Voltage sag

Voltage sags are defined by their magnitude or depth, and duration. A voltage sag happens when the RMS voltage decreases between 10 and 90 percent of nominal voltage for one-half cycle to one minute. Some references define the duration of a sag for a period of 0.5 cycle to a few seconds, and a longer duration of low voltage would be called a sustained sag. The definition of voltage sag can be found in IEEE 1159, 3.1.73 as "A variation of the RMS value of the voltage from nominal voltage for a time greater than 0.5 cycles of the power frequency but less than or equal to 1 minute. Usually further described using a modifier indicating the magnitude of a voltage variation (e.g. sag, swell, or interruption) and possibly a modifier indicating the duration of the variation (e.g., instantaneous, momentary, or temporary)."

Voltage sag in large power system

The main goal of the power system is to provide reliable and high-quality electricity for its customers. One of the main measures of power quality is the voltage magnitude. Therefore, Monitoring the power system to ensure its performance is one of the highest priorities. However, since power systems are usually grids including hundreds of buses, installing measuring instruments at every single busbar of the system is not cost-efficient. In this regard, various approaches have been suggested to estimate the voltage of different buses merely based on the measured voltage on a few buses.