Consulting-Specifying Engineer - March 2008 - (Page 35)

• Generator fundamentals • UPS fundamentals • Key electrical concepts. Having established this foundation, we are ready for a step-by-step analysis of the cyclical phenomenon. Generator fundamentals A generator is a motor operating in reverse. But whereas a motor receives ac electrical power and converts it into mechanical energy, a generator converts mechanical energy (e.g., from a diesel engine) into electrical power. The generator accomplishes this by turning a set of coils through a magnetic field, thereby inducing current in those coil. A coil has an impedance consisting primarily of inductance. As this is characteristic of a coil, so too is it characteristic of a motor or generator. A typical generator impedance is 11%. Contrast the generator’s impedance with that of a pad-mounted or substation transformer, which typically have impedances of between 5% and 6%. UPS fundamentals This article’s discussion of UPS is limited to the on-line double-conversion type. UPS work by converting ac power to dc power and then inverting it back to ac power to feed the load (see Figure 1). The primary reason for this “double conversion” is to create a dc link via which batteries are able to provide replacement power in the event of a primary power loss. Because the batteries are always connected into the dc bus, there is no power interruption at the output of the UPS when they are called upon to feed the load. Let’s take a closer look at the UPS front end—the converter. Three-phase or singlephase ac power is provided to the converter. Integrated circuit switches, either silicon-controlled rectifiers (SCR) or insulated-gate bipolar transistors (IGBT) switch A typical generator impedance is 11%. Contrast that with the impedance of a pad-mounted or substation transformer which is typically between 5% and 6%. on and off at speeds of 450 Hz to 4 kHz and in a manner that converts the ac voltage to dc voltage. A byproduct of this switching is current harmonics. The percent of harmonics generated is inversely proportional to switching speed. The phenomenon that is of concern here is most pronounced with the slower SCR-type converters. These originate at the converter and affect everything upstream of it. Consequently, these harmonics are commonly referred to as “reflected harmonics”—reflecting back on and affecting the source of power. The current harmonic orders that are most significant (in percent of harmonic distortion) depend on the particular design of the converter. For converters with the slowest switching speeds, these would be the fifth and seventh harmonics. Static Bypass AC Input DC Link AC Output Converter Batteries Inverter Figure 1: Uninterruptible power supply systems work by converting ac power to dc power and then inverting it back to ac power to feed the load in order to provide replacement power in case of a power loss. Source: Michael Shields Consulting-Specifying Engineer • MARCH 2008 35

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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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