Mind Your P's & Q's

One of the expressions I remember hearing during my childhood was “Mind your P’s and Q’s.” When I would ask where that expression came from, it usually resulted in another popular expression, such as, “Don’t talk back to your elders.” With the benefit of the Internet, the answers are there, but they are no clearer than they were back then. While most people will agree the expression has to do with paying attention or being careful to behave properly, maintaining proper etiquette and so on, no one seems to know where the expression really originated.

You might ask how this relates to an article on maintaining power quality. The most obvious answer is the letters, as power quality is often abbreviated with the two-letter acronym PQ. It could be the often-quoted expression in the industry: “No one really cares about power quality. What they care about is cold beer and warm pizza.” P’s and Q’s could have originated from “pints and quarts,” which could refer to the serving of beer to unruly patrons in English pubs (though it is served at room temperature by choice across the pond). This leads to another play on words (or letters)—PQ could also stand for process quality, which is what the aforementioned saying is really all about.

Maintaining the quality of the process at a facility is what keeps productivity high and the revenue stream flowing smoothly. It is the charter of the facility maintenance person and the electrical shop staff, and power quality is just one of the factors in this mission.

Maintenance, schmaintenance

Many aspects of maintenance have begun to change from regularly scheduled programs to condition-based (CBM) or reliability-centered maintenance (RCM). Rather than using time as the basis for carrying out a task, either the actual condition of the equipment—or the statistical data on what piece of equipment is most likely to fail and when—is used to carry out or delay maintenance activity. The NFPA 70B-2006 Recommended Practice on Electrical Equipment Maintenance added an entire new chapter on RCM and tables of extensive availability indices on many types of equipment.

In the PQ realm, all three methods of time, condition and reliability are intertwined in a well-designed PQ maintenance program.

First, monitoring to determine the present status or condition needs to be conducted periodically (if not continuously) over a business cycle, which is how long it takes for the process in the facility to repeat itself. In a 24/7 data center, one day is probably very similar to the next from the perspective of the electrical loads in the facility. However, the quality of the electrical supply will not necessarily be the same, as the other consumers in the area are likely to have a different load profile on a weekend and a weekday. Also, the regulation of the voltage levels in the summer heat will likely be different than during the winter cold.

Monitoring during these different times when the operations are running smoothly provides a baseline to compare for trends in the future or when quality of the process takes a turn for the worse. It is like going to the doctor’s office to get an electrocardiogram when you feel fine, and your heart is operating normally. Should you experience any cardiac problem in the future, it will be easier for the doctor to determine what the cause is. In your facility, having a 7.4 percent voltage total harmonic distortion might be normal and not disruptive to your process, whereas at another facility where it is usually less than 3 percent, a sudden jump to more than 7 percent would cause a drop in the quality of the product or even interrupt the process. This difference in the before and after data is a valuable clue toward resolving the situation.

Indications of a problem

High levels of voltage harmonic distortion almost always indicate high current harmonics, which can cause premature aging of electromagnetic devices such as motors and transformers. A motor operating in such an environment will most likely run hotter, and heat is not the friend of a motor. Harmonic currents flowing to the ground through the bearings can cause fluting, requiring extensive maintenance at more frequent intervals. As shown in Figure 1, the repetitive negative transients from the commutation period of the rectifiers in the power supplies of the drives can cause premature failure of the outer windings of a motor as well, which could be seen in different current waveforms per winding.

Harmonics aren’t the only misbehaviors of the process. In most facilities, voltage sags (decreases in the voltage amplitude lasting from half cycle to 30 seconds) are the most common type of power quality disturbances. The level of the remaining voltage during the sags may not cause a problem with the facility the last time it occurred. But there is no guarantee that they will not get more severe in depth and duration and more frequent to a point that it will eventually cause a decrease in productivity. The first step is again to monitor for the baseline data and compare against the susceptibility of the equipment in the facility.

Determining the source or direction of the problem is the next step. If the monitoring point is at the point of common coupling between the electric utility and the facility (often the watt-hour meter), then any phenomena with a source upstream or back toward the source will result in a phone call to the local electric utility customer service department or, in some areas, the power quality group. Being armed with the data showing the problem will help the utility resolve the problem quickly as well.

In Figures 2A and 2B, the sags are the result of tree branch contact with the distribution wires. The key to determining that it was the utility’s problem is evident when the current does not increase significantly when the randomly occurring sag begins, which would indicate a load downstream in the facility caused the sag. Additional information pointing toward the cause is seen in the voltage waveform, where the decrease in voltage occurs at the peak of the waveform. This often indicates the potential has to reach a high voltage level before the arc-over or breakdown occurs.

Some power quality problems are solved with simple tools, such as a screwdriver or a wrench. Loose connections can cause overheating conditions, so periodic tightening with the appropriate tool can prevent waveforms, as shown in Figure 3. Each time a large truck rumbled past the facility, the lights would blink, and occasionally, some equipment would trip off line. Remember to retorque to the manufacturer’s specifications, and work de-energized whenever possible with proper personal protective equipment and safety precautions.

So, the next time you are enjoying your beer cold (or warm, if you prefer) and your pizza warm (or cold, if you prefer), you can thank those fellow members of the trade who are minding the P’s and Q’s.     EC

BINGHAM, a contributing editor for power quality, can be reached at 732.287.3680.




About the Author

Richard P. Bingham

Power Quality Columnist
Richard P. Bingham, a contributing editor for power quality, can be reached at 732.287.3680.

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