Speaking the Same Language--Part I of III

As I was preparing to disembark in the mother country after instructing a tutorial on flicker at the Institute of Electrical and Electronic Engineers (IEEE) Power Engineering Society winter meeting, I reflected upon the difficulties with communication, even among English speakers. It gave me the idea of reviewing power quality industry terminology. This multipart article covers the “proper” terminology, which hopefully will help the reader to understand future articles (and some past ones). The standards organizations are making a concerted effort to harmonize power quality terminology and methods. In the United States, these include the IEEE and the National Fire Protection Association (NFPA), authors of National Electrical Code (NEC) and other standards. Overseas, the International Electrotechnical Committee (IEC) writes the standards. For the most part, this unification effort has made good progress (although a “sag” is a “dip,” still). It may not always be the best word used to describe something, but it was the least objectionable one. Transients There are probably more synonyms for “transient” than for any other power quality term. Initially, the power quality community used the word “surge,” which is still used by transient voltage surge suppressor (TVSS) vendors, such as for those used in outlet strips. “Impulse” and “spike” were other terms frequently used. “Transient” was selected as the standard term, as it describes the time duration of the event. A transient event (not a gathering of hobos) is a change in the waveshape that typically lasts less than one quarter of a cycle, or less than 4 milliseconds. There are impulsive transients, such as lightning, which can last less than a microsecond if you are close to the lightning strike. Oscillatory transients are ones where there is a ringing signal that decays quickly after the initial waveshape change. The switching in of a power factor correction capacitor (PF cap) is the classic oscillatory transient, as shown in Figure 1. Since the capacitor is initially uncharged, it steals energy from the power line when first connected, resulting in an initial transient, followed by the oscillation that typically rings at 400 to 1,400 Hz. “Negative” and “positive” are additional characteristics used to describe transients. They refer to whether the transient adds energy to the curve (positive) or subtracts energy from the curve (negative). The initial transient in the PF cap switching waveform is a negative transient, as it subtracts energy from the waveform, even though it is pointing “up” in the graph. There are low-, medium-, and high-frequency transients, which is another way to describe the duration. High-frequency transients are measured in microseconds or even nanoseconds. Low and medium frequencies are the most common, because the wiring impedance attenuates the high frequency. The PF cap switching event would also be called a medium-frequency transient. Some power quality monitors will capture low- and medium-frequency transients, but few monitors have the special circuitry needed to capture high-frequency transients. The effects of transients can be sneaky. The dramatic ones, such as lightning striking a power conductor or facility directly, have obvious results. But the repetitive transients (also called notching as shown in Figure 2) from the overlapping commutation period of silicon-controlled rectifier (SCR)-driven, rectified-input, three-phase power supplies that are typically found in adjustible speed drives (ASD) can chip away at the silicon layers of semiconductor devices, the insulating material in capacitors, and the wire insulation in motors. This continues until what seems like an “innocent” transient causes a catastrophic equipment failure. High-frequency transients can cross-couple between adjacent wires and cause problems on circuits that aren’t supposed to be electrically connected. The PF cap-switching event is often the source of an ASD tripping off line because of what it perceives as a potential supply problem. Next month’s article will be about root mean square (rms) variations, what are typically called sags, swells, interruptions, and long duration over and under voltages. BINGHAM, manager of products and technology for Dranetz-BMI in Edison, N.J., can be reached at (732) 287-3680.

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