Only you can prevent ‘PQ fires’:
The primary function of a fire department is to prevent and extinguish fires in its district, so if you don’t have any fires, why do you need a fire department? Because with flammable fuels, electricity and people in the mix, fires are inevitable. So fire departments may train continuously, but they also train the public on how to prevent the fires, reducing the chances of a fire by educating the people in the fire equation. This is the key to prevention, and it’s important because once a fire crew has to respond to a fire, it is too late—the damage has already been done. The question then becomes how to limit the damage. And the same thing applies in the power quality realm.
Many facilities only think of power quality when their processes go down. Dollars are lost, equipment and/or product is damaged, productivity comes to a halt. Instead of calling the fire department, they call in either the staff electricians or outside electrical contractors to find out what happened. Needless to say, it is even more difficult than a fire investigator’s job. In the latter case, there is usually visual evidence that points to the origin of the fire by following some basic rules, such as fire burns upwards and the fact that it can’t start without an ignition source. Doing a power quality investigation after-the-fact isn’t as straightforward, since the evidence, the “misbehaving electrons,” are no longer around. There might not even be any physical damage, just the unplanned reboot of computers and PLCs.
However, just like with fires, being proactive is the best way to prevention. The NJATC offers training classes on power quality analysis (X053) and has developed an informative study guide on the topic. Once you learn about the typical power quality phenomena, what causes them and what problems they can create in a facility, you can pass that knowledge onto the facility managers to prevent the problems, rather than trying to do the post-event analysis. A proactive monitoring program either monitors the quality of supply at the service entrance and critical loads on a continuous basis, or at least periodically to look for trends that indicate potential problems before they cause losses.
The table gives a brief overview on the typical phenomena and their possible effects. Remember it is not just the quality of the supply that matters, but the susceptibility of the equipment to the various types of phenomena. If the load is a resistive element such as an electric heater, harmonic distortion can be higher than the table indicates and still not be a problem. Or, a photocell that will misoperate during a sag to 95 percent of nominal for ¼ cycle will trip off sooner than the table would suggest.
Unfortunately, getting a susceptibility data sheet from an equipment vendor isn’t always easy; though, there has been significant effort by NEMA with adjustable speed drives along with semiconductor industry with its SEMI F47 curve. In some instances, they will be rated with reference to more obscure classes within standards, such as “Installation Category 3, Pollution Degree 2.” Instead, having a baseline of data when things are all working well with periodic monitoring to look for changes or trends as well as correlating them with equipment misoperation will be the key to a proactive power quality program and preventing future “PQ fires.” EC
BINGHAM, a contributing editor for power quality, can be reached at 732.287.3680.