Performance Improvement Technology Of EC-WPT System Under Variation Of Coupling Capacitance And Load

Electric-filed coupled wireless power transfer(EC-WPT)technology utilizes high-frequency electric-field to realize the power transmission from the power grid or battery to the electrical equipment.EC-WPT system adopts metal plates as the capacitive coupler,which results in the high flexibility of the coupling structure.When the EC-WPT system works,most of the electric flux is distributed between metal plates,resulting in low electromagnetic interference(EMI).According to the theory of electrostatic field,EC-WPT system shows the ability to transfer power through metal barriers.In addition,with the metal conductors around the coupler,the eddy current loss caused by EC-WPT system is very small.In EC-WPT applications,the transmission performance of the system is largely affected by the coupling capacitance and equivalent load,which is inevitable and offen happened during the operation.On the one hand,the movement of the device will lead to the coupling misalignments or change the transmission distance,affecting the coupling capacitance of the capacitive coupler.On the other hand,even if the capacitive coupler is perfectly aligned,the coupling capacitance will be changed due to environmental factors.In addition,the equivalent load of many electrical equipment changes during the power supply,especially for the wireless power supply equipment with lithium battery.The changes of coupling capacitance and equivalent load have many influences on the transmission performance of EC-WPT systems: 1)the output voltage/current of the system is seriously affected,leading to the change of the output characteristics of the system;2)the input impedance characteristics of the system is affected,leading to the reduction of the power factor and resulting in the increase of system loss,volt-ampere capacity,and other problems;3)the frequency characteristics of the system is changed,resulting in the frequency drift,frequency splitting,and the frequency bifurcation.To improve the transmission performance of the EC-WPT system under the change of coupling capacitance and equivalent load,this paper studies the parameter design method,compensation network,and system control,aiming at the change of output characteristics,power factor reduction and frequency characteristics of the EC-WPT system caused by coupling capacitance and equivalent load.The research work contains:This paper proposes a parameter design method with both constant voltage(CV)characteristic and high power factor.It can solve the problem of output characteristic change and power factor decrease caused by the change of coupling capacitance and equivalent load.Based on the fundamental harmonic approximation(FHA),the output voltage gain and input impedance model are established considering the equivalent series resistance.This paper gives the limit condition and parameter condition of CV characteristic under the change of coupling capacitance and equivalent load.On this basis,considering the limitations and requirements of the actual system,a parameter design method considering the CV characteristics and power factor of the system is given.Finally,simulation and experiment verify the CV characteristics and high power factor under the change of coupling capacitance and equivalent load.In addition,the correctness and effectiveness of the parameter design method are verified by simulation and experiment.To solve the problem of output variaiton caused by the changing coupling capacitance and equivalent load,an EC-WPT system based on LC-LC/LCLC hybrid compensation network was proposed to fulfill the characteristic of CV/CC and output conversion.Based on the fundamental harmonic approximation,considering the equivalent series resistance,the model of voltage/current gain is established,thus the limit condition and parameter condition of CC/CV characteristic are given.This paper also analysis the influence of the change of coupling capacitance and the equivalent load on the input impedance angle.Combined with the topological characteristics of LC network and LCLC network,the combination mode and control method of hybrid compensation network are given.Finally,both simulation model and experimental device are established.The simulation and experimental results verify that the proposed system has CC/CV characteristic when the coupling capacitance and equivalent load change,and also verify the effectiveness of the switching of the CC/CV operation modes.To solve the problem of frequency characteristic and output variation caused by the change of coupling capacitance and equivalent load,a dual-loop control method combining frequency tracking and phase shift control is proposed.Based on the full capacitance model,the output voltage/current gain model of the EC-WPT system is established.The parameter condition of the load-independent output voltage/current is derived,the frequency bifurcation and frequency splitting characteristics of the system are analyzed,and the phase relationship between the input and output of the system is analyzed.Based on the finite element simulation software COMSOL,the influence of coupling misalignment on the system frequency is analyzed.Finally,both simulation model and experimental device are established.The simulation and experiment prove that the system always works at CC/CV frequency when the coupling capacitance and equivalent load change,and the output voltage/current of the system is always the target value.Simulation and experimental results also verify that the proposed frequency tracking strategy can effectively track the frequency splitting frequency(i.e.constant voltage frequency)of the system in CV mode,and it can also avoid the system operating at other resonant frequencies caused by frequency bifurcations in CC mode.