Another Tool for the Toolbox

While the screwdriver may still be one of the best tools for troubleshooting power- quality-related problems, another tool is very useful in determining the equipment’s susceptibility to the quality of the electrical supply. In the grand scheme of things, what really matters is whether the equipment can operate properly with the electricity that is supplied to it. IEEE Standard 1346, “Electrical Power System Compatibility with Electronic Process Equipment,” is such a tool. The document provides a standard methodology for the technical and economic analysis of process equipment compatibility with the electric power system. It doesn’t try to set hard limits for the different power quality phenomena, as IEEE 519 does for harmonics. In the real world, too many factors are involved to say, “if the quality is above value X, then the process will run smoothly.” Different pieces of equipment have different susceptibilities. Different building wiring results in different disturbance levels from the same event. As with most things in life, it is the weakest link in the chain that can break the process flow. IEEE 1346 shows how to correlate performance data of the equipment to the voltage, especially as it relates to voltage sags. Voltage sags are the most common power quality phenomena in most facilities, and result in process problems ranging from decreased quality of the product to a complete shutdown with a lengthy restart time. The key to using this method is in two charts that are developed and then overlaid on top of each other. The first step is to monitor a site for typically a month or longer. It can be useful to conduct the survey over different seasonal periods if applicable in your area, since lightning and ice storms can contribute significantly to the data. A histogram detailing the number of occurrences of voltage sags, along with their duration, needs to be generated next. Some power-quality-analysis software programs provide this feature; otherwise, you will have to manually sort through the data and generate such a table. The contour lines can be in whatever increments that are desired. Dots are placed on the graph corresponding with the data collected and connected with contour lines, like a topographical map that civil engineers use when designing roads. The next step is to obtain data from the equipment manufacturer on the equipment’s susceptibility in similar terms. For example, the relay will drop out if there is a sag below 75 percent of nominal lasting two cycles or longer, whereas the programmable logic controller (PLC) will drop out or malfunction with a sag below 47 percent of nominal for 37 cycles or longer. This clearly shows that the PLC is far more tolerant of sags than the relay (contrary to some people’s intuition). Note that the photo-eye, often used as an emergency safety shut-off in stamping presses or cutting machines, will drop out with just a one-cycle sag to 87 percent of nominal. Hence, the PLC and other equipment has no problem running during such an event, yet the safety stop would malfunction, shutting down the process. The final step is to overlay the two graphs. This shows the probability of the specific equipment having a problem in this particular electrical environment. The relay is likely to experience problems 25 to 30 times per year, whereas the PLC is only likely to see sags that would affect it zero to five times per year. The photo-eye may be shutting down the process more than 70 times a year. If the cost of the downtime is calculated, now there is a method to assign the economic impact of such, and justification for taking steps to prevent such needless productivity and profit losses. It is as easy as 1-2-3, with IEEE 1346 in your toolbox. BINGHAM, manager of products and technology for Dranetz-BMI in Edison, N.J., can be reached at (732) 287-3680.

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