Research On Interharmonic Of Traction Power Grid

With the rapid development of China’s high-speed railway,the harmonic and interharmonic problems caused by locomotive-traction network electrical coupling in traction power grid have become increasingly prominent.Although the wide use of AC-DC-AC electric locomotives has effectively improved the harmonic characteristics of traditional locomotives,it has also brought about a new problem of broadening the harmonic spectrum,among which there is a special form of harmonic:interharmonic which is not an integer multiple of the fundamental frequency.Interharmonics,as a characteristic parameter of power supply network,have a special influence on the power quality of traction power grid.Therefore,this paper studies the interharmonics of traction power grid by combining theoretical analysis and simulation experiments,and provides theoretical support for solving the problems of harmonics and interharmonics in locomotive-traction network system.In this paper,the harmonic and interharmonic problems of traction power grid are deduced and analyzed from the theoretical point of view.Aiming at the harmonic problems,the generation mechanism and influencing factors are analyzed from the aspects of high-frequency harmonic and low-frequency oscillation.Aiming at the problem of interharmonics,the mechanism characteristics of different types of interharmonics are analyzed from three aspects:external power supply,natural frequency of traction network and working characteristics of ferromagnetic elements..By studying the interharmonic transmission law in the process of vehicle-network coupling,it is found that when there is interharmonic in traction network,new interharmonic will be generated by superposition and diffusion in the whole spectrum under the alternating action of AC side and DC side of traction network.The joint simulation model of traction power grid is built based on PSCAD simulation software,and the modeling process of traction power supply system and electric locomotive is introduced,which provides a simulation platform for exploring the interharmonic characteristics of traction power grid.Three situations of interharmonic generation in traction power grid are simulated,and fast Fourier transform algorithm is used to analyze the network voltage of traction power grid,and the distribution characteristics and laws of different types of interharmonic generation are summarized.For the linear resonance phenomenon of locomotive-traction network coupling process,the resonance phenomenon of roof grid voltage and head-end grid voltage is the most obvious,and the resonance phenomenon of power supply side grid voltage is reduced,but there are still some harmonics and interharmonics exceeding the standard;For the phenomenon of ferromagnetic resonance in over-phase process,the spectrum distribution is single,and there are mainly interharmonics with specific frequency;As to the low-frequency oscillation phenomenon during the simultaneous start-up of multiple vehicles,the increase of locomotive number,equivalent impedance of traction network,K_{i P} of current inner loop,K_{v P} of voltage outer loop and K_{v I} of voltage outer loop will aggravate the low-frequency oscillation of network voltage.According to the characteristic that linear resonance is mainly high-order harmonic,C-type filter is designed for simulation verification to effectively suppress harmonic and interharmonic content.According to the characteristics of undisturbed stability of ferromagnetic resonance,RC protection device is designed for simulation verification,which effectively reduces the occurrence probability of ferromagnetic resonance phenomenon.According to the characteristics that low-frequency oscillation is affected by locomotive rectifier control mode,the rectifier control mode based on sliding mode control is optimized and verified by simulation,and the suppression effect is good.