| Current power supply methods for on-line monitoring device on high voltage transmission line have severe disadvantages in energy feeding reliability and stability and magnetic coupling resonant wireless power transfer(MCR-WPT)method is proposed in this paper as a novel solution.Moreover,a multi-coil MCR-WPT structure is presented for higher voltage level transmission lines and better performance in wireless power transfer distance and system efficiency.The research of this paper is organized as follows.In the first stage,a mathematical model of an N-coil MCR-WPT system is established on the basis of the circuit theory and the systematic impedance matrix equation,which only has two variables of k(coupling coefficient)and Q(quality factor),is obtained,deducing the mathematical expressions of key basic parameters,such as power transfer efficiency,output power,secondary current and optimized load.Then make the N-coil system specific to be a two-coil and three-coil system respectively and followed their various characteristic studies with inclusion of frequency response of secondary current and efficiency,load reaction,coupling effect for both systems and distance behaviors as well as cross-coupling effect for the three-coil system.Furthermore,the criteria for max power transfer and calculation for intrinsic frequency are all presented in detail.On the other hand,a helix coil,specifically designed for an 110kV high voltage transmission lines,is structured in Maxwell with a final diameter of 40cm,24 turns and 2mm turn spacing after detail relationship analyses among coil inductance,resistance and turn spacing as well as coil radius.Magnetic strength distribution of the designed coil in radial and axial direction are all studied and the empirical formula for mutual-inductance calculation is restricted to be true only in some conditions.Additionally,ADS is utilized for the two-coil and three-coil WPT system simulation and results show that when the load is set as 7Ω and distance is 1.2m,wireless power transfer efficiency and the corresponding output power are 31.4%,60.5%and 58.2W,26.2W respectively.Besides,systematic characteristics in theoretical analyses for these two WPT systems are all validated.Finally,a practical platform is set up and experimental results illustrate that for the two-coil WPT system,max power transfer efficiencies reach 56.8%and 31.2%for a distance of 0.6m and 1.2m respectively and the corresponding max output power are 197.5W and 83.3W for a load of 7Ω.However,for a three-coil WPT system,the system efficiency gently fluctuates around 49%for a 1.2m distance and the output power is only determined by output properties of the power source.In order to study the displacement effect of the relay coil away from the predefined middle point,a 20cm left and right offset as well as a 25cm perpendicular offset are all examined and results indicate that efficiency can stably maintain over 40%.Even when the relay coil is set paralleled with the horizontal axial line,the system efficiency is still higher than the value for the two-coil WPT system at the same output power level performance of the source.The paper validates the feasibility of MCR-WPT system on high voltage transmission line in transferred power and distance.Emphases are put on the three-coil WPT system for a longer distance access,higher power transfer efficiency,lower requirement for power source output properties and stronger resistance to frequency fluctuations.The practice can provide significant references for future researches of multi-coil WPT applications on higher voltage transmission lines. |
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