Power Quality Detection In Traction Power System Based On Hilbert-Huang Transform

With the pervasion of power electronics apparatus and nonlinear loads, recently the power quality problems have been more and more deteriorated by the distorted waveform of voltages and currents in power network. Traditional power quality analysis methed is Fourier transform. As a traditional analysis tool in frequency domain, it is not suitable for the non-stationary and non-linear signals processing. Thus new analysis methods are required to detect and analysis power quality disturbances accurately.Hilbert-Huang Transform (HHT) is applied in detection and analysis of power quality in this paper. Short time power quality disturbances and harmotics are taken more consideration. For the first time, Hilbert-Huang Transform is applied to detect harmonic in traction power systemin this paper. And simulation results show that this method is effective to extract harmonic. The main contribution is gaven as follows:Hilbert-Huang Transform is studied deeply and it is applied to power quality in power system, such as short time power qulity disturbances including voltage sags, voltage swells, interruption, transient oscillasion and transient interruption, and harmonics including integer harmonic, non-integer harmonic, transient harmonic distortion, and so on.In order to restrain the end effects of Hilbert-Huang transform, two new methods, combination of support vector regression machines and mirrorizing extension, combination of BP neutral network and mirrorizing extension, are proposed in this paper based on the original extension methods. The merits of support vector regression machines, BP neutral network and mirrorizing extension are discussed, and the disadvantages of them are listed. Simulation results show that the two methods are effective to improve the end effects of HHT. Comparision of the two methods is given in this paper, and the results show that the former new method is better than the latter for most of simulation and real signals. The charateritsic of harmonics in traction power system is introduced and analyzed, and the corresponding applied methods for measuring harmonics are analyzed and discussed in this paper.Because the energy of fundamental is much bigger than the other harmonics' in traction power system, mode-mixing appears seriously during the sifting by Empirical Mode Decomposition (EMD). Harmonic with any frequency in traction power system can not be directly abstracted with EMD. In order to sift the mode-mixing harmonic signal into good mode, an improved method named EMD based on Fourier transform, which is proposed by Yang, is used in this paper. This method is with the combination of support vector regression machines and mirrorizing extension method. The improved HHT method is applied to harmonics analysis in traction power system, and it is effective to extract harmonic with any frequency. Accurate instantaneous frequency and instantaneous amplitude of harmonic components can be obtained by using Hilbert transform.