Voltage Sags and Equipment Sensitivity: a Practical Investigation

Abstract

In recent years, interruption of manufacturing processes due to power quality degradation has become a major focal point for many power utilities. The most prominent power quality issue plaguing utility customers is voltage sag or dip. It is a sudden decrease in voltage amplitude followed by a return to its initial level after a short time. The use of automation and energy efficient equipment with electronic control would greatly improve industrial production. However, since these new devices are more sensitive to supply voltage deviations, characteristics of the power system that were previously ignored are now becoming a nuisance. To evaluate the technical aspects and economic issues related to voltage sags, the process and equipment immunity level has to be known. However, there is little available information related to equipment sensitivity due to voltage sags. Studies assessing sensitivity of voltage sags on customer loads are divided into practical and theoretical approaches. The practical approaches investigate the effects of voltage sag by monitoring and conducting experiments on customers’ sensitive loads, as well as by performing pertinent surveys (Bollen, 2000). Equipment sensitivity to voltage sag can also be considered and presented in the form of power acceptability curves. These curves are plots of bus voltage deviation versus time duration which separate the bus voltage deviation time duration plane into two regions namely, “acceptable” and “unacceptable” regions. The lower limb of the power acceptability curve relates to voltage sags and momentary outages. The latest power acceptability standards are the SEMI F47 issued by the Semiconductor Equipment and Materials International (SEMI) in the year 2000 (Djokic et al., 2005) and ITIC curve of the Information Technology Industry Council (ITIC) (Kyei et al., 2002). The SEMI F47 specification simply states that semiconductor processing, metrology, and automated test equipment must be designed and built to confirm to the voltage sag ride-through capability as per the defined curve. Equipment must continue to operate without interruption during conditions identified in the area above the defined acceptable region (Institute of Electrical and Electronics Engineers Inc, 2005). As an effort to understand the voltage immunity level of sensitive equipment, some works have been reported in the past. The categories of sensitive equipment commonly evaluated for voltage sags are personal computers (PCs) that control the on line and off line processes, 2