A wavelet-based procedure for automatically determining the beginning and end of transmission system voltage sags

Voltage sag disturbances are characterized by several parameters, including but not limited to sag duration and depth. An accurate determination of fault inception and clearing times is needed to properly determine these and other characterizing parameters, such as point-on-wave, and to verify proper breaker operation. This paper presents a discrete wavelet-based approach for determining the beginning and end times of sags. Several types of wavelets are compared using recorded transmission sag data. A recommendation is made for the best overall wavelet choice. The procedure is fully described, and the results are compared with other methods for determining sag duration, such as the specified RMS voltage and waveform envelope methods.