IEEE Std 1564-2014 pdf download – IEEE Guide for Voltage Sag Indices.
3. Definitions For the purposes of this document, the following terms and definitions apply. The IEEE Standards Dictionary Online should be consulted for terms not defined in this clause. 4 characteristic voltage (of a voltage sag): A single-event characteristic that combines the voltages in the three phases providing one single value corresponding to the sliding-window root-mean-square (rms) voltage as defined for single-phase recordings. The characteristic voltage is the lowest of the three per-unit phase-to-phase rms voltages, and the three per-unit phase-to-ground rms voltages after removing the zero- sequence component. declared voltage: The nominal voltage or a value of a voltage different from the nominal voltage obtained by agreement between the electricity supplier and a consumer. NOTE—See IEC 61000-4-30. 5, 6 depth (of a voltage sag): The difference between the reference voltage (either V din or V SR ) and the residual voltage. It is generally expressed in percentage of the reference voltage. NOTE—See IEC 61000-4-30. duration (of a voltage sag): The length of the interval between the time when the root-mean-square (rms) voltage drops below the sag threshold and the time it subsequently rises above the sag threshold.
6. Site indices 6.1 General As input to the site indices, the single-event characteristics are used as obtained from all events recorded at a given site over a given period, typically one month or one year. The clauses that follow present a number of alternatives for the two-index method. Each alternative can be summarized as a count of events within a certain range of retained voltage and duration. For single-index methods, the site index is the sum of the single-event indices of all events recorded within the given period. When publishing site indices, it is important to indicate which method has been used to obtain the single- event characteristics: for example, the lowest of the three phase-to-ground voltages, the lowest of the three phase-to-phase voltages, or the characteristic voltage obtained from three-phase measurements. 6.2 SARFI indices System Average RMS Variation Frequency Index (SARFI) is a power-quality index that provides a count or rate of voltage sags, swells, and/or interruptions for a system. The size of the system is scalable: it can be defined as a single monitoring location, a single customer service, a feeder, a substation, a group of substations, or an entire power delivery system. There are two types of SARFI indices: SARFI-X and SARFI-Curve. In this clause, SARFI is used as a single-site index (a one-site system). In the next clause, it is used as a system index. The index provides how many (annual) events there are when the magnitude of a voltage sag is below a specified threshold.