APTP Articles

Power Quality RMS Voltage Events

This article briefly describes the detection algorithms of four different RMS voltage events listed in the IEC61000-4-30 [1] standard, suitable for a Class A Power Quality analyser (PQA).

The four different power quality parameters in relation to RMS voltage events mentioned in the IEC61000-4-30 are:

  • Voltage Dips (sag)
  • Voltage Swells
  • Voltage interruption
  • Rapid Voltage Change (RVC)

Voltage Dips and Swells

A Voltage Dip or Swell is a temporary reduction or increase of the voltage magnitude at a point in the electrical system below or above a threshold [1]; with the Dip or Swell event ending when the RMS is above or below the threshold plus or minus the hysteresis voltage. The threshold and hysteresis values are based on the declared input voltage (Udin) and where Udin is normally set to the nominal RMS voltage. Hysteresis is used to prevent nuisance triggering. The detection of a Voltage Dip is shown in Figure 1.

An alternative approach for detecting Voltage Dips and Swells is based on the Sliding Reference (USR) [1]. This is a non-fixed value that is based on the 10/12 cycle (10 for 50Hz or 12 for 60Hz power system) RMS voltage measurements after being passed through a low pass filter with a 1-minute time constant. In this case, the threshold levels are a percentage of USR. The Sliding Reference method may be suitable where the system voltage varies, for example as a result of dynamic load changes.

Voltage Interruptions

“A Voltage Interruption begins when the RMS voltages of all channels fall below the voltage interruption threshold, and ends when the RMS voltage on any one channel is equal to, or greater than, the voltage interruption threshold plus the hysteresis” [1]. The detection of a Voltage Interruption is shown in Figure 2.

Rapid Voltage Change (RVC)

An RVC is defined as a “quick transition in RMS voltage occurring between two steady-state conditions, and during which the RMS voltage does not exceed the dip/swell thresholds” [1]. The detection of a RVC is shown in Figure 3. The RVC event detection requires 100/120 samples of URMS(1/2) (value of the RMS voltage measured over 1 cycle, commencing at a fundamental zero crossing, and refreshed each half-cycle) for 50 Hz/60Hz systems [1] (blue trace in Figure 3). 

About the Author

Richard Lam