Voltage dips and flicker

POWER SYSTEMS of electrical utilities are nominally constant-speed (mechanical) and constant-voltage (electric) types of system. Therefore, the degree to which these two quantities are maintained constant at all times determines the quality of service supplied by the utility. Changes in mechanical torques and in electric currents on the system cause deviations from normal speed and voltage. Adjustments in input to prime movers, in excitation of synchronous machines, and in ratio of transformation are required to restore balance on the system with normal speed and voltage. Large capital investments are made by the utilities for equipment designed for automatic control of speed and voltage. All such equipment has an inherent time-delay characteristic. First, a deviation from normal value must take place before the need for correction is detected by the regulating device. Then, the corrective action must be damped or delayed enough to prevent oscillations. The magnitude of the controlled quantity is held within a band between a minimum and a maximum value for relatively slow changes of loading.