During this past week, as my business partner was getting ready to teach a middle school science class, and I was preparing for my third career retirement, we were discussing the importance of starting out the students with sets of rules and definitions of terms before delving into the actual science. Everyone needs to be on the same page, and not just on their Chromebooks. This is also true in the realm of power quality.

When I was first developing a set of PowerPoint slides for a half-day PQ seminar in the early 1990s, the first chapter was on definitions and terms. Some attendees had limited PQ knowledge and knew even less about electrical engineering, and there were even those for whom “sine wave” was a new term. I wasn’t attempting to turn them into PQ geeks in four hours, but I wanted them to be able to comprehend more of the material covered in the presentations and not have that glassy-eyed look after the first hour.

Part of the teaching process is to say it once, then say it again. With that idea in mind, this month we will do a quick review of the most common words and phrases used when discussing power quality, starting with voltage and current. 

Definitions to the rescue!

Voltage is the potential to do work and current is the force that does the work. Both parameters can either be AC or DC. Without getting into the famous Edison and Westinghouse debate, most electrical distribution and facility infrastructures are AC, though there are PQ disturbances and issues on both types of systems. AC voltage and current has been generated, transmitted, distributed and consumed as sine waveforms for decades. 

The impedance of electrical loads are combinations of inductance, capacitance or resistance, as is the electrical infrastructure (wiring, transformers, contacts, etc.). In its simplest form, voltage equals current multiplied by the resistance. Until the latter part of the 20th century, loads were linear, meaning that the sine wave shape was preserved when the current was drawn by the load, though it may be shifted in phase based on the amount of inductance or capacitance.

These systems did have power quality problems, though the dominant loads were electric motors, which were less susceptible to PQ phenomena than the majority of today’s loads. 

Electronic loads take the AC voltage and convert it into DC voltage, sometimes converting it back into AC voltage at a different frequency than the one originally provided. The frequency is defined as how many times the sine wave completes a full cycle each second. Typically, that fundamental power frequency is 50, 60 or 400 hertz (Hz). The operators of electric generators powering the electrical grid (transmission and distribution systems) try to keep that frequency as close as possible to those rates, or bad things can happen, such as sustained voltage interruptions (blackouts).

To quantify the AC sine wave effective energy compared to a DC value, a mathematical process called root mean squared (rms) is used. There is also the peak value that the sine wave reaches in the positive and negative half-cycles. The potential of 120V AC rms is equivalent to 120V DC rms, though the AC voltage has values as high as 170V peak. When this rms voltage decreases below a threshold—typically 90% of the nominal—it’s classified as a voltage sag, which is a type of rms variation. When it goes below a much lower threshold—typically 10% of nominal—it is a voltage interruption. Equipment starts to malfunction below the sag threshold, and almost everything is inoperable at the voltage interruption level. 

The flip side of the rms variation is a voltage swell, where the Vrms increases above 110% of nominal. Swells above 130% can cause serious damage to equipment.

Changes in the peak value of the voltage waveform are usually classified as voltage transients. They are much shorter in duration than rms variations but can do significant damage, such as a lightning strike that couples into the electrical system. Transients can be positive or negative, which means they add energy to the waveform or subtract it. A continuous series of negative transients is called notching, often associated with the commutation period in three-phase power converters, such as those found in adjustable speed drives (ASD). This can also be destructive. In fact, I used to call it the Woody Woodpecker phenomena, as they can eat away at wiring, capacitors, semiconductors and other components.

Those linear loads in the past have been replaced with the nonlinear loads, whose primary issue is distortion in the sine wave, especially the current waveform. Harmonics are the term given to the signals that result from this nonlinear operation and are multiples of the fundamental power frequency. A typical single-phase electronic load, such as the “brick” that powers a laptop or printer, has 3rd, 5th, 7th and 9th harmonics in decreasing percentages. A three-phase power converter used in ASDs will typically have 5th, 7th, 11th, 13th, 17th and 19th harmonics, though newer units can have much higher ones. Harmonics are generally wasted energy, causing motors and transformers, and even the neutral conductor on three-phase wye circuits, to overheat.

These 25 are among the most common PQ terms, which hopefully will make future articles easier to comprehend. A good resource can be found in Chapter 3 of the IEEE Std 1159-2019 Recommended Practice for Monitoring Electric Power Quality. Your vocabulary quiz will be tomorrow!

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