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Development & Implementation of Electric Tram System with Wireless Charging Technology
Development & Implementation of Electric Tram System with Wireless Charging Technology
ICT Express. 2014. Jan, 1(1): 34-38
Copyright © 2014, The Korea Institute of Communications and Information Sciences
This is an Open Access article under the terms of the Creative Commons Attribution (CC-BY-NC) License, which permits unrestricted use, distribution and reproduction in any medium, provided that the original work is properly cited.
  • Received : August 23, 2014
  • Accepted : September 22, 2014
  • Published : January 30, 2014
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About the Authors
DongHo Cho
Wireless Power Transfer Research Center(WPTRC), KAIST, 291 Daehak-ro, Yuseong-gu, Daejeon, Korea
GuHo Jung
Wireless Power Transfer Research Center(WPTRC), KAIST, 291 Daehak-ro, Yuseong-gu, Daejeon, Korea
Uooyeol Yoon
Wireless Power Transfer Research Center(WPTRC), KAIST, 291 Daehak-ro, Yuseong-gu, Daejeon, Korea
Byungsong Lee
Korea Railroad Research Institute(KRRI), Korea

Abstract
In this paper, an electric tram system with a wireless power transfer system based on SMFIR technology is presented. The detailed technology of power-line infra, regulator, and pick-up device is described for train application, respectively. Furthermore, implementation and experimental results for wireless power transfer electric tram are presented
I. Introduction
Studies for applying wireless inductive power technology to vehicle system have been being made [1] .. [5] . From a few years ago, KAIST also has developed and commercialized On-Line Electric Vehicle(OLEV) system using wireless magnetic resonance technologies to reduce weight, volume and cost of battery in conventional battery powered electric vehicles [2] .. [5] .
For the first time, the main target for our study focused on on-line electric bus system because bus moves on a regular route and the optimal length of road embedded rail can be calculated considering bus speed, number of bus stops and stoppage time. After that, we applied the developed OLEV technology to train. Fig. 1 shows the overview of a proposed OLEV tram system.
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Overview of proposed OLEV tram system..
As shown in Fig. 1 , this system consists of four components, that is, three-phase power inverter, road embedded power line module, pickup module with rectifier and regulator. The power inverter supplies 60kHz high frequency current to the power line module which is composed of high frequency cable and magnetic core. In case that power inverter supplies high frequency current to the cable, the power line module generates a high frequency magnetic field through power cable and core, and pickup module with rectifier converts this magnetic field to low DC voltage. And power line consists of two 15m power tracks and the regulator outputs high DC voltage appropriate for battery inside tram using this low DC voltage.
II. Overall System
The rated power of motor for electric tram is 180kW, which is very high compared to a small electric car. And an airgap, that is, a distance between power line module and bottom side of the pickup module should be about 70mm for protecting collisions with obstacles on the rail. From this, target development requirements of our OLEV tram system were determined as shown in Table. I
Target development requirements and basic spec. for OLEV tram system
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Target development requirements and basic spec. for OLEV tram system
Fig. 2 shows an overall block diagram and the power circuits of wireless charging system for a proposed OLEV tram system. As illustrated in the internal circuit of power inverter in Fig. 2 , three-phase AC or DC voltage is rectified and controlled as a variable DC-link voltage by phase controlled rectifier or DC/DC converter. This DC-link voltage is converted as an isolated single-phase voltage source by single-phase inverter and high-frequency transformer, where DC blocking capacitor Cb exists for protecting the saturation of transformer and the turn-ratio of for this transformer varies according to the current of power cable flowing inside the power line module.
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Overall block diagram and power circuits of proposed system for electric tram.
III. Wireless Power Transfer Structure
Wireless power transfer of simple inductive coupling technology is realized with two coupled coils. Two coils are mutually coupled with the same resonant frequency and are placed within a very short distance to obtain sufficient efficiency of transferring power. When two coils become more distant each other, transfer efficiency is decreased since magnetic flux generated by primary coil cannot reach a secondary coil, resulting in a decrease of mutual coupling. The trace of magnetic flux has to be modified in order to obtain higher transfer efficiency. An appropriate way is to place material with high permeability, especially ferrite core. Magnetic fluxes tend to be absorbed in the ferrite core, and are radiated at the edge as shown in Fig. 3 . In designing these core structures, primary module width is minimized to reduce the cost of power line module construction, and the thickness of pickup is minimized to reduce the weight of the pickup modules. The designs were supported by theory, finite-element method based computer simulations, and practical experiments. In the power line module, the ferrite cores are not continuous but separated at regular distances. This reduces the cost of the power line module embedded within the rail.
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Shaped magnetic field in resonance(SMFIR) technology
IV. Design of Power Transfer Infrastructure
As shown in the secondary part of transformer, equivalent inductance Lr1 is measured by power cable and core in the power line module, whose value varies with length of rail and is about 60 uH in case of 15 m track length and is higher uH in longer length of track. In case of the power line module shown in Fig. 2 , it is divided into two tracks, that is, #1, #2. Because this power infrastructure has two tracks and power inverter can supply current to each track, the common cable is used. In our system, each track consists of 6 turns of high frequency power cables with internal ritz wires and dual-rail type(E-type) ferrite core.
As shown in Fig. 4(a) , L1 and R1 at the primary side of the high-frequency transformer are the inductance and resistance created by a power line track and transformer, respectively. Cr1 is the resonant capacitance that makes the resonant frequency based on L1 and Cr1 equal to inverter switching frequency finv. Thus, the impedance made by Cr1 and L1 becomes almost zero. Here, M is the mutual inductance between the track and the pickup module and has a low value in this presented system due to large air gap distance. The equivalent circuit at a resonant frequency which is the same as the inverter switching frequency is shown in Fig. 4(b) in the phasor domain. All the inductances are canceled by additional resonant capacitances, and thus, only resistance component exists in this equivalent circuit. A power line module consists of ferrite cores, track power cables and FRP tubes as shown in Fig. 5 , where the FRP tubes are used to protect physically the power cables. We use several turns of power cables to reduce power line power loss and get higher magnetic field for electric tram.
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Equivalent circuits of proposed wireless power transfer system: (a) Actual equivalent circuit and (b) Ideal equivalent circuit in perfect resonance.
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Power line module(transmitter) structure and shape of magnetic field.
V. Design of Power Collection System
Fig. 6 shows the structure of OLEV Wireless Power Transfer (WPT) system. The WPT system in this figure consists of power line and pick-up. The power-line part consists of coil and E-shaped ferrite core. The coils of the pick-up part can be divided into three parts which are left part, right part, and center part. Ferrite core in pick-up has a meander-line shape across loop coil. The ferrite material used here is PL-13 ferrite with relative permeability of 3200. The power-line and pick-up models were simulated by Ansys Maxwell tool which is based on finite element method (FEM). The induced voltage of pick-up depends on the magnetic field generated by excited AC current of power line and this voltage is important parameter in view of evaluating the performance of WPT system. Here, the electromagnetic field (EMF) value should be considered because safety is very important. Fig. 7 shows the simulated result of magnetic field distribution for power collector system, which shows the shaping of magnetic flux to guarantee the safety. Table 2 shows the specification of final design of the power collector. Total power capacity for wireless tram is 180kW which consists of three power collector modules. This pickup system has the air gap of 70 mm and 4 channels.
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Structure of power transmitter and collector system
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Simulated result of magnetic field distribution
Electric specification of power collector system
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Electric specification of power collector system
VI. Implementation and Discussion
- A. Power Inverter
We use a 200kW power inverter with 60kHz switching frequency for electric tram. Controller in power inverter has a dual-loop structure, where outer loop is used for controlling inverter output current to be constant and inner loop is used for pulse width modulation (PWM) control loop to follow the pulse width reference obtained from the outer current control loop. By this dual-loop structure of controller, it is possible to obtain constant inverter output current. Fig. 8 is a pilot system of power inverter used in wireless charging system for electric tram, where input voltage is 750 Vdc.
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Pilot system of power inverter used for electric tram.
- B. Power Line Module
Power line module for electric tram uses 6 turn power cables connected in series inside the rail shown in Fig. 9 . Like electric bus, inductance Lr1 is measured by power cable and core in the power line, which is divided into two segments (or tracks) , that is, #1, #2. Length of each segment is 15 m and resonant freq. of its power line is determined as 60 kHz to obtain higher power efficiency because smaller current of power line module in case of 60 kHz can flow to get the same output voltage of pickup module compared to 20 kHz.
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Power line module for electric tram
- C. Pick-up module
Pick-up module for train application consists of ferrite core, pick-up cable, compensation capacitor, and pick-up case. Differential factors between bus and train application for wireless power transfer are air-gap, resonance frequency, and power capacity. The air-gap of train application is smaller than that of bus application. For example, the air-gap value of bus application is 170 mm, but that of train application is about 70 mm. This is because train is operated along the railway unlike bus driving a general road. The required power of train is larger than that of bus. OLEV bus system needs 100 kW power capacity. However, tram system requires power capacity over 180kW. To achieve high power capacity within similar size of pick-up device, we select the operating frequency of 60 kHz because the increase of resonance frequency in power electronics is proportional to increase of power capacity.
Fig. 10 shows a pilot pick-up module for electric tram application. Fig. 11 shows the pick-up module installed under the body of tram. Pick-up module has the power capacity of 60 kW for each module. The wireless charging tram has 3 pick-up modules, and total capacity is 180 kW.
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Implementation of pick-up module for train application.
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Pick-up module of wireless charging tram.
- D. Experimental Results
As shown in the experimental waveforms of Fig. 12 , the input voltage of the regulator is about 300V at charge-off operation and increases to about 500V in the beginning of charge-on period due to the increase of output current of power inverter. After that, regulator output current is changed from almost zero to 100A. In this case, we can see that the output voltage of regulator is controlled to maintain 800 Vdc of rated voltage.
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Input and output experimental waveforms measured in regulator ; (a) Waveform of regulator output current[50A/div], (b) Waveform of regulator output voltage[200V/div], (c) Waveform of regulator input voltage[200V/div].
The total power efficiency for wireless charging system of electric tram becomes 85 ~ 90% depending on input power as shown in Fig. 13 .
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Total power efficiency measured for electric tram.
VII. Conclusion
In this paper, wireless power transfer tram system based on SMFIR technology has been presented. The detailed technology of inverter, power-line module, regulator, and pick-up device has been described for train application, respectively. Furthermore, implementation and test for wireless power transfer electric tram have been explained. The developed pilot system has 180kW capacity and up to 90% efficiency under the air gap of 70mm and 60 kHz operating frequency. KAIST and KRRI demonstrated that WPT pilot system could support the specification and function for tram application sufficiently.
Through further research, the presented wireless power technology for tram can be extended to trains with higher power capacity like low-weight train, MTR, high speed train and other applications such as green ship, harbor, airplane and robot systems.
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California Partners for Advanced Transit and Highways http://www.path.berkeley.edu/
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