Undervoltage

Undervoltage is a condition in which the voltage supplied to electrical equipment or systems falls below the level required for normal operation. It is a type of power quality issue and is often distinguished from overvoltage by the direction of the deviation. The assessment of undervoltage depends on the nominal voltage of the system and the defined voltage limits for that system.

Causes of undervoltage include increased load on the network during peak periods, long distribution feeders with significant impedance, transformer tap settings, interruptions in generation, faults, or rapid switching events. In grids with distributed generation, voltage can drop if generation does not match load or if line impedance is high.

Effects of undervoltage can range from nuisance to hazardous for equipment. Electrical devices may operate erratically, lights can flicker or dim, motors may stall or run inefficiently, and sensitive electronics can reset or malfunction. Protective devices and relays may trigger to prevent damage when voltage remains too low for a specified duration.

Measurement and standards are used to identify undervoltage. Voltage is typically measured as the rms value. Utilities and equipment standards define acceptable voltage ranges around the nominal voltage, often aiming to keep voltage within a band such as ±5% to ±10%. When voltage stays below the lower limit for a defined period, it is classified as undervoltage or a voltage sag.

Mitigation can be implemented at different levels. Consumers may use voltage regulators, uninterruptible power supplies, or appropriately rated equipment. Utilities can adjust transformer taps, manage capacitor banks, or modify network topology. Industrial facilities may employ motor soft starters or variable-frequency drives to dampen voltage changes.

See also: power quality, brownout, voltage sag, overvoltage, transient voltage suppression.