Research On Variable Parameter Adaptability Of Electric Vehicle Wireless Charging System

As a new type of intelligent transmission technology,magnetic induction radio energy transmission technology can get rid of the cable constraints,has obvious advantages,and has attracted widespread attention.Especially in the application of electric vehicles,wireless charging system can not only effectively solve the problem of short mileage of power battery,save the trouble of centralized charging in charging station,promote the development of electric vehicle industry,but also have the convenience without plug and plug,and the safety of no spark and no leakage.Variable parameters are an important feature of the wireless charging system of electric vehicles.The adaptability of variable parameters is one of the bottlenecks in the development and popularization of wireless charging technology of electric vehicles.Among them,as the load of the system,the charging characteristics of the electric vehicle power battery determine that the equivalent load parameters are changing continuously.At the same time,because the transmission coil on both sides can offset horizontally or longitudinally,and there is no fixed connection between the transmitting side and the receiving side,the change of mutual inductance parameters is the most prominent.Moreover,the offset characteristics of the transmission coil can not be overcome.In order to improve the advantages of wireless charging system,we can only improve the offset resistance of the output power by optimizing the circuit structure and control strategy of the system,thus increasing the system offset allowance.As an important part of the wireless charging system of electric vehicle,compensation network plays an important role,which directly affects the power transmission characteristics of the system.This paper focuses on the research and optimization of the constant current / constant voltage output and anti offset characteristics of the system,so as to improve the adaptability of the system parameters.Based on the analysis of three kinds of modeling theory of energy transmission module in the system,the power transmission characteristics of four kinds of basic compensation networks and the high-order compensation network with T-type LCC network at the transmitting side are analyzed respectively.Through mathematical analysis,the design values of four basic compensation networks are obtained under the condition that the resonance occurs at the receiving side and the capacitance at the transmitting side fully compensates the self inductance of the transmitting coil and the parameters reflecting the reactance.The expressions of output power and efficiency of various compensation networks are obtained,and the variation characteristics of output power with mutual inductance and frequency are deeply analyzed,and the circuit principle of the variation characteristics of output power is revealed.The input and output characteristics of forward / reverse LC resonant network,T-type and π type sub-networks are analyzed.The constant current / constant voltage output characteristics of four basic compensation networks and several high-order compensation networks are analyzed in detail.The constant current / constant voltage output capacity,parameter design conditions and output current / voltage expressions of different compensation networks are obtained.In addition,combining with mathematical analysis and circuit theory analysis,the anti-shifting and stability of output current of SS and double LCC compensation network in constant current output mode and the anti-shifting and stability of output voltage of S-LCC and LCC-S compensation network in constant voltage output mode are compared respectively.Aiming at the problem that the transmission coil is easy to shift in static electric vehicle wireless charging system,which results in the change of charging current and charging voltage,a system scheme with anti offset property of constant current before constant voltage output is designed.Firstly,a mutual inductance identification method based on SS compensation network is proposed to determine the actual mutual inductance value under different offset states.This method only needs to detect the output voltage and current of the inverter at the transmitting side,and identify according to the electrical relationship,which saves the wireless communication link between the two sides.The method is simple and reliable.On the basis of determining the actual mutual inductance,a set of phase-shifting control scheme is designed.If transmission coil has a large offset,the system can still achieve rated current / voltage charging by adjusting the phase-shifting angle of the inverter at the transmitting side.Then,according to the output characteristics of SS compensation network,the frequency splitting phenomenon of SS compensation network in constant current output mode is analyzed,and a set of compensation parameter design process considering various indicators is proposed.The output mode is changed by switching the frequency to meet the demand of constant current and then constant voltage charging of electric vehicle power battery.Finally,an experimental prototype is built to verify the effectiveness of the phase-shift control method based on mutual inductance identification and the feasibility of the switching mode control method.In order to further increase the offset tolerance of the magnetic induction wireless transmission system,the maximum output power of the transmission coils on both sides in the opposite state is taken as the basic condition.The rated working point corresponds to the extreme point of the output power.The relationship between the compensation parameters of SS compensation network and the anti offset of output power is found,and an optimal design method of compensation parameters is proposed.In order to reduce the fluctuation of output power with coil offset.In order to minimize the input apparent power,the PS-S compensation structure and parameter design method are proposed.Moreover,an experimental platform is built to verify the effectiveness of the compensation parameter optimization method.Through simulation analysis and experimental verification,the scheme of constant current before constant voltage output system with anti offset is designed in this paper,which can automatically identify the mutual inductance at the transmitting side and obtain accurate actual mutual inductance value.Under different offsets,the rated current and rated voltage can be maintained by phase-shifting control.Moreover,the frequency can be changed automatically to switch from constant current mode to constant voltage mode.In addition,for the proposed optimization design method of SS compensation network parameters,the experimental results show that this method can reduce the fluctuation of output power under large offset,and can effectively improve the anti offset performance of output power.