Electromagnetic Energy Flow And Bioelectromagnetic Safety Of High Power Wireless Power Transfer System

Wireless power transfer(WPT)techniques can transfer power without direct electrical contact.As early as the late 19th century,Tesla and other pioneers began to study the WPT techniques.After the 1960s,especially after MIT proposed magnetically coupled resonant wireless power transfer(MCR WPT)techniques in 2007,WPT techniques have developed explosively.Currently,the techniques for mid-and short-range wireless charging applications have been applied in various fields,but they also face many challenges.Among them,the electromagnetic safety of the system is a prominent problem,and the current electromagnetic safety research mainly focuses on the evaluation of the electromagnetic environment around the system and the electromagnetic shielding of the system to reduce the leakage magnetic field,but these researches are still superficial.And with the increasing power rating of the system applying MCR WPT techniques,the electromagnetic safety problem tends to be more serious.Therefore,this paper attempts to explore the method of improving the electromagnetic safety level of the system from the perspective of electromagnetic energy flow,and intensively studies the electromagnetic safety of the high-power MCR WPT system.The main work is as follows:(1)The electric and magnetic field distribution around the coupler of general MCR WPT system is analyzed by the analytical method,and the electromagnetic energy flow of three most basic MCR WPT with coil alignment,coil lateral offset and angular rotation is obtained based on the Poynting vector theory,and unified expressions of active and reactive power density are provided.Also,it is found that the active power density is mainly distributed on the transmission path,and the reactive power density is composed of three parts.And it is revealed that the phase difference of?/2 would contribute to the transmission of electrical energy.In addition,the effect of four basic reactive power compensations on electromagnetic energy flow is considered,and it is proposed that for systems with parallel compensation on the secondary side,heavy loads should be used so that the phase difference tends to be?/2.(2)To improve the electromagnetic safety of the MCR WPT system,the electromagnetic energy flow of the aligned MCR WPT system coupler with a current phase difference of?/2 is intensively analyzed,and the concept of energy-concentrating effect is proposed.Firstly,the expression of the transmission power is derived for the first time from the perspective of the field.Then it is found from the energy flow results that the active power density is concentrated on the transmission path and presents a 3D hollow cylindrical distribution,which discloses that there are large electromagnetic fields near the path and thus it would pose a threat to biological electromagnetic safety.Therefore,an evaluation index R_t of the energy-concentrating effect of transmitting unit active power is proposed and the inner-outer ring and disc couplers are designed and optimized by the directly-discretizing sweeping method based on the index.Then,it is verified that the two optimized couplers have better energy-concentrating effect by finite element simulation and experiment.And the maximum allowable transmission power of the system can be increased under the premise of ensuring the electromagnetic safety of the system.(3)Mathematical models for evaluating the complex electromagnetic field boundary value problem of bio-electromagnetic safety of MCR WPT systems in electric vehicles are established.Finite element preprocessing models of adults in standing(or lying)and sitting posture and a child with various tissues and organs,and an electric vehicle(including couplers)have been built by joint simulation techniques.Also,four human electromagnetic exposure schemes are considered and the electromagnetic safety of the MCR WPT system in the electric vehicle with a transmission power of 10 k W is evaluated applying finite element numerical analysis,and the differences in electromagnetic exposure between an adult and a child are compared.Finally,the corresponding relationship between the output power of the charging system and the electromagnetic exposure values in the human body models is obtained through the circuit and electromagnetic field theory,and the maximum allowable transmission power of the MCR WPT system when the electromagnetic safety requirements are satisfied is obtained.(4)To solve the contradiction between the maximum allowable transmission power of the system and the electromagnetic safety requirements of the human body,four feasible measures are proposed:(1)Select optimized couplers with high energy-concentrating effect to minimize the impact of leaked magnetic fields on the surrounding environment under the same transmission power of the charging systems;(2)The safety distance of the personnel when the system is working normally is defined,and the safe and dangerous areas when the system is working are given;(3)More sub-regions in the dangerous area are defined,and the calculation method of the maximum allowable power of the system when personnel stay in these sub-regions is provided;(4)A novel method for detecting the offset distance of the system is proposed,which can be utilized to assist the driver of the electric vehicles to adjust the charging position of the vehicle,thereby reducing the leaked electromagnetic field of the charging system.