Four power system parameters—frequency, amplitude, waveform, and symmetry—can serve as frames of reference to classify the voltage and power disturbances according to their impact on the quality of the normal sine wave of system voltage.

A brief discussion is given below of the need for evaluation of their impact on sensitive loads.

a) Frequency variations are rare on utility-connected systems, but engine-generator based distribution systems can experience frequency variations due to load variations and equipment malfunctions.

b) Amplitude variations can occur in several forms; their description is inextricably associated with their duration. They range from extremely brief duration to steady state conditions, making the description and definition difficult, even controversial at times.

Their causes and effects need close examination to understand the mechanisms and to define an appropriate solution.

c) Waveform variations occur when nonlinear loads draw a current that is not sinusoidal. One could also describe an amplitude variation as momentary waveform variation, but the intended meaning of the term is a steady variation of the waveform, or lasting at least over several cycles.

This type of disturbance may be described as harmonic distortion because it is easy to analyze as the superposition of harmonics to the nominal frequency of the power system.

d) Dissymmetry, also called unbalance, occurs when unequal single-phase loads are connected to a three-phase system and cause a loss of symmetry. This type of disturbance primarily concerns rotating machines and three-phase rectifiers and, as such, is not receiving broad attention. It is important, however, for machine designers and users.

The percentage by which one-phase voltage differs from the average of all three is the usual description of this type of disturbance. A detailed definition of various measures of voltage and power quality by magnitude, duration, and spectral content is provided in IEEE Std 1159-1995.

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