Fundamental-Harmonic Power Transmission Strategy Of Bidirectional Inductive Power Transfer System

With the popularization of smart grids and electric vehicles(EVs),bidirectional inductive power transfer(BIPT)has developed rapidly on EVs.Based on a series-series(SS)compensation topology,the harmonic power transmission strategies of BIPT are proposed to solve the collaborative optimization between the transmission power and efficiency.The proposed methods can improve the efficiency under the light load conditions and the realization of zero voltage switching(ZVS).Then the smooth switch strategy from the fundamental power to the harmonics power transmission is proposed,which can improve the system transmission efficiency within the full load power range.A 3.7k W rated power BIPT platform is applied.The transmission efficiency is as high as 97% under the 100% load power and 25% load power conditions.Firstly,based on the First Harmonic Approximation(FHA),the relationship between the transmission power and internal phase angle βp,βs and the external angle δ of the SS-BIPT is analyzed.The ZVS conditions of MOSFETs of full-bridge converters are calculated;Based on the equivalent loss circuit model,the relationship between the transmission efficiency η and the excitation voltages ratio κac and δ is obtained.Then the system power adjustment and efficiency optimization are realized by the symmetrical triple phase shift(STPS)modulation strategy,and the experiments are performed to verify the STPS strategy.When the fundamental power transmission with STPS is applied,as system transmission power decreases,it is necessary to reduce δ to achieve ZVS.However,it will result in a decrease in power factor and transmission efficiency.The power transmission model based on the third harmonic is established.Then the impedance matching and ZVS are analyzed.Both strategies of dual third harmonics power transmission and single third harmonic hybrid power transmission are proposed.The transmission efficiency of the BIPT system with proposed methods is improved significantly under the light load conditions.To expand the power range of harmonic power transmission,a power transmission model based on the second harmonic is established.Combining the switching modes of both fullbridge converters in one switching period,the conditions of impedance matching and ZVS realization are obtained.The strategy based on second harmonic power transmission is proposed.Then comparative experiments are performed to prove that the second harmonic power transmission can significantly improve the efficiency of the BIPT system under light conditions.When the power transmission method changes from the fundamental to the second harmonic or third harmonic,the resonance current will oscillate.Therefore,a smooth switching strategy is proposed to solve the problem,which is based on the symmetrical pulse width modulation,and the implementation method is obtained.The relationship between output pulse width αp,αs and fundamental amplitude |Vpm|,|Vsm| and phase φp,φs is analyzed.Based on the3.7k W BIPT platform,the comparison experiment of symmetrical dual-phase-shift(SDPS),STPS,and the combination strategy of asymmetric even harmonic and fundamental smooth switching are compared,and the combination strategy is proved to significantly improve the transmission efficiency in the full load power range.