Research On Harmonic Resonance Characteristics And Suppression Of Traction Power Supply System In Weak Grid

In recent years,China’s railway construction and development have achieved historic achievements and changes.With the development of the western region,large-scale electrified railway construction is underway in areas with weak power sources.The conventional traction power supply system(TPSS)always powered by a robust external power source.Differently,the weak external power supply has a long railway line and a few power points,which makes the external power system more and more exhibits the characteristics of a weak grid.Furthermore,the short-circuit capacity of the external weak power grid is small,and the transmission distance is long.It results in a large number of distributed inductance and distributed capacitance on high-voltage transmission lines.Once the inductive equipment parameters in the system match the capacitive parameters,the TPSS under weak grid conditions may generate multiple resonance frequencies,which further complicates the resonance problem of the TPSS.Besides,electrified railways run electric locomotives or EMUs with different principles and working methods,which inject a large number of widely distributed harmonics into the TPSS.Harmonic resonance will generate harmonic overvoltage,and at the same time,amplify the harmonic current,which will further cause severe distortion of the system harmonic voltage and harmonic current.Therefore,this thesis focuses on the theoretical research of harmonic resonance of traction power supply systems under weak grid conditions.Furthermore,it dedicates to solving theoretical issues such as harmonic resonance analysis methods,system resonance characteristics,and harmonic suppression evaluation that take into account the types of harmonic source branches.Firstly,it establishes a complete mathematical model of the traction power supply system,which includes the external weak grid-traction transformer,secondary side feeder,traction network,AT substation/section post,and traction load.In which,the weak external grid is built as a transmission line model with short-circuit impedance and distributed parameter characteristics.Then,considering the actual types of harmonic sources in the power system,four branch types of harmonic source are formed according to whether the harmonic source branch has impedance or not,and the branch is a series or parallel branch.Moreover,it deduced the method of establishing the node admittance matrix,considering the branch types of different harmonic sources is deduced,which makes this method applies to both series resonance and parallel resonance analysis.Furthermore,It analyzed the parallel and series resonance,respectively,with resonance modal analysis and node voltage method.Ultimately,it reveals more potential series and parallel resonant frequencies in the system.Using the established system model and established series and parallel resonance analysis methods,the influence of different equipment or component parameter(such as external weak power grid,traction transformer,etc.)fluctuations on the TPSS resonance under weak grid conditions is studied;based on the resonance modal sensitivity analysis method,the normalized modal impedance/frequency sensitivity information of the system’s key equipment component parameters is calculated,and the component that has the greatest influence on a certain resonance is revealed;the basis of the normalized modal sensitivity.In the above,a standardized modal sensitivity index is introduced,which can achieve the purpose of selecting the component parameters to suppress this resonance without intensifying the resonance at other frequencies.This paper is aiming at the harmonic phenomenon of the traction power supply system under the condition of the weak power grid,with the aim of not introducing a new resonance point for the passive filter,C-type and third-order high-pass filters were designed as filtering schemes.The impedance-frequency characteristics of the two suppression schemes are studied,both can achieve practical control effects.and the high-order characteristic harmonic filtering efficiency of the third-order high-pass filter is better than that of the C-type filter.