Inner Resonance and Outer Current Based Control Strategy for Inductive Power Transfer System Used in Wireless Charging for Electric Vehicles

Shi W., Dong J., Soeiro T.B., Riekerk C., Grazian F., Yu G., Bauer P.

Due to the urgent desire for a fast, convenient, and efficient battery charging technology for electric vehicle (EV) users, extensive research has been conducted into the design of high-power inductive power transfer (IPT) systems. However, there are few studies that formulate the design as a multiobjective optimization (MOO) research question considering both the aligned and misaligned performances and validate the optimal results in a full-scale prototype. This article presents a comprehensive MOO design guideline for highly efficient IPT systems and demonstrates it by a highly efficient 20-kW IPT system with the dc–dc efficiency of 97.2% at the aligned condition and 94.1% at 150-mm lateral misalignment. This achievement is a leading power conversion efficiency metric compared to IPT EV charging systems disseminated in today’s literature. Herein, a general analytical method is proposed to compare the performances of different compensation circuits in terms of the maximum efficiency, voltage/current stresses, and misalignment tolerance. An MOO method is proposed to find the optimal design of the charging pads, taking the aligned/misaligned efficiency and area/gravimetric power density as the objectives. Finally, a prototype is built according to the MOO results. The charging pad dimension and total weight, including the housing material, are 516*552*60 mm 3 /25 kg for the transmitter and 514*562*60 mm 3 /21 kg for the receiver. Correspondingly, the gravimetric, volumetric, and area power density are 0.435 kW/kg, 581 kW/m 3 , and 69.1 kW/m 2 , respectively. The measured efficiency agrees with the anticipated value derived from the given analytical models.

Ouyang Q., Xu G., Liu K., Wang Z.

This study presents a user-involved wireless battery charging approach for electric vehicles, which enables the battery to reach the user-specified state by regulating the charging current provided by a wireless charger with double-sided inductor–capacitor–capacitor compensation topology. Specifically, an optimal state of charge (SOC) trajectory is first generated through formulating and solving an optimisation issue with the consideration of user demand and operating constraints. Next, a cascade control strategy is proposed to regulate the wireless charger to make the battery track the scheduled charging trajectory. The out loop control designs suitable charging current for SOC trajectory tracking, which can effectively suppress the violation of the charging constraints by on-line compensating the battery model bias. In the inner loop, a fuzzy proportion–integration (PI) algorithm is proposed to control the wireless charger to provide the charging current designed by the out loop, in which fuzzy logic algorithm is utilised to on-line tune the PI gains to improve the control performance. Extensive real-time results demonstrate the effectiveness and promise of the proposed user-involved wireless battery charging control strategy.

Goud J.S., R. K., R. K., Singh B.

Li-ion batteries are the most popular rechargeable batteries in applications like consumer electronics and electric vehicles in comparison with other rechargeable battery chemistries due to its high specific energy and longer life cycle. The reliability and life of the battery power devices depends on state of charge (SOC) and the charging protocol used for charging the battery. In this paper, low frequency ripple current (LFRC) charging of Li-Ion battery using the Zeta bi-directional DC-DC converter is presented. Charging the battery at the optimal frequency with ripple current results in improved charge transfer efficiency, improved charging time and thereby improves the life of the battery. To realize the LFRC charging technique, Zeta bidirectional converter and mathematical model of 48V, 4Ah Li-Ion battery is designed and simulated in MATLAB/ Simulink platform. Moreover, an experimental prototype is developed to validate the LFRC charging technique.

Peng W., Chen Z.

Three ferrite component structures are presented and incorporated into the planar magnetically coupled resonance (MCR) wireless power transfer (WPT) systems in this paper. They are then employed to build two WPT systems which are simulated and tested. The ferrite core and ferrite plate being introduced improve the transmission efficiency by 16.2%. The ferrite plate and ferrite sheet effectively mitigate the magnetic leakage and restore the transmission efficiency when metal object placed in proximity of the receiver. Our results show that the proposed ferrite configurations can benefit the planar WPT systems for small electronic devices.

Minnaert B., Stevens N.

Nowadays, near-field wireless power transfer is realized by inductive or capacitive coupling. Power transmission is respectively accomplished by a time-varying magnetic or electric field as medium. Recently, mixed or hybrid near-field wireless power transfer is being developed as a possible mean to increase the power density of the system by utilizing both the magnetic and the electric near-field. The fundamental basics of mixed coupling are well understood. However, the implications of the mixed coupling theory on wireless power transfer applications is not rigorously described. Moreover, no general description is available that allows for a detailed comparison between current hybrid systems, especially for a series topology of inductive and capacitive coupling. In this work, we analytically solve a general mixed wireless power transfer configuration with series topology. We determine the optimal load to maximize the amount of power transfer and calculate the maximum achievable output power. The analytical derivation is validated by numerical simulation in SPICE. Our solution provides insight in the mixed wireless link and can serve as a reference point to evaluate the performance of mixed systems with regard to power transfer.

Zahedmanesh A., Sutanto D., Muttaqi K.M.

Despite the significant advantages of Plugin Electric Vehicles (PEVs), the mass charging of PEVs can cause an increase of the peak load demand and worsen the voltage quality specifically in Low Voltage (LV) distribution networks. In this context, different technical approaches and charging control strategies have been proposed. Nevertheless, most of these methods necessitate advanced communication systems. In this work, based on the SAE J1772 standard, initially an interface unit for controlling the charging current of PEVs is introduced. A droop charging control system, which only needs local measurements, is proposed for the AC charging of PEVs. Using the field measurements and existing statistical studies, the PEV charging loads are modelled and the capability of the proposed system is evaluated for an LV test system with different electric loads and high penetration of PEVs. The results of the simulations confirm the feasibility of the proposed method.

Gao Y., Ho Tse Z.T., Ginart A.

The switching frequency of inductive power transfer (IPT) is sensitive to the air gap between the primary and secondary coils. The mutual inductance might change significantly for different air gaps, resulting in a shift for the switching frequency. The mutual inductance is usually acquired through experimental measurement, finite element analysis, or frequency tracking control. In this paper, Neumann's formula is simplified to compute the mutual inductance of two coils. A frequency-gap model is presented to offer a simple way to calculate the unity-gain frequency for a symmetrical series-series (SS) IPT. Finite element simulation and experiment were conducted to verify the model. The experimental results show that the unity-gain frequency can be output within an average computational time of 9.8ms, and the gain fluctuates from 0.95 to 1.05 when the air gap changes from 7.5cm to 25cm.

Li M., Yao G., Zhou L., Yin Z.

The specific harmonic analysis of bidirectional LCL-IPT system is presented in the paper. A mathematical model is built to obtain both current and power harmonic contents. The converter loss, devices properties, charging performance and harmonic power flow have been investigated to discuss the effect of harmonics on the system performance. Simulation and experiment results reveal that there are high harmonic currents in the system, which results in efficiency reduction and devices damage. The harmonic power flow is negligibly low compared to the fundamental.

Liu M., Fu M., Ma C.

Magnetic resonance coupling working at megahertz (MHz) is widely considered as a promising technology for the mid-range transfer of a medium amount of power. It is known that the soft-switching-based Class E rectifiers are suitable for high-frequency rectification, and thus potentially improve the overall efficiency of MHz wireless power transfer (WPT) systems. This paper reports new results on optimized parameter design of a MHz WPT system based on the analytical derivation of a Class E current-driven rectifier. The input impedance of the Class E rectifier is accurately derived, for the first time, considering the on-resistance of the diode and the equivalent series resistance of the filter inductor. This derived input impedance is then used to develop and guide design procedures that determine the optimal parameters of the rectifier, coupling coils, and a Class E PA in an example 6.78-MHz WPT system. Furthermore, the efficiencies of these three components and the overall WPT system are also analytically derived for design and evaluation purposes. In the final experiments, the analytical results are found to well match the experimental results. With loosely coupled coils (mutual inductance coefficient $k$ =0.1327), the experimental 6.78-MHz WPT system can achieve 84% efficiency at a power level of 20 Watts.

Waltrich G., Hendrix M.A., Duarte J.L.

The success of electric vehicles (EVs) greatly depends on battery size, cost, lifetime, and capacity. The battery capacity of EVs has to contain enough energy to allow EV users to drive, at least, the minimum distance that internal combustion engine vehicles take. However, even with very efficient EVs and combining all the available highly efficient EV technologies, the battery capacity still needs to be large. Thus, one alternative to reduce the battery size is to improve the charging infrastructure by implementing sufficient public/private fast-charging locations, which will significantly increase the EV driving range with relatively low battery capacities. However, this kind of infrastructure requires charging with higher current capability, due to the high power levels involved in the fast-charging process. In this paper, a bidirectional dc/dc converter with six inverter legs connected to a three-phase output is proposed. The converter is similar to a three-phase dual-active-bridge (DAB) converter with more inverter legs in parallel. These additional inverter legs increase the converter current capability, without affecting the DAB main characteristics, preserving similar modulation techniques and DAB advantages. An accurate study about the proposed topology is carried out in this paper, and simulations and experimental results are shown for a 20-kW prototype, validating the theoretical analysis.

Dai J., Ludois D.C.

Inductive power transfer (IPT) and capacitive power transfer (CPT) are the two most pervasive methods of wireless power transfer (WPT). IPT is the most common and is applicable to many power levels and gap distances. Conversely, CPT is only applicable for power transfer applications with inherently small gap distances due to constraints on the developed voltage. Despite limitations on gap distance, CPT has been shown to be viable in kilowatt power level applications. This paper provides a critical comparison of IPT and CPT for small gap applications, wherein the theoretical and empirical limitations of each approach are established. A survey of empirical WPT data across diverse applications in the last decade using IPT and CPT technology graphically compares the two approaches in power level, gap distance, operational frequency, and efficiency, among other aspects. The coupler volumetric power density constrained to small gap sizes is analytically established through theoretical physical limitations of IPT and CPT. Finally, guidelines for selecting IPT or CPT in small gap systems are presented.

Zhang Y., Lu T., Zhao Z., He F., Chen K., Yuan L.

In multiple receivers of resonant wireless power transfer, selective power flow among the loads is an important issue. This paper proposes a new method to control power division. The two-coil structure with different resonant frequencies of the sending and receiving loops is modeled and analyzed. The efficiency is proved to peak at the resonant frequency of the receiving loop, regardless of the resonant frequency of the sending loop. Using this feature, selective power transfer can be achieved by setting the receiving loops at different resonant frequencies. The efficiency of a particular load is greatly influenced by the driving frequency. The multiple-load system with different resonant frequencies is modeled and the efficiency expression of each load is deduced. The mutual inductances of the receiving coils have a small impact on the efficiency distribution. The closer the resonant frequencies of the receiving loops, the less isolated the related loads. The calculations and the experiments confirm the analysis.

Ni Q., Quan L., Zhu X., Du Y., Shi K.

In this paper, energy management control strategies are proposed for hybrid electric vehicle with brushless dual-rotor flux switching permanent magnet (BDRFSPM) motor. The proposed methods not only make the output speed correspond to the working condition, but also make the internal combustion engine operate in the fuel saving mode. The corresponding battery and internal combustion engine performances are analyzed in the combined working conditions of CYC_CBDTRUCK and CYC_HWFET in detail. Finally the reasonableness and real effect of the control method is verified by the co-simulation of MATLAB and advisor analysis.

Boys J.T., Covic G.A.

Inductive power transfer (IPT) is now recognised as one of the ?hottest? research areas in Electrical Engineering combining the EE foundation studies of electricity and magnetism with power electronics and microprocessor control. But this acceptance is very recent ? as recently as 20 years ago IPT was widely supposed to be completely impractical and papers on it were in very short supply. This paper describes how one research group at the University of Auckland went from power electronics and motor control to international recognition in IPT over these past 20 years. It is a simple story combining technology and people and especially people with the vision to see what can be done and how systems can be built on those principles to offer solutions to engineering problems that hitherto had no possibility of a solution. The paper covers the story in a people oriented foreword that describes the original development from serendipity to a Daifuku prototype, and then as a more conventional paper from that prototype to a wide variety of IPT systems up to the possibility of roadway at power levels of 10-30 kW or more, with high efficiency and wide tolerance to misalignment.