What Is Power Quality?

22:20 / Posted by tech data /

There can be completely different definitions for power quality, depend-ing on one’s frame of reference. For example, a utility may define powerquality as reliability and show statistics demonstrating that its systemis 99.98 percent reliable. Criteria established by regulatory agenciesare usually in this vein. Amanufacturer of load equipment may definepower quality as those characteristics of the power supply that enablethe equipment to work properly. These characteristics can be very dif-ferent for different criteria.Power quality is ultimately a consumer-driven issue, and the enduser’s point of reference takes precedence. Therefore, the following def-inition of a power quality problem is used in this book:Any power problem manifested in voltage, current, or frequency devia-tions that results in failure or misoperation of customer equipment.There are many misunderstandings regarding the causes of powerquality problems. The charts in Fig. 1.1 show the results of one surveyconducted by the Georgia Power Company in which both utility per-sonnel and customers were polled about what causes power qualityproblems. While surveys of other market sectors might indicate differ-ent splits between the categories, these charts clearly illustrate onecommon theme that arises repeatedly in such surveys: The utility’s andcustomer’s perspectives are often much different. While both tend toblame about two-thirds of the events on natural phenomena (e.g., light-ning), customers, much more frequently than utility personnel, thinkthat the utility is at fault.



When there is a power problem with a piece of equipment, end usersmay be quick to complain to the utility of an “outage” or “glitch” that hascaused the problem. However, the utility records may indicate no abnor-mal events on the feed to the customer. We recently investigated a casewhere the end-use equipment was knocked off line 30 times in 9 months,but there were only five operations on the utility substation breaker. Itmust be realized that there are many events resulting in end-user prob-lems that never show up in the utility statistics. One example is capaci-tor switching, which is quite common and normal on the utility system,but can cause transient overvoltages that disrupt manufacturingmachinery. Another example is a momentary fault elsewhere in the sys-tem that causes the voltage to sag briefly at the location of the customerin question. This might cause an adjustable-speed drive or a distributedgenerator to trip off, but the utility will have no indication that anythingwas amiss on the feeder unless it has a power quality monitor installed.In addition to real power quality problems, there are also perceivedpower quality problems that may actually be related to hardware, soft-

ware, or control system malfunctions. Electronic components candegrade over time due to repeated transient voltages and eventuallyfail due to a relatively low magnitude event. Thus, it is sometimes dif-ficult to associate a failure with a specific cause. It is becoming morecommon that designers of control software for microprocessor-basedequipment have an incomplete knowledge of how power systems oper-ate and do not anticipate all types of malfunction events. Thus, a devicecan misbehave because of a deficiency in the embedded software. Thisis particularly common with early versions of new computer-controlled

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