Research On Impedance-Frequency Characteristic Test Technology Of Traction Power Supply Systems

In traction power supply systems(TPSSs),high-order harmonic resonance accidents occur frequently.These resonant accidents would destroy ground or on-board equipment.In some severe cases,these accidents would result in traffic interruptions which seriously threaten power supply safety of railways.Impedance-frequency characteristics of TPSSs have a direct effect on the high-order harmonic resonance,but it is hardly to be acquired accurately by calculations.Thus,it is necessary to obtain the impedance-frequency characteristics of TPSSs by test methods.This dissertation intensively studies key technologies of the test method,system design,and actual application.Research results provide valuable references for existing line operating,new line debugging,and new locomotive commissioning.The test system topology suitable for TPSSs is proposed and the work principles are analyzed.A harmonic generator served as the harmonic current source is connected between overhead contact lines and steel rails.In a specific frequency range,harmonic currents both fully controlled in magnitude and frequency are injected into TPSSs.Voltage and current data are recorded.Then,the input impedance varying with frequencies at the test point is obtained,which is called the impedance-frequency characteristic curve.When the curve achieves its local maximum,the corresponding frequency is the resonant frequency of the TPSS.In order to adapt to TPSSs,which are high voltage,huge capacity and wide frequency range systems,the circuit topology of the test system and associated parameter constraint conditions are proposed.Power flow relations are analyzed when harmonics are generating.Control methods of the test system are proposed to inject full controlled harmonic currents into TPSSs.The hierarchical control strategy of the test device is proposed consisting of three levels.A first-level active power control(APC)enables the converter to absorb a certain amount of active power at the fundamental frequency for compensating power loss.A second-level voltage balancing control(VBC)equalizes the voltage among dc capacitors.And a third-level harmonic generation control(HGC)delivers a specific harmonic interference.The harmonic reference is added to the total reference directly avoiding parameters coupling with other control parts.In the modulation strategy aspect,output voltage spectrum analytical expression of cascaded H-bridge converters is deduced and given out for the first time when the modulating wave includes both the fundamental and harmonic components.By proper design,the dominant sideband harmonics can be move out of the frequency range of interest to achieve more pure fundamental and desired harmonic outputs in this frequency range.Simulation models and an experimental setup in a small power scale were built to verify the main circuit topology,hierarchical control strategy,and modulation strategy.The harmonic impedance test calculating method is put forward to reduce the background harmonic influence and increase test result accuracy.Harmonic impedance test is easily influenced by background harmonics.Features of background harmonics are analyzed based on measuring data.Considering impact factors on errors of the traditional fluctuation quantity based method,inter-harmonics are injected into TPSSs to increase fluctuations at the user side.Least square method and inter-harmonic impedance interpolation are used to calculate harmonic impedances.Under the support of key research project of China Railways Corporation and the national key research and development plan project,the first actual test equipment was constructed.The test equipment's rating voltage is 27.5 kV.Its rating capacity is 150 kVA,and the maximum frequency is 5000 Hz.During the research and development,key issues of the test prototype designs and constructions were solved including main circuit device integrations,control system design,software systems development,and type tests.At last,actual tests were carried out on the passenger transport line from Beijing to Shenyang.Functional validity and operational reliability of the test equipment are verified.The impedance-frequency characteristics of the outer power system and auto-transformer all parallel connection railways were obtained.Distribution rules of inherent resonant frequencies of TPSSs are uncovered.The research results contribute to suppress locomotive-network resonance and solve locomotive-network electrical coupling problems.