Published In July 2001
Three more categories are identified in the IEEE Standard 1159 table of power quality (PQ) phenomena: voltage imbalance, waveform distortion, and power frequency variations. In the United States, voltage imbalance is typically the worst-case difference between the voltage of each phase as compared to the average of the three. International standards calculate imbalance using the ratio of negative-to-positive sequencing components. Even a small imbalance can cause motor overheating. A 3 percent imbalance results in 10 percent derating of motors, as per the NEMA MG1/ANSI C112 standard. Imbalance can be caused by unequal loading, unequal source impedance, unequal source voltage, or an unbalanced fault. Due to the interconnection of many generating sources on a relatively low-source impedance network, power frequency variations aren’t that common in North America, although variations can occur during faults on the distribution or transmission systems. However, frequency variations are a different issue when operating off a backup generator, where a sudden increase in load current will most likely result in a decrease in the power frequency. Equipment that derives its time clock from the power line may have problems when such happens. Waveform variations take on many different shapes. The IEEE Standard 1159 breaks distortion into six categories. We covered the “ideal” waveshape in a previous article on sine waves, along with another article on harmonics, which are frequency components that are integer multiples of the power frequency. Notching was covered in the first part of this series, as repetitive transients, typically due to the commutation period where two power switching devices, such as silicon-controlled rectifiers (SCRs) in the front ends of adjustible speed drives (ASDs) are on briefly at the same time. DC offset in AC systems is generally a bad thing, as it pushes transformers and other electromagnetic devices towards saturation; hence, reduced operating range. Interharmonics are frequencies that aren’t integer multiples of the power frequency, as harmonics are. In a 60Hz system, the 120, 180, 240, 300, ... frequency components are the harmonics. But some systems have frequencies that fall between these, such as 185Hz. These are often due to equipment that runs asynchronously to the power line frequency, such as cycloconverters or electric arc-welding equipment. Some of the effects are similar to harmonics, such as increased losses in transformers and motors. Interharmonics can be a source of another phenomena, called voltage fluctuation. A 185Hz component will modulate with the third harmonic (180Hz ) and produce a 5Hz component. This 5Hz frequency falls in the range of subharmonics that can result in light flicker if the voltage fluctuation level is large enough. At 9Hz, it takes only 1/4 percent of a change for most people to perceive the flicker in a 60W incandescent light bulb. It also can cause interference on displays, as can noise. Noise tends to cause errors in communication signals, although this can result in process interruptions if the noise levels are large enough and last long enough. There are also a number of words that we no longer use in the industry, as their definitions varied from person to person, which made communications difficult. These include spike, surge, outage, and glitch. Having a common language improves the chances of quickly and effectively explaining what someone observed to another person, so that a solution is found quickly and the process interruptions are reduced. BINGHAM, manager of products and technology for Dranetz-BMI in Edison, N.J., can be reached at (732) 287-3680.