Long-Duration Voltage Variations

Long-duration voltage variations encompass root-mean-square (rms) deviations at power frequencies for longer than 1 min. ANSI C84.1 specifies the steady-state voltage tolerances expected on a power system. A volt age variation is considered to be long duration when the ANSI limits are exceeded for greater than 1 min.

Long-duration variations can be either overvoltage’s or under voltages. Overvoltage’s and under voltages generally are not the result of system faults, but are caused by load variations on the system and system switching operations. Such variations are typically displayed as plots of rms voltage versus time.

Over voltage:

  • An overvoltage is an increase in the rms ac voltage greater than 110 percent at the power frequency for duration longer than 1 min.
  • Over voltage is usually the result of load switching (e.g., switching off a large load or energizing a capacitor bank). The over voltages result because either the system is too weak for the desired voltage regulation or voltage controls are inadequate.
  • Incorrect tap settings on transformers can also result in system over voltages.

Under voltage:

  • An under voltage is a decrease in the rms ac voltage to less than 90 percent at the power frequency for a duration longer than 1 min.
  • Under voltages are the result of switching events that are the opposite of the events that cause over voltages.
  • A load switching on or a capacitor bank switching off can cause an under voltage until voltage regulation equipment on the system can bring the voltage back to within tolerances.
  • Overloaded circuits can result in under voltages also.

Sustained Interruptions:

  • When the supply voltage has been zero for a period of time in excess of 1 min, the long-duration voltage variation is considered a sustained interruption.
  • Voltage interruptions longer than 1 min are often permanent and require human intervention to repair the system for restoration.  The term sustained interruption refers to specific power system phenomena and, in general, has no relation to the usage of the term outage.
  • Outage, as defined in IEEE Standard 100, does not refer to a specific phenomenon, but rather to the state of a component in a system that has failed to function.
  • In power quality monitoring, the term interruption has no relation to reliability or service continuity.
  • Thus interruption is only referred to the absence of voltage for long periods.

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