Research On Harmonic Filtering And Utilization In Wireless Power Transfer System

The requirement of modern power devices and electronic equipment for stable operation is increasing,so people pay more attention to the electromagnetic interference near the equipment.Electromagnetic sensitive occasions have clear specifications for electromagnetic emission in the environment,such as CISPR25 of automotive electronics and GB6364 of aviation radio.For the wireless power transfer(WPT)technology which transmits energy through electromagnetic,it will produce harmonic interference when it works,which will affect the surrounding electromagnetic sensitive equipment.The emission limit of WPT harmonic interference is clearly defined in the latest opinion draft issued by Ministry of industry and information technology of China.So filtering out the harmonics in WPT can effectively expand its application scope,which is also one of the research contents of this paper.There are two main reasons for system instability caused by electromagnetic compatibility(EMC),which are electromagnetic interference(EMI)and electromagnetic tolerance(EMS).EMS refers to the ability that the equipment is not affected by electromagnetic interference during stable operation.Electromagnetic interference exists everywhere in the environment.Because of its unique technical characteristics,electrical,industrial and mechanical equipment has strong EMS,and most non precision instrument equipment also have certain EMS,so the harmonic interference in WPT has no great influence on non electromagnetic sensitive equipment,so it is not necessary to filter harmonic in many applications.At the same time,because the energy carried by harmonics is very small,its impact on the system characteristics is very small,so researchers often ignore harmonics,and rarely explore its own value.In the non electromagnetic sensitive environment,the harmonic does not need to be filtered,in this case,the harmonic always exists and is easy to be ignored.Harmonic has the characteristics of rich order,high frequency and low energy.After research,the author found that using these characteristics to solve some important problems in WPT will show great potential,so harmonic utilization is another innovative research content of this paper.The main contents of this paper are as follows:1)Harmonic elimination.Adding filters to eliminate harmonic interference is a general and effective method,but the position and parameters of filters in WPT system will greatly affect the system characteristics of WPT.The main reason for the harmonic interference in WPT is that the harmonic in the output square wave is loaded on the resonant coil of the transmitter,so this paper studies a method of harmonic elimination by adding a second-order LCR filter to the resonant part(LCR-WPT).However,the circut of WPT resonant part is precisely matched and designed;if a filter is added at the resonant part,it inevitably affects the resonance state,system output power and efficiency of the WPT.To ensure that the LCR-WPT has good system characteristics,this paper analyzes the mathematical model of the LCR-WPT and deduces a design method for the filter and an efficiency optimization formula.This paper also introduces LCR-WPT applications in two different control schemes.The experimental results show that the low-order harmonic interference is attenuated by 15 d Bu V/m,and the highorder harmonic is basically eliminated and the system still maintains good characteristics.2)Harmonic utilization.In the WPT control system,estimating the coupling coefficient(k)is an essential step to achieve the optimal control over a wide charging range.The signal of traditional WPT system is mainly the current and voltage generated by fundamental.It needs a lot of parameters and complex calculation to estimate k by analyzing these electrical signals,which makes the traditional method of estimating k difficult to have good robustness and apply in practice.In order to simplify the process of estimation,a method of estimating k only by harmonic current is proposed in this paper.Compared with the traditional method,the amount of calculation is greatly reduced.At the same time,the influence of parameter disturbance on the robustness of the results is basically kept within 1%.According to this method,a WPT control system with wide charging range is designed with harmonic current as feedback signal.The main feature of the control system is that it is very easy to estimate k,because the high frequency harmonics can simplify the circuit.By analyzing the system characteristics under different harmonic currents,the system can achieve optimal control in a wider charging range.