CBEMA curves specifying acceptable voltage sensitivity levels

Inthe recent past, dramatic improvements in productivity have been realized in the high technology sector as well as in the traditional industries. For the electric power supply to these industries, this hasled to a concomitant increase in the number of loads that are sensitive to power quality. Some of the industries that have such large sensitive loads include semiconductor manufacturing, textile mills, paper millsand plastic injection molding.Of course, a number of smaller but equally critical loads such as computers and electronic data processing equipment are also sensitive to power quality.Thetolerance
levels of computer equipment are specified by the Information Technology Industry/Computer and Business Equipment Manufacturers’ Association (ITI/CBEMA) curves. Figure illustrates theCBEMA curves. This figure gives thepercent of nominal voltage versus duration in (60-Hz) cycles. The CBEMA curves represent the boundary of the ac input voltage envelope that can be tolerated (typically) by most
computer-based equipment. The upper curve represents the maximum voltage below which the equipment will continue to function normally. The lower curve is the minimum voltage above which the equipment will continue to function normally.

As seen in Figure, the steady state range of tolerance for computer equipmentis ±10% from the nominal voltage, i.e., the equipment continues to operate normally when sourced by any voltages in this range for an indefinite period of time. Similarly, voltages wells to a magnitude of 120% of the nominal value can be tolerated for about 0.5 s or 30 cycles; voltage sags to 80% of nominal for 10 s, or 600 cycles, can be tolerated. When the supply voltage is outside the boundaries of the susceptibility curves, improvement of the quality of power supplied to sensitive loads is essential to avoid a possible failure in their operation.

G. Venkataramanan, M.S. Illindala, C. Houle, and R.H. Lasseter. “Hardware Development of a  Laboratory-Scale Microgrid Phase 1—Single Inverter in Island Mode Operation”. NREL. November 2002 • NREL/SR-560-32527


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