2014 년도 한국철도학회 추계학술대회 논문집 KSR2014A172
Conceptual design of contactless power transfer system for high speed maglev 손연*†, 이창영*, 한영재** Yan Sun *†, Changyoung Lee*, Youngjae Han*
Abstract Contactless power transfer (CPT) is a promising technology for various applications and contactless power transfer system based on resonance coupling method is proposed to solve mid-range power transfer between transmitting antenna and receiving antenna. There are many researches about wireless charging for electrical vehicle or smart phone and 180kW/60kHz wireless power supply has already been realized to provide power for wireless tram. Based on current research of CPT, this paper will introduce a conceptual design of CPT system for high speed maglev and high temperature superconducting (HTS) antenna is proposed to realize higher power transfer efficiency.
Keywords : High speed maglev, Contactless power transfer, high temperature superconducting, Conceptual design
1. Introduction
Wireless energy transfer is a process whereby electrical energy is transmitted from a power source to an electrical load that does not have a built-in power source, without the use of interconnecting wires. Wireless energy transmission is useful in cases where interconnecting wires are inconvenient, hazardous, or impossible [1]-[2]. According to power transfer principle, there are three different fundamental methods for wireless energy transfer. They can be transferred using radiative methods that involve beam power/lasers, radio or microwave transmissions, electromagnetic induction, and magnetic coupling resonance [3]. CPT has been an active research topic since 1960s, and CPT technology has been proposed to a variety of applications, such as biomedical devices, automotive systems, industrial manufacturing and test systems, and in some consumer electronics as well [4]. Regarding the application of CPT systems to transportation applications, important achievements have been made by KAIST. Online electric vehicle (OLVE) developed by KAIST is now in operation at Seoul Grand park, which realized wireless dynamic charging while a vehicle is in motion on a road. In this scheme, a road-embedded power supply system
† 교신저자: 과학기술연합대학원대학교 한국철도기술연구원 철도시스템공학 ([email protected]) * 한국철도기술연구원, 자기부상철도연구팀
and a power receiver system in a vehicle are required [4]. Based on KAIST’s research, 180kW/60kHz CPT system for wireless tram has been successfully developed and tested [5]. Inductive power supply (IPS) system was developed for Tr09 as an external source for on board power consumption while vehicle travels below 100km/h and during stops [6]. To improve power transfer efficiency, wireless charging system for electric vehicle from HTS antenna to normal conducting receiver was proposed [7]. Although there are different application requirements, power transfer distance and power transfer efficiency are two important factors for CPT research. In this paper, we will introduce principle of magnetically coupled resonant CPT and analyze power transfer efficiency of this method first, and then proposed a conceptual design of CPT model for high speed maglev. HTS antenna is considered to realize higher power transfer efficiency.
2. Basic principle of CPT using magnetically coupled resonant
2.1 CPT based on magnetically coupled resonant introduction
Contactless power transfer (CPT) based on the magnetic resonance and near-field coupling of two-loop resonators was reported by Tesla a century ago. Compared with radiative power transfer methods, MCR- CPT can realized several hundreds of watts power transfer while radiative method only can transfer several mill watt power, because long distance radiative power transfer has great energy loss due to the omni-directional nature of radiative power emission. And compared with electromagnetic induction method, MCR-CPT can extend power transfer distance. As a mid-range CPT technology, magnetically coupled resonant CPT (MCR-CPT) has become a hot issue since the research team of MIT released their research findings in 2007 [8]. In this paper, mid-range applications refer to the situation that the transmission distance between the power source and the load is larger than the dimension of the antenna coil.
2.2 Principle of CPT using magnetically coupled resonant
2.2.1 Power transfer efficiency analysis A resonant CPT system consists of magnetically coupled transmitting and receiving coils as well as power electronic circuits as shown in Fig. 1(a). Equivalent circuit model is illustrated in Fig. 1(b), where
L1 and L2 are, respectively, the inductances of the transmitting and receiving coils. When a transmitting coil is excited by the source, the transmitting and receiving coils are coupled magnetically (M is the mutual inductance). For the maximum amount of power transfer, capacitors C1 and C2 are connected in series with each coil in the model. R1 and R2 are the internal resistances of each coil, and RL is the load resistance. According to Kirchholf’s voltage law, we can get circuit loop equation as Eq. (1). From the model, the loop impedance of transmitter Z1 and the loop impedance of receiver Z2 can be calculated as Eq. (2). After deriving current in each loop, we can calculate power transfer efficiency by Eq. (5). Magnetic resonance can be utilized with a capacitor to minimize the magnitude of the reactance and
maximize the power transfer capability between magnetically coupled coils. Only when primary and secondary coil has resonance at same time ( Z11 R Z22 RR L ), the total coupled CPT system can have highest power transfer frequency as calculated by Eq. (6).
(a) Configuration of CPT system base on MCR (b) Equivalent circuit modle of CPT system based on MCR Fig. 1 Principle of MCR-CPT system
1 1 RjL11 jM ZRjL11 1 U jC I jC in 1 1 (1) 1 (2) 01I 1 jM R jL R 2 Z RjL R 22jC L 22 2 L 2 jC 2
ZU2 in I1 2 ZZ12 M PUIin in 1 (3) jMU 2 (4) in PIRout 2 L I2 2 ZZ12 M
2 2 P MR MR out L 100% (5) L 100% (6) 2 2 Pin ZZ M Z RR R M R R 12 2 12LL 2 From Eq. (6), suppose inductances L, capacitors C and internal resistances R of each coil are fixed, load
resistance RL, mutual inductance M, and resonant frequency f0 ( 2 f0 ) are the variables that affect power transfer efficiency. is decided by power source frequency fs , mutual inductance M of two coil shown in Fig. 1(a) can be calculated with Neumann formula as [3]
2sin2 1 M NN r r b2 d 01212air 0 22 1sinb (7) 4rr b2 12 2 2 rr12 d
Where, N1 N2 are number of turns, r1 r2 are coil radius and d is distance between transmitter coil and receiver coil. From the above analysis we can derive that power source frequency are function of load
resistance RL, power source frequency fs and distance between transmitter coil and receiver coil d .
f RfdLs,, (8)
2.2.2 Quality factor Quality factor Q is defined in terms of the ratio of the energy stored in the resonator to the energy supplied by a generator. In electrical systems, the stored energy is the sum of energies stored in lossless inductors and capacitors; the lost energy is the sum of the energies dissipated in resistors per cycle. In ideal series RLC circuit, Q factor is
1 LL Energy stored Qf2 (9) R CR0 Powerloss
Higher Q factor enables larger amount of power transfer since it indicates less power loss.
3. Conceptual design of CPT system for high speed maglev
According to analysis in section 2 and considering high speed maglev configuration, transmitting coil will be installed along the track and mobile pick-up coil will be loaded on the vehicle. Since superconducting wire has a merit for lower material resistance, quality factor is higher than coil made by other material, also it can tune the impedance matching effectively. Further, since superconducting coils have an enough current density, superconducting receiver coil can receive a mass amount of energy [9]. So HTS pick-up coil are proposed. Fig. 2 shows the design of MCR-CPT system for maglev train. When maglev train is in operation, distance between transmitting coil and mobile pick-up coil will not change a lot, so variables that affect power transfer efficiency would be load resistance RL and power source frequency fs . To maximize power transfer efficiency, number of coil turns and coil dimension needs to be considered as well according to Eq. (6).
Fig. 2 Configuration of MCR-CPT system and interface between transmitting and receiving coil
4. Conclusion
MCR-CPT is a hot research issue and many potential applications are proposed by using this power transfer method. For transportation, many research achievements have been made, including power supply for tram, bus and contactless charging for EV. Based on these research MCR-CPT system with HTS pick-up coil is proposed for high speed maglev. By analyzing factors that affect power transfer efficiency, considerations of the conceptual design of MCR-CPT system for high speed maglev are discussed. Future studies will focus on detailed coil design, theoretical calculation of , and experiment analysis.
Reference
[1] http://en.wikipedia.org/wiki/Wireless#Wireless_energy_transfer [2] http://en.wikipedia.org/wiki/Wireless_energy_transfer [3] Huang shizong (2013) Research on wireless energy transmission efficiency based on magnetic coupling resonant theory, Electrical Engineering, PP.23-26 [4] J. Kim, S. Kong, H. Kim, I. S. Suh, N. P. Suh, D. H. Cho, et al., "Coil Design and Shielding Methods for a Magnetic Resonant Wireless Power Transfer System," Proceedings of the Ieee, vol. 101, pp. 1332-1342, Jun 2013. [5] K. Kim, (2013)"A Investigation on the Characteristics in Accordance with Changes in Operating conditions of Wireless Power Transfer for Railway Vehicles," The Korean Institute of Electrical Engineerings
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