Voltage Swell

Voltage SwellVoltage Swellis defined by IEEE 1159 as the increase in the RMS voltage level to 110% - 180% of nominal, at the power frequency for durations of cycle to one (1) minute. Itis classified as a short duration voltage variation phenomena, which is one of the general categories ofpower quality problemsmentioned in the second post of the power quality basics series of this site. Voltage swell is basically the opposite ofvoltage sag or dip.

Voltage Swell

The disturbance is also described by IEEE C62.41-1991 as A momentary increase in the power-frequency voltage delivered by the mains, outside of the normal tolerances, with a duration of more than one cycle and less than a few seconds. However, this definition is not preferred by the power quality community.Swellsare subdivided into three categories:

Voltage Swell Categories

Voltage swells are characterized by their RMS magnitude and duration. The gravity of the PQ problem during a fault condition is a function of the system impedance (i.e.relation of the zero-sequence impedance to the positive-sequence impedance of the system), location of the fault and thecircuit grounding configuration. As an example, on an ungrounded system, the line-to-ground voltages on the unfaulted phases can go as high as 1.73 pu during a SLG fault. On the contrary, on a grounded system close to the substation, there will be no voltage rise on the unfaulted phases because the substation transformer is usually connected delta-wye, providing a low impedance zero-sequence path for the fault current.

Voltage Swell Terminology Usage

The term "momentary overvoltage" is used as a synonym for the term swell.According to IEEE 1159-1995, voltage swell magnitude is to be described by its remaining voltage, in this case, always greater than 1.0 pu. For example, a swell to 150% meansthat the line voltage is amplified to 150% of the normal value.

Causes and Effects of Voltage SwellsVoltage swells are usually associated with system fault conditions - just like voltage sags but are much less common.This is particularly true for ungrounded or floating delta systems, where the sudden change in ground reference result in a voltage rise on the ungrounded phases.In the case of a voltage swell due to a single line-to-ground (SLG) fault on the system, the result is a temporary voltage rise on the unfaulted phases, which last for the duration of the fault. This is shown in the figure below:

Instantaneous Voltage Swell Due to SLG fault

Voltage swells can also be caused by the de-energization of a very large load.The abrupt interruption of current can generate a large voltage, per the formula: V = L di/dt, where L is the inductance of the line and di/dt is the change in current flow. Moreover, theenergization of a large capacitor bankcan also cause a voltage swell, though it more often causes anoscillatory transient.

Although the effects of a sag are more noticeable, the effects of a voltage swell are often more destructive. It may cause breakdown of components on the power supplies of the equipment, though the effect may be a gradual, accumulative effect. It can cause control problems andhardware failure in the equipment, due to overheatingthat could eventually result to shutdown. Also, electronics and other sensitive equipment are prone to damage due to voltage swell.

Capacitor switching

Capacitor switching is one of the most common switching events on utility systems. Capacitors are used to provide reactive power (in units of vars) to correct the power factor, which reduces losses and supports the voltage on the system. They are a very economical and generally trouble-free means of accomplishing these goals.Whenever capacitor bank is switched, voltage dip is followed by a rise in voltage by 1.5 2 pu resisted by the system impedance. In case the capacitive reactance matches with system impedance, the resonance occur which decays very slow and causes severe damage to the equipment associated.