Electrical Bus

Aswellis defined as an increase to between 1.1 and 1.8 pu in rms voltageor current at the power frequency for durations from 0.5 cycle to 1 min.As with sags, swells are usually associated with system fault condi-tions, but they are not as common as voltage sags. One way that a swellcan occur is from the temporary voltage rise on the unfaulted phasesduring an SLG fault. Figure illustrates a voltage swell caused by anSLG fault. Swells can also be caused by switching off a large load orenergizing a large capacitor bank.Swells are characterized by their magnitude (rms value) and dura-tion. The severity of a voltage swell during a fault condition is a func-tion of the fault location, system impedance, and grounding. On anungrounded system, with an infinite zero-sequence impedance, theline-to-ground voltages on the ungrounded phases will be 1.73 pu dur-ing an SLG fault condition. Close to the substation on a grounded sys-tem, there will be little or no voltage rise on the unfaulted phasesbecause the substation transformer is usually connected delta-wye,providing a low-impedance zero-sequence path for the fault current.Faults at different points along four-wire, multigrounded feeders willhave varying degrees of voltage swells on the unfaulted phases. A15percent swell, like that shown in Fig. 2.8, is common on U.S. utilityfeeders.The term momentary overvoltageis used by many writers as a syn-onym for the term swell.