Research On Capacitive Power Transfer System In Different Media

Wireless power transfer(WPT)technology can realize both mechanical and electrical isolation in the process of power transmission and avoid the problems of mess of wires,destruction of insulation,and damage of sockets.Then,the safety and reliability of the power supply system and the electrical equipment are greatly improved.CPT has the advantages of low cost,light weight,no eddy current losses,small electromagnetic interference,and excellent misalignment performance,which attracts more and more attention.The commonly used media of a CPT coupler can be divided into two categories:insulating media and conductive media.Air and water are the typical representatives of the two kinds of media.Based on different power transmission media,the equivalent circuits and working characteristics of a CPT coupler would be different,which can further affect the performance of CPT systems.In air,the equivalent capacitances of the CPT coupler are usually in unit of pF due to a very small dielectric constant of air.Such small equivalent capacitances would induce large amounts of reactive power in CPT circuits and high voltage stresses would be generated among the CPT coupler.Thus,a suitable compensation circuit is needed to compensate for the reactive power.Meanwhile,if a single-stage CPT system can realize load-independent constant current or constant voltage output as loads required,the CPT system is effectively simplified and the transfer efficiency is improved.In water,since water is conductive,the equivalent model of CPT couplers and the characteristics of CPT systems are changed,which would affect the system design.Thus,further research is needed.Based on the aforementioned problems,the research of this dissertation consists of four main parts:(1)The structures and characteristics of the coupler and basic compensation circuits are analyzed and a parameter design method to reduce the voltage stresses of the CPT coupler is proposed.Based on the equivalent models of the CPT coupler,series-series,series-parallel,parallel-series,and parallel-parallel CPT circuits are analyzed.And the constant current(CC)or constant voltage(CV)frequencies with zero-phase angle(ZPA)and load-independent output are derived respectively.Taking the CC frequency of the double-sided LC compensated CPT circuit as an example,a parameter design method is proposed to reduce the voltage stresses of the CPT coupler.Finally,the single-stage CPT circuit can not only achieve CC output,but also reduce the voltage stresses of the CPT coupler,alleviating the difficulties of insulation design.(2)Based on a higher-order compensation circuit,a parameter design method is proposed to reduce electromagnetic radiation.Due to the fringe effect,voltages on the main capacitors and the leakage capacitors should be further constrained.Since basic compensation circuits have no design freedom for further voltage optimization,a voltage optimization method for higher-order compensation circuit should be proposed to reduce electromagnetic radiation.After a systematical analysis of the double-sided LCLC compensated CPT circuit,three operating frequencies with load-independent CC output and input ZPA have been found.A parameter design method is proposed for the double-sided LCLC compensated CPT converter at each frequency to satisfy the desired CC output and the predesigned voltage limitations.(3)A LCLC-LC compensation circuit is proposed to fulfill the charging demands of Lithium-ion batteries and reduce the redundancy of the compensation circuit.After a systematical analysis of LCLC-LC compensated CPT converters,two frequencies for CC output and one frequency for CV output are found,while input ZPA at these three frequencies can be well achieved.Parameters of the LCLC-LC compensated CPT circuit are designed to realize the CC-to-CV charging transition.Finally,an LCLC-LC based CPT prototype with the CV frequency of 1 MHz and the CC frequency of 893 kHz is built to charge the 48 V/12 Ah Lithium-ion batteries.The maximum efficiency is 85.636 %.(4)The performances of the CPT systems in air and water are compared and the reasons for the low efficiency of underwater CPT systems are explored.The two-plate coupler and the four-plate coupler have been analyzed.The influences of the water dielectric conductivity on the equivalent capacitances,misalignment performance,and operating characteristics of the underwater CPT coupler are also studied.Based on freshwater,a novel underwater CPT coupler is proposed and a double-sided LC compensated CPT converter is built.The equivalent parameters of the novel underwater CPT coupler are derived and the reasons for power losses of CPT couplers in freshwater are summarized.