An Ounce of Prevention

When I went through “Firefighter 101” training, the instructors emphasized that the primary purpose of a fire department is to prevent fires, not to put them out. The old adage that, “An ounce of prevention is worth a pound of cure,” applies to the power quality world as well. Recording disturbances that shut down processes or disrupt the quality of the product is valuable, but preventing them is even more so. The same power quality monitor used to do the former can provide the information necessary for the latter. It just takes a little attention to the details. Many people are too busy in today’s Internet-speed economy to step back and look at things before they become a crisis. However, one should examine the data in order to determine if things are getting worse, if marginal operating conditions are present, and if there could be an accident just waiting to happen.” RMS variation data may be viewed quickly by using a 3-D magnitude-duration (mag-dur) chart. The mag-dur chart plots the magnitude or amplitude of the voltage versus the duration of the sag or swell versus how many times that value or range of values occurs. Figure 1 shows an example of such over a one-week period. Note that most of the sags have a magnitude of 80 to 89 percent of nomimal and last one to five cycles. While these sags are often not enough to cause a process interruption, this information can be a valuable clue that something is about to happen. Closer examination of the information from a timeline in Figure 2 shows that the events don’t seem to occur at any particular time or pattern. Eventually, an event occurred that was severe enough to cause computers that weren’t on a UPS to reset themselves. Closer examination found that the majority of the events themselves were sags of one to two cycles long that only involved one phase and were self-clearing, as shown in Figures 3 and 4. Eventually, a sag occurred that involved all three phases, and caused the problem with the computers. By looking at the load current at the time of the sags, you see evidence that the problem was caused upstream, back towards the source of the electricity, as the current decreased when the voltage decreased. Further correlation to other conditions determined that it was a very windy day. A likely scenario, given the numerous single-phase sags leading up the disruptive event, was that some conductors on the poles that were swinging in the wind brushed up against a high-impedance ground, such as a tree branch. Eventually, two conductors were involved at the same time, and then all three, as shown in Figure 5. Armed with this data, the facility manager could have asked the local electric utility to dispatch a crew to find the source of the ground before it became a problem for that facility and others fed by that circuit. Remember Smokey the Bear’s motto, “Only you can prevent [power quality problems].” Bingham, manager of products and technology for Dranetz-BMI in Edison, N.J., can be reached at (732) 287-3680.

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