| The balise transmission system is an important part of Chinese Train Control System to ensure high-speed and reliable operation of trains.As a spot transmission system,it is one of the main channels for transmitting vehicle control information between trains and ground equipments.The system consists of ground equipments and on-board equipments.As an essential core component of the system,the Lineside Electronic Unit is responsible for obtaining important driving information from Train Control Center or Computer Based Interlocking,including line parameters,positioning,temporary speed restriction,routes and other safety-related variable information,and continuously sending those to the controlled balise in accordance with the requirements of interface‘C’.It is sent to the controlled balise and received by the on-board equipments when the train passes above the balise.However,the abnormal communication between the Lineside Electronic Unit and balise often occurs in engineering applications,especially when the quality of the waveform of the telegram received by the balise deteriorates due to the large scale of the station and the long transmission distance,which causes the balise to send the default telegram information to the on-board equipment,resulting in train braking and seriously affecting the train operation efficiency.The existing research on this key issue is mainly carried out from the aspects of functional realization,experimental testing,engineering practice,etc.,but less research has been conducted on these topics,which relate to the quality of the Lineside Electronic Unit output telegram waveform,including the optimal design of the core component of the interface‘C’circuit,channel transmission characteristics,and electromagnetic interference intrusion mechanisms.Therefore,this paper will focus on the above-mentioned key issues,and the main points are as follows:(1)Optimized design of the coding output circuit of the Lineside Electronic Unit.The Lineside Electronic Unit coupling transformer is the core device of the coding output circuit.The relationship equations between the output pulse rise time of the coupling transformer,the overshoot of the coupling transformer,and the distribution parameters of the transformer are established respectively based on the interface‘C1’signal waveform index proposed in the TB/T 3485-2017 specification standard,so as to obtain the reference range of the design value of the distribution parameters of the transformer.By establishing the distribution parameters calculation model of the coupling transformer of the Lineside Electronic Unit in COMSOL Multihysics,the above conclusions can be used to achieve rapid adjustment and optimization of the transformer fabrication process design.The model fully considers the influence of the eddy current effect and core hysteresis effect on the distribution parameters of the transformer under the action of high-frequency pulses,which greatly reduces the design cost.(2)The three-dimensional finite element calculation model of the twisted cable type LEU BSYYP is established.By setting the rotational period boundary conditions at both ends of the cable model with one twisted pitch length,the calculation results of the distribution parameters that are approximate to the target length cable model can be obtained.This modeling approach significantly reduces the time required to simulate long-distance cables with periodic characteristics,saves computational costs,and takes full account of the effects of skin effect,proximity effect,and twisting effect on the current density distribution of the conductor.The results of the above distribution parameter calculation model are used to create a lumped equivalent circuit cascaded by lossy T-circuits to achieve the physical simulation of the transmission characteristics of an arbitrary length of cable.And the cut-off length is determined based on the error coefficient between the characteristic impedance of the equivalent circuit described above and the characteristic impedance of the transmission line,as well as the frequency characteristics of the excitation signal on the cable.Compared with the traditional empirical value method based on the wavelength corresponding to the maximum operating frequency,the error value can be selected according to the actual engineering design requirements to achieve flexible configuration of the number of cascaded equivalent circuits.The above simulation method can also be applied to the analysis of the transmission characteristics of long-distance cables with more complex structures(types LEU BSYL23 and LEU BSYA23),which are covered externally with armor and sheathing,providing ideas for modeling various types of long-distance cables.(3)When the lightning current intruded into the through-ground wire and discharged to the soil along them,the coupled lightning electromagnetic fields model considering the frequency-dependent effect of the soil electrical parameters was established,which realized the quantitative analysis of the induced voltage and the induced current on the nearby cable typed LEU BSYYP and the qualitative analysis of influencing factors of the lightning electromagnetic radiant intensity in the soil.The following conclusions can be drawn from the numerical calculations:1)When the lightning strike point is close to the observation point,the electromagnetic field strength of lightning radiation in the soil is not significantly influenced by the frequency-dependent effect of the soil conductivity,but the value is significantly influenced by the air-soil interface;2)The electromagnetic coupling intensity of the lightning radiation electromagnetic field on the nearby signal cable is less correlated with the frequency-dependent effect of the soil electrical parameters,but is more influenced by the lightning current waveform parameters,the distance between the air-soil interface and the cable,and the distance between the cable and the lightning current injection points;3)The double-terminal grounding of the cable shield provides protection against the lightning electromagnetic field radiation in the frequency range 0to 10~6 Hz.The above conclusions suggest that:1)Minimize the buried depth of the through-ground wire as much as possible and consider burying the signal cable below the through ground wire;2)Given the close proximity of the through-ground wire to the signal cable in actual construction,the effect of the frequency-dependent effect of the soil conductivity on the strength of the lightning electromagnetic field can be disregarded;3)When protecting electronic equipment against lightning,the spectral characteristics of lightning waveforms with different parameters should be examined in order to estimate the amplitude and energy of the lightning strikes within the signal bandwidth.At the same time,the frequency-dependent effect of the soil cannot be ignored when examining the coupling effect of lightning electromagnetic fields on nearby conductors in the soil excited by lightning waveforms of different parameters. |
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