A lower sideband EMD and its application in power systems

Low frequency oscillation assessment methods based on measured data have attracted increasing attention from research and industry stakeholders in power systems engineering. Among those existing methods, empirical mode decomposition (EMD) is a multi-resolution signal processing method used for analysis of non-stationary signals. In recent years, EMD has been a most widely used method in practice. However, it has some disadvantages such as end effect, mode mixing, etc. If a signal contains more than two individual components whose frequencies fall within an octave, it cannot be decomposed by standard and the frequency heterodyne EMD methods. To improve the frequency resolution, a lower sideband frequency shift EMD (LSB-EMD) is proposed in this paper. The method includes an advanced approach on how to select an appropriate modulation frequency so as to can enlarge the component frequency ratios and avoid frequencies turn over. The improved frequency shift EMD is integrated with the procedure of the standard EMD. Consequently, the complex signal can be decomposed by the method. This method can be applied for the analysis of low-frequency oscillations, as well as other power-system signals such as power quality signals and inrush currents. The effectiveness of the method to separate closely spaced modal components is demonstrated by both numerical simulation and field measurements.