A swell is outlined as a rise to between 1.1 and 1.8 Pu in rms current or voltage at the power frequency for duration's 0.5 cycle to one minute.
As with sags, swells are sometimes related to system fault conditions, however they're not as communal as voltage sags. A technique that a swell will arise is from the short-term voltage increase on the unfaulted phases throughout an SLG fault. Figure above shows a voltage swell caused by an SLG (Substation Line to Ground) fault. Swells will additionally be caused by switch off an oversized load or energizing an over-sized capacitance bank.
Swells are characterized by their magnitude (rms value) and interval. The severity of a voltage swell throughout a fault condition may be a function of the fault location, system impedance, and grounding. On an un-grounded system, with an infinite zero-sequence impedance, the line-to-ground voltages on the un-grounded phases are going to be 1.73 Pu during an SLG (Substation Line to Ground) fault condition. Near the station on a grounded system, there'll be very little or no voltage rise on the unfaulted phases as a result of the substation transformer is sometimes connected delta-wye, providing a low-impedance zero-sequence path for the fault current. Faults at totally different points on four-wire, foregrounded feeders can have varied degrees of voltage swells on the unfaulted phases.
The term short over-voltage is employed by several writers as an alternative expression for the word swell.