Assessment of Voltage Sag Indices Based on Scaling and Wavelet Coefficient Energy Analysis

The two main voltage sag indices are magnitude and duration, defined in terms of the well-known rms voltages. The spectral energy of the voltages provides the same voltage sag indices of the rms voltage analysis and less computational effort is required. However, neither of them provide point on wave of sag initiation and recovery. This paper presents a wavelet-based methodology for the characterization of voltage sags, where the spectral energy of a voltage is decomposed in terms of the scaling and wavelet coefficient energies. The scaling coefficient energies of the phase voltages are used for voltage sag characterization, providing sag indices (magnitude and duration) in agreement with the definition. However, the analysis of the wavelet coefficient energies of such voltages provides additional information for the identification of the point on wave of voltage sag initiation and recovery as well as important parameters for power system protection and voltage sag mitigation devices. The performance of the proposed wavelet-based methodology was assessed with actual data and it was scarcely affected by the choice of the mother wavelet. Therefore, a compact mother wavelet can be used for voltage sag analysis with computational effort equivalent to the rms method and in agreement with practical applications. The maximal overlap discrete wavelet transform presented better performance than the discrete wavelet transform. All of the equations provided in this paper were developed for real-time analysis.