The Impedance-Frequency Characteristics And Harmonic Resonance Of The Locomotive-Network System In Electric Railways

With a large number of AC-DC-AC locomotives and EMUs being put into service in the electric railways,the harmonic resonance has occurred in different power supply sections.It gives rise to the overvoltage of the traction network.As a result,it has become an urgent problem of the electric railways.To address the harmonic resonance,this dissertation focuses on the impedance modeling and the impedance-frequency characteristics of the traction power supply system and the network-side converter.And the mechanism of harmonic resonance is further analyzed based on the impedance matching of the locomotive-network interacted system.Firstly,this dissertation studies the field test data,analyzes the locomotive-network harmonic interaction,and then elaborates the mechanism of the overvoltage caused by the harmonic resonance.The harmonic transfer characteristics and harmonic interaction between the traction power supply system and the network-side converter is studied through analyzing the power transfer between ac-side and dc-side and the harmonic spectrum of the pulse width modulation.The mechanism of the overvoltage,caused by the harmonic resonance,is investigated through deriving the transfer function including the traction power supply system and the network-side converter controller.And it is obtained qualitatively that both the impedance of the traction power supply system and the impedance of the network-side contribute to the overvoltage of the traction network.The impedance model of the traction power supply system is established and the impedance-frequency characteristics are analyzed.The on-site test is designed to validate the impedance model and the analysis result.By taking the distributed parameters of the traction network into consideration,the detailed impedance model of the traction power supply system is obtained based on the multi-conductor transmission line of the traction network.Then the simulation is carried out to analyze the impedance characteristics for multiple coupling factors.The R-X distribution,the transit frequency of the impedance,the frequency range of the capacitive impedance and inductive impedance are obtained when the length of the traction network,the location of the locomotive,and the parameters of the traction network vary.The on-site test in the new-built railway is conducted by the impedance measuring system.The impedance-frequency characteristics are acquired through the small-signal harmonic disturbance(frequency?5000Hz)injected into the traction network.To validate the theoretical analysis and the simulation result,the impedance of the traction power supply system is measured for different length of the traction network,different locations of the harmonics injection,different feeding modes of the traction network.This method overcomes the difficulty in verifying the impedance characteristic.The method of small-signal frequency-domain dq impedance modeling is proposed to establish the model of the network-side converter with single-phase transient current control algorithm.Combined with the impedance of the traction power supply system,the impedance bode diagram is introduced to study the mechanism of the harmonic resonance.In addition,the frequency-domain analysis and the time-domain simulation are carried out.The dq-frame input impedance of the converter is derived through applying the generation of the fictitious ?? signals and the ??/dq transformation to all loops of the single-phase transient current control algorithm.By plotting bode diagrams of the impedance,three cases representing different locomotive-network systems are investigated by the frequency-domain analysis and the time-domain simulation.Furthermore,the frequency-domain analysis and the on-site test of an actual electric railway are carried out.The analysis result shows that the harmonic resonance will occur in the locomotive-network system with the negative phase margin.The analysis approach based on the bode diagram is simple,flexible,and highly variable without the need for the impedance ratio.Finally,the total impedance modeling of the locomotive-network system is proposed.Then the passivity and dissipativeness theory is introduced to evaluate the influence of the digital pulse width modulation(DPWM)on the harmonic resonance of the locomotive-network system.The DPWM is modeled as the time delay and zero-order-holder(ZOH).After that,the total impedance is derived including the converter controller,the DPWM,and the main circuit.Consequently,based on the total impedance,the passivity and dissipativeness theory is adopted to conduct the frequency-domain analysis for six cases.In addition,the time-domain simulation of these cases is carried out.It obtains that the harmonic resonance will occur in the system with non-dissipative property.Moreover,the frequency of the harmonic resonance is determined by the parameters of the traction power supply system.This approach provides an idea for the design of the controller parameters matching the traction power supply system.The mechanism of harmonic resonance is analyzed based on the impedance matching of the locomotive-network system,and the approximate frequency range of the harmonic resonance can be obtained before a new electric railway or a new locomotive is put into use.And some mitigation measures can be taken in advance for the electric railway or the locomotive which holds the possibility of harmonic resonance,thereby avoiding threats to the normal and safe operation of the electric railways.