Tuesday, April 19, 2011

Voltage Swell is defined by IEEE 1159 as the increase in the RMS voltage level to 110% - 180% of nominal, at the power frequency for durations of ½ cycle to one (1) minute. It is classified as a short duration voltage variation phenomena, which is one of the general categories of power quality problems mentioned in the second post of the power quality basics series of this site. Voltage swell is basically the opposite of voltage sag or dip.
Voltage Swell
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.

Swells are subdivided into three categories:

Voltage Swell 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 the circuit 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%” means that the line voltage is amplified to 150% of the normal value.

Causes and Effects of Voltage Swells

Voltage 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
Instantaneous Voltage Swell Due to SLG fault
Voltage swells can also be caused by the deenergization 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, the energization of a large capacitor bank can also cause a voltage swell, though it more often causes an oscillatory 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 and hardware failure in the equipment, due to overheating that could eventually result to shutdown. Also, electronics and other sensitive equipment are prone to damage due to voltage swell.


Magnitude: 1.1 to 1.8 pu 
Duration: ½ cycle to 1 minute
Source: Utility or facility
Symptoms: Malfunction or Shutdown
Occurrence: Very low
Protection: Voltage isolation
Mitigating Devices: Constant Voltage Transformer (CVT), Dynamic Voltage Restorer (DVR)

Bingham, R. (1998). SAGs and SWELLs. New Jersey: Dranetz-BMI
Dugan, R., McGranaghan, M., Santoso, S., and Beaty, H.W. (2004). Electrical Power Systems Quality (2nd ed.). New York: McGraw-Hill.
IEEE 1159-1995. Recommended Practice For Monitoring Electric Power Quality. New York: IEEE, Inc.
Leng, O.S. (2001). Simulating Power Quality Problems
Utility Systems Technologies, Inc. (2009). Power Quality Basics


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I am a Professional Electrical Engineer with a Masters Degree in Business Administration. My interest is in Power Quality, Diagnostic Testing and Protective Relaying. I have been working in an electric distribution utility for more than a decade. I handle PQ studies, power system analysis, diagnostic testing, protective relaying and capital budgeting for company projects.