| The telemetry drill string transmits the drilling and geological parameters obtained by MWD to the ground through the high-speed drill string formed by telemetry drill pipe.The electromagnetic coupling joint in the telemetry drill pipe is similar to the high-frequency transformer,which is used for the coupling connection of high-frequency electromagnetic signals between drill pipes.However,its voltage transfer function seriously affects the signal transmission characteristics of the telemetry drill pipe channel,so it is necessary to analyze the signal transmission of electromagnetic coupling joint.The establishment of equivalent circuit and circuit parameters of electromagnetic coupling joint is the key of signal transmission function modeling.Through the electromagnetic simulation analysis of electromagnetic coupling joint,the equivalent relationship between circuit parameters and electromagnetic parameters of coupling joint magnetic core material can be established,which is very important for the theoretical analysis of signal transmission of coupling joint.Based on NDT electromagnetic detection analysis method,the paper studies the generation mechanism of eddy current in core materials and the interaction mechanism between eddy current and coupling coil.It makes reasonable circuit equivalence on the energy loss caused by the core,and establishes the high frequency equivalent circuit model of electromagnetic coupling joint.The mathematical models of input impedance,voltage transfer function and power transfer function of coupling joint are established by circuit analysis,We use COMSOL software to simulate and analyze the electromagnetic field of electromagnetic coupling joint,and study the influence of magnetic core structure and parameters on circuit parameters.Firstly,the structure of the electromagnetic coupling joint is determined,and the equivalent relationship between the electromagnetic parameters of the magnetic material and the circuit parameters of the electromagnetic coupling joint is established by combining the simulation results with the mathematical models of input impedance and voltage transfer function.Then,the voltage transfer function and power transfer function under the influence of electromagnetic parameters and signal frequency are calculated and analyzed.Finally,the modeling effect of the equivalent circuit of the electromagnetic coupling joint is evaluated by comparing with the electromagnetic simulation results.The results show that the voltage transfer function and power transfer function of the electromagnetic coupling joint are basically consistent with the electromagnetic simulation results,which indicates the correctness of the equivalent circuit model.The relative permeability of the magnetic core is about 50,which is good for signal transmission.The influence of core conductivity on voltage transmission is very small,so the conductivity of core material can be ignored when selecting electromagnetic parameters.When the frequency is higher than 3MHz,the power transfer function of the electromagnetic coupling joint is higher than 95%.The conclusion indicates that the eddy current loss generated by the magnetic core has little effect on the power transmission at high frequency.In other words,it is conducive to the long-distance transmission of the signal.The voltage transmission characteristics of the electromagnetic coupling connector show obvious low-pass characteristics,which is not good for the transmission of high-frequency signals.Therefore,the corresponding compensation must be made to the transmission characteristics of the channel to meet the transmission requirements of the frequency band signal.The research in this paper can provide theoretical guidance for electromagnetic design of intelligent drill pipe and improvement of signal transmission characteristics of electric telemetry drill string. |
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