What Is Power Quality?
There can be completely different definitions for power quality, depending on one’s frame of reference. For example, a utility may define power quality as reliability and show statistics demonstrating that its system
is 99.98 percent reliable. Criteria established by regulatory agencies are usually in this vein. Amanufacturer of load equipment may define power quality as those characteristics of the power supply that enable
the equipment to work properly. These characteristics can be very different for different criteria.
Power quality is ultimately a consumer-driven issue, and the end user’s point of reference takes precedence.
Any power problem manifested in voltage, current, or frequency deviations that results in failure or misoperation of customer equipment. There are many misunderstandings regarding the causes of power
quality problems. The charts in Fig. 1.1 show the results of one survey conducted by the Georgia Power Company in which both utility personnel and customers were polled about what causes power quality
problems. While surveys of other market sectors might indicate different splits between the categories, these charts clearly illustrate one common theme that arises repeatedly in such surveys: The utility’s and
customer’s perspectives are often much different. While both tend to blame about two-thirds of the events on natural phenomena (e.g., lightning), customers, much more frequently than utility personnel, think
that the utility is at fault.
When there is a power problem with a piece of equipment, end users may be quick to complain to the utility of an “outage” or “glitch” that has caused the problem. However, the utility records may indicate no abnor-
mal events on the feed to the customer. We recently investigated a case where the end-use equipment was knocked off line 30 times in 9 months, but there were only five operations on the utility substation breaker. It
must be realized that there are many events resulting in end-user problems that never show up in the utility statistics. One example is capacitor switching, which is quite common and normal on the utility system,
but can cause transient overvoltages that disrupt manufacturing machinery. Another example is a momentary fault elsewhere in the system that causes the voltage to sag briefly at the location of the customer
in question. This might cause an adjustable-speed drive or a distributed generator to trip off, but the utility will have no indication that anything was amiss on the feeder unless it has a power quality monitor installed.
In addition to real power quality problems, there are also perceived power quality problems that may actually be related to hardware.