Power Quality Disturbances

Power quality disturbances can be categorized into the following seven basic groups.

Outage

An outage is a complete loss of power. A temporary outage lasts anywhere from 3 seconds to 1 minute. A long term outage would last longer than 1 minute.

Possible Causes

  • Accidents, acts of nature, etc. which require the proper operation of utility equipment (like fuses)
  • Internal short circuit resulting in the proper operation of a customer's breakers and fuses

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Momentary Outage

A momentary outage is a very short loss of power lasting from 1/120th of a second to 3 seconds. This is often seen as a "blink" in your lights.

Possible Causes

  • Accidents, acts of nature, etc. which require the proper automatic operation of utility protective equipment

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Undervoltage / Overvoltage

An undervoltage or overvoltage is any long-term change (lasting more than a minute) below or above normal voltage.

Possible Causes

  • Overloaded wiring or equipment
  • Poor voltage control possibly due to large load swings or improper transformer settings
  • Voltage drop due to undersized wiring and faulty or poor electrical connections

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Sag / Swell

A sag or swell is any short-term (lasting less than one minute) decrease (sag) or increase (swell) in voltage.

Possible Causes

  • Major equipment start-up or shutdown
  • Short circuits
  • Improper electrical connections
  • Sudden load reduction

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Surge / Transient

A surge or transient is a sudden change in voltage up to several thousand volts lasting a few microseconds (a microsecond is one millionth of a second).

Possible Causes

  • Lightning
  • Proper operation of utility fuses, reclosers, and breakers
  • Turning on or off large equipment
  • Operation of welding equipment

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Noise

Noise is an unwanted electrical signal of high frequency that alters the normal voltage pattern (sine wave). An example of the result of noise would be the distorted picture you may get on a computer screen when a microwave is turned on.

Possible Causes

  • Electronic equipment
  • Radar transmitters
  • Radio and television broadcasts
  • Operation of welding equipment
  • Heaters, thermostats, and loose wiring

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Harmonic Distortion

Harmonic distortion is the alteration of the normal voltage pattern (sine wave) due to "non-linear" loads (like electronic equipment). All electronic devices (those having transistors for example) draw electricity differently than non-electronic equipment and distort the normal voltage pattern.

Possible Causes

  • Operation of non-linear loads such as fluorescent ballasts and computer power supplies
  • Operation of battery chargers