Equipment lock-ups, restarts, and the loss of control programs are common symptoms of power quality problems. Fortunately, although these events are vexing and annoying, they seldom damage equipment directly. This is not to say, however, that they should be ignored. These symptoms indicate the presence of power anomalies that can cause more severe problems now and in the future.
Power anomalies come in many forms and go by many names. Briefly, these disturbances are broken down into six general categories. Although not mentioned as a separate category, please note that the power system itself is a major contributor to existing power quality problems, especially the continuity and quality of the grounding connections.
Transients, the first category, are sub cycle disturbances with a very fast voltage change. They typically have frequencies in the tens to hundreds of kilohertz, some even into megahertz. The voltage excursions range from hundreds to thousands of volts. Transients are also called spikes, impulses, and surges.
The second category, momentary interruptions, refers to a loss of voltage for periods from less than a cycle to several cycles. They do not last long enough to be called outages, but in effect, are.
Voltage sags and swells are the third category. These are variations in the RMS voltage from about one half cycle up to several seconds. They are characterized by changes in the RMS value of the voltage. Sags refer to a reduction in the voltage, while swells refer to an increase.
Should a sag or swell last for a longer period of time than several seconds, it becomes an undervoltage or overvoltage. This is the fourth category, and these disturbances may last indefinitely.
The fifth category of power anomalies is harmonic distortion. If the voltage or current waveshape is not sinusoidal, it is described as distorted. Since our entire power system was developed with sine waves in mind, harmonic distortion challenges many of our basic concepts about how to transmit, distribute, use and maintain electric power.
The last category is electrical noise - sometimes called electromagnetic interference, or EMI. EMI consists of high frequency, low voltage signals coupled onto the power lines. Frequencies may vary from the kilohertz range to the megahertz level, with magnitudes up to 10 or 20 volts.