Undervoltage, Overvoltage, Sags, Swells
The second most simple type occurs when voltage at the load drops below a minimum rated voltage or climbs above a maximum rated voltage for some period of time. Depending on how long these conditions last, they may be referred to as undervoltage or overvoltage and sags or swells.
- An undervoltage occurs when the rms voltage drops below 90% of the nominal rms voltage and stays at that level for more than one minute. The term “brownout” often refers to an intentional or unintentional drop in voltage in an electrical power supply system.
- An overvoltage is an event where the rms voltage rises above 110% of the nominal rms voltage and stays there for more than one minute.
Fig1. Undervoltage and overvoltage occur when voltage at the load drops below a minimum rated voltage or climbs above a maximum rated voltage for longer than a minute.
- Sags occur when the rms voltage decreases between 10% and 90% for a duration of a half-cycle to one minute. In a 60Hz power system, a complete sine wave lasts approximately 16 milliseconds, a half cycle is approximately 8 milliseconds.
- Swells are defined as an increase in the rms voltage to over 110% for a duration of a half-cycle to one minute.
Fig2. Sags and Swells occur on the power system when voltage drops below or exceeds nominal voltage for a short duration.
Reductions in voltage and sags usually occur when the rms current to the load increases significantly. There are 3 categories of sags and swells, depending on their duration:
- 0.5 cycles to 30 cycles: Instantaneous
- 30 cycles to 3 seconds: Momentary
- 3 seconds to 1 minute: Temporary
- 1 minute+: Sustained Undervoltage or Overvoltage