Electric Power, Quality of

a group of characteristics of electric power that determine the operating conditions of power users (electric motors, heating units, illumination equipment, electronic devices, and so on).

The indicators used to assess the quality of electric power vary according to the type of system. For single-phase AC systems, they include frequency and voltage deviation, frequency and voltage fluctuation, and nonsinusoidal wave form of the voltage curve. For three-phase AC systems they include the same indicators used for single-phase systems together with asymmetry of the phase voltages at the power frequency (the phase voltages are not equal to one another, and their phase shifts differ from 120°). For DC systems, they include voltage deviation, voltage fluctuation, and the ripple factor (the ratio of the amplitude of the alternating component to the rectified voltage).

Frequency deviation is the difference between the nominal and the actual value of the power frequency averaged over 10 min; under normal conditions a frequency deviation of ±0.1 hertz (Hz) is permissible, and a temporary deviation of ±0.2 Hz is sometimes allowed. Frequency fluctuation is the difference between the largest and the smallest values of the power frequency at a rate of change not less than 0.2 Hz/sec; under normal conditions such fluctuations should be no greater than 0.2 Hz above the permissible deviations cited above. Voltage deviation is the difference between the nominal and the actual (for a given system) voltage value registered during a comparatively slow change in operating conditions when the rate of voltage change is less than 1 percent per sec. Voltage fluctuation is the difference between the highest and the lowest effective voltage value of the system registered during a fairly rapid change in operating conditions when the rate of change is not less than 1 percent per sec. A nonsinusoidal voltage wave form (nonconformity with the wave form of a harmonic oscillation) is permitted at the terminals of a power user for a long period of time if the effective value of all the higher harmonics does not exceed 5 percent of the effective voltage value for the power frequency.

The quality of electric power may vary with the time of day, weather and climatic conditions, changes in the load on the power system, the occurrence of emergency conditions in the system, and so on. A decline in quality may cause a substantial change in the operating conditions for power users and, consequently, a reduction in the productivity of actuating mechanisms, a decline in the quality of a product, a shortening of the service life of electrical equipment, and a higher probability of breakdowns. The quality indicators of electric power can be maintained within prescribed limits under actual conditions most effectively by means of automatic voltage control and automatic frequency control.