Power Quality Evaluation

April 8, 2002
Designing a Safe and Reliable Electrical System - Part 4
A complete power quality evaluation is another tool that can be used to maintain a safe and reliable electrical distribution system. Power quality concerns increase as the system design and processes become more complicated. In an effort to reduce energy costs, facilities managers can manage energy usage through load shedding and peak shaving options. However, because of these energy management options, many facilities managers are being faced with more power quality issues than before.The increasing number of harmonic producing loads can dramatically affect the quality of power on the system. The operation of switch-mode power supplies found in laboratory equipment, operating room monitors, and computers are prime examples of products and activities that can create the harmonic currents that flow throughout the 120/208V circuits in a power system. Other sources of harmonics include variable speed drives (VSD), uninterruptible power supplies (UPS systems), and emergency/ standby generators.A complete power quality evaluation will generally involve one or more of the following power systems studies: a harmonic analysis study; grounding analysis; voltage flicker; and/or transient voltage surge suppression.A harmonic analysis study will accurately determine the sources and magnitudes of harmonic currents and voltages that are present in the electrical power system. Up-to-date measurements are essential in verifying harmonic generation from all significant harmonic sources. Harmonic measurements demonstrate the effect of system resonance caused by power system configuration and characteristics. Power system engineers use the recorded measurement data in the analytical modeling of the system. When internal expansion or external utility changes affect a facility’s power system, comprehensive power systems studies will ensure greater protection and reliability of the entire power system. Following the study, it is imperative to annually maintain, test, and update all equipment within the one-line diagram.Jill Revolt is Product Line Manager for Power System Engineering and Automation at Eaton Corp. for its Cutler-Hammer Engineering Systems and Services business unit (www.ch.cutler-hammer.com), Pittsburgh. Additional information was contributed by Bill Vilcheck, Power System   Engineer at Eaton|Cutler-Hammer.Attribution: Information taken from a case study entitled “Power Systems Outage in Critical Care” by Daniel P. O’Reilly, Power Systems Engineer, Eaton|Cutler-Hammer.

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