A swell is a temporary increase in RMS voltage, typically to between 110 and 180 percent of nominal for half a cycle to a minute. The classic cause is a single-line-to-earth fault, which lifts the voltage on the two unfaulted phases, especially on resistance-earthed or unearthed systems. Sudden load rejection and capacitor bank energisation also produce swells, which stress surge protective devices and electronic power supplies.
When B phase went to ground the other two swelled to about 1.3 per unit, and that's what killed the surge protectors in the MCC.
Also heard as
- voltage swell
Do not mix it up with
- Sag: A sag is the opposite event, a temporary drop in RMS voltage.
Related terms
Sag
Brief drop in RMS supply voltage, usually to between 10 and 90 percent of nominal, lasting up to about a minute.
Earth fault
Fault in which a live conductor makes unintended contact with earth or with earthed metalwork.
Per-unit system
Shorthand that states voltages, currents and impedances relative to chosen base values, so 1.0 means nominal.