IEEE 1564:2014 pdf free download – IEEE Guide for Voltage Sag Indices
5.3.1 Definitions
A voltage sag or voltage swell should be characterized by its duration and its retained voltage.The durationis the time that the rms voltage stays below the threshold. The retained voltage is the lowest rms voltageduring the event. Instead of retained voltage, the depth may be used, which is the difference between theretained voltage and a reference or declared voltage.
To determine the duration of the sag， a threshold setting is needed. This threshold can be defined inmultiple ways, such as a percentage of the nominal voltage, a percentage of the long-term average voltageat the location, or a percentage of the rms voltage just prior to the beginning of the event.
For measurements in low-voltage and medium-voltage networks, the declared or nominal voltage should beused. This value is the most relevant one for the performance of end-use equipment. In low-voltagenetworks, the nominal voltage should be used in all cases. In medium voltage networks, a different declaredvoltage may be used to incorporate the turns ratio of the step-down transformers.
Different threshold values may be used for obtaining the starting and ending instants of the sag.The endingthreshold could be higher than the starting threshold by an amount that is referred to as the“hysteresisvoltage.”In that case, the voltage sag would begin when VrTmaq.uy fell below the sag threshold, and end whenVnes(1/zy was equal to or above the sag threshold plus the hysteresis voltage.
The recommended value for the starting threshold is 90% of the declared voltage or of the sliding-referencevoltage. In case in which a different ending threshold is used,the recommended value is 91% of thedeclared voltage or of the sliding-reference voltage.
Voltage swells can be characterized in the same way as voltage sags. The rms voltage is again used as acharacteristic versus time. The single-event indices are the duration and the retained voltage.The durationequals the amount of time the rms voltage is above the swell threshold. The retained voltage is the highestvalue of the rms voltage. The recommended value for the swell threshold is 110% of the declared voltageor of the sliding-reference voltage.
Voltage sags with a retained voltage below the interruption threshold can be classified as momentary ortemporary interruptions. The duration of the event is the amount of time the rms voltage is below the sagthreshold. Alternatively the duration of an interruption can be defined as the amount of time the rmsvoltage is below the interruption threshold.The difference between these two definitions is normally smallcompared to the duration of the interruption.
The recommended value of the interruption threshold is 10% of the declared voltage or of the sliding-reference voltage.