The costly effects of power quality problems are most clearly seen when equipment or products suffer damage. Not only is there downtime for the affected equipment, there are also repair and replacement costs. In addition, loss of product means expensive rework, loss of productivity, and higher overhead costs.
While most people think that power quality problems only affect sensitive electronic components, other aspects, in particular a new disturbance known as harmonic distortion, also affect normal loads such as motors, and transformers. The entire system, from transmission, to distribution, to utilization, is now subject to damage and destruction from various power quality phenomena.
Power anomalies come in many forms and go by many names. Briefly, these disturbances are broken down into six 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 continuity, and the quality of grounding connections.
The first category is Transients. These are subcycle disturbances with a very fast voltage change. They typically have frequencies of tens to hundreds of kilohertz, with 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 ranging from less than a cycle to several cycles. Though they do not last long enough to be called outages, in reality that's what they are.
Voltage Sags And Swells are the third category. These are variations in the RMS voltage from about one half cycle to several seconds. They are characterized by changes in the RMS voltage value. Sags refer to a reduction in the voltage, while swells deal with a voltage increase.
Should a sag or swell last for longer than several seconds, it is referred to as an Undervoltage Or Overvoltage. This is the fourth category. These disturbances may last indefinitely.
The fifth category is Harmonic Distortion. If the voltage or current waveshape is not sinusoidal, it is considered 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 to the megahertz range, and magnitudes may be up to 10 or 20 volts.