Study On Wireless Power Transfer Resonant Reactive Shielding Systems

Wireless power transfer(WPT)technology relies on high-frequency magnetic field to transfer electric energy from the primary side to the secondary side through the air,realizing the non-contact transmission of the electric energy.Due to its advantages of safety,environmental protection,convenience and stability,this technology has been deeply studied by domestic and foreign scholars and has been widely applied in production practice.Although most of the magnetic fields assume the role of energy transfer,there is still an inevitable leakage of some magnetic fields,which will threaten human safety and interfere with electronic devices.How to reduce magnetic leakage has become a very important research content.As a new magnetic shielding technology,resonant reactive shield technology has been applied in WPT system in recent years and has the advantages of low cost,light weight,convenient installation and good magnetic shielding effect.In order to realize the magnectic shielding,this technology uses the principle of the leakage magnetic field to excite the offset magnetic field.Changing the equivalent inductance of the shielding coil can control the inductive current in the shielding coil and thus change the magnetic shielding effect.However,there is no complete criterion for the selection of the equivalent inductance.In addition,the existing methods have disadvantages such as cumbersome capacitor array control,simple observation samples and lack of accurate calculation model,so it is necessary to conduct in-depth research on resonant reactive shield technology.Firstly,this paper introduces the typical topology of WPT system,analyzes the circuit of U-SS structure and U-SP structure,deduces the expression of magnetic flux density at any point in the space from the primary and secondary side current,and establishes the expression of magnetic flux density at any point in the space from circuit parameters based on U-SS structure.Then,the functional relationship between the sum of square of magnetic flux density on the target surface and equivalent inductance is established.The expression of optimal inductance is obtained when the sum of square of magnetic flux density is minimized.It is found that the optimal inductance is related to input voltage,output voltage and the number of turns of shielding coil.By using the expression of the sum of square of magnetic flux density,the input voltage and output voltage are optimized and by using the finite element simulation,the number of turns of shielding coil is optimized,thus,the optimal inductance is determined.At last,three groups of shielding parameters and without shielding condition are selected for comparison through simulation and experiment.The results show that the magnetic shielding effect decreases first and then increases with the decrease of the equivalent inductance,and the sum of square of magnetic flux density of the measuring points accounts for 16.05% of that without shielding when the equivalent inductance is the optimal inductance.In addition,the proposed method has little effect on original system,and the efficiency changes only 0.177%.