulting-Specifying Engineer - November 2007

Consulting-Specifying Engineer Contents Editor's Viewpoint Letters In The News M/E Roundtable Emergency Power for Healthcare 2007 Products of the Year Industrial-strength Lighting Hazardous HVAC Codes and Standards Management Report Jobs/Classifieds Specifier's Notebook On-Peak Performance Contents Best Battery Selector in Engine Starting Applications NEC 708: Practical Impact on Backup Power Systems Protecting Power: Specifying Outdoor Generator Enclosures Investing in Backup Power Systems Coes, Consultants, Manufacturers, and Standby Power Systems

Consulting-Specifying Engineer - November 2007

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