**2. Power rails and pickups**

The researchers from the Japan Railway Technical Research Institute proposed a different design of coupling mechanism for the ET. The transmitters are long bipolar coils, and "figure‐ 8" coils are used as the matching pickups as shown in **Figure 6**. The system is able to transfer

Bombardier Primove from Germany is currently leading in WPT technology for EV and ET. Studies have been primarily conducted for better exploitation of the technology. Apparently, the technical information of the WPT system developed by Bombardier Primove has not been published. In 2013, the company proposed a design shown in **Figure 7** to ensure high reliability when powering the EV. The main DC bus is supplied by k‐number of AC/DC substations connected in parallel. This configuration is used to increase the robustness of the system. If one of the AC/DC substations breaks down, that particular substation will be disconnected from the system and other neighboring substations can continue functioning normally, thus avoiding power interruption. Each transmitter cluster is supplied by multiple high‐frequency DC/AC inverters in parallel. Similar to the DC bus, the power supply at the AC bus will not be interrupted if an inverter breaks down. At the receiver side, the train contains a DC bus as shown in **Figure 7**. Multiple receivers are supplying to the DC bus simultaneously via AC/DC rectification. The DC bus powers the motor through a controller. If any of the rectifiers is

damaged, other receivers can continue providing sufficient power to the DC bus [7].

50 kW of power across a 7.5‐mm gap with 10‐kHz frequency [6].

112 Wireless Power Transfer - Fundamentals and Technologies

**Figure 6.** The non‐contact power supply system for railway vehicle.

**Figure 7.** DWPT system for railway vehicle.

Core‐less rectangular coils and bipolar coils are the two general types of coils used in WPT. The University of Auckland proposed using long rectangular rails to transfer power. A larger surface area for road construction necessitates less amount of power to be transferred per surface area. The design is also sensitive to lateral displacement of the electric vehicles. Moreover, a high level of magnetic field leakage occurs at both sides of the rail [10]. KAIST proposed an improved version by adding a magnetic core with an optimized design. Com‐ pared to the transmitter rail proposed by the University of Auckland, the transfer efficiency and transfer distance are increased. However, the construction cost is also higher.

KAIST presented an advanced coupling mechanism design and optimization technology in their past research. In 2009, the first‐generation OLEV was successfully produced. An E‐shaped magnetic core is used as the power transmission rail. The air gap is only 1 cm and the transfer efficiency 80% [2]. A U‐shaped transmission rail was also proposed in the same year by significantly increasing the transmission gap to 17 cm with an efficiency of 72%. In 2010, a skeleton‐type W‐shaped magnetic core is proposed, thus further increasing the transfer distance to 20 cm and efficiency to 83% [2]. From 2011 to 2015, researchers from KAIST designed fourth‐generation I‐shaped bipolar rails and fifth‐generation S‐shaped bipolar rails with even larger transfer gap, narrower frame, and higher efficiency [2]. With bipolar rails, the magnetic field path is parallel to the moving direction of the vehicle instead of being orthogonal to the moving direction. The new design is well suited for DWPT due to its advantages such as high power density, narrow frame, and therefore lower construction complexity, robust to lateral displacement, and lower magnetic field exposure on both sides of the rail [10–12] (**Tables 1** and **2**).

In 2015, KAIST proposed using a dq‐two‐phase transmitter rail for cancelling the zero coupling points along the moving direction [13] using the control method which is relatively complex. A double loop control is implemented by detecting the phase of the primary current. The amplitudes and phases of the d‐q currents are controlled using a phase‐locked loop and DC chopper according to the position of the receiver.


**Table 2.** Wireless power rails and receiving pickups developed by KAIST (From generation 1 to 6).

#### **3. Segment and power supply scheme**

In order to overcome the issues of low transfer efficiency and high sensitivity to the changing parameters in a centralized power supply system, a new segmented scheme is proposed [14]. The voltage at the 50 Hz AC bus is first stepped up to reduce transmission loss. Then, before the segmented transmitters, the voltage is stepped down via the inverter. Constant current is also used at the transmitters. Efficient converter topologies are also reviewed for implementing a centralized power supply system.

(1) Centralized power supply scheme (**Figure 8**)

With the increasing length of the transmitter rail, the bandwidth of the primary side channel becomes narrower. Therefore, the system is more sensitive to the variations of parameters, and the robustness is decreased. The controller for the centralized power supply is relatively


**Type Coreless long coil Bipolar rail**

High power density, narrow design, robust to lateral displacement, low construction complexity, and low

Uneven magnetic field distribution, zero coupling point. High cost due to the usage of ferrite core

level of magnetic field exposure

transfer, coreless, and low manufacturing cost

displacement, large surface area is needed for

**Table 1.** Advantages and disadvantages of commonly used powering rail.

**Table 2.** Wireless power rails and receiving pickups developed by KAIST (From generation 1 to 6).

In order to overcome the issues of low transfer efficiency and high sensitivity to the changing parameters in a centralized power supply system, a new segmented scheme is proposed [14]. The voltage at the 50 Hz AC bus is first stepped up to reduce transmission loss. Then, before the segmented transmitters, the voltage is stepped down via the inverter. Constant current is also used at the transmitters. Efficient converter topologies are also reviewed for implementing

**3. Segment and power supply scheme**

(1) Centralized power supply scheme (**Figure 8**)

a centralized power supply system.

high level of magnetic field exposure

Merits Even magnetic field distribution, stable power

114 Wireless Power Transfer - Fundamentals and Technologies

Demerits Low power density, sensitive to lateral

construction, and


**Figure 8.** Centralized power supply scheme.

**Figure 9.** Power frequency scheme—segmented rail mode.

(2) Power frequency scheme—segmented rail mode (**Figure 9**)

The advantages of segmented rails are as follows:


**d.** Lower self‐inductance, less sensitive to variations in parameters, and therefore increasing the system stability.

However, segmented rails also have the following disadvantages:


(3) High frequency scheme—segmented rail mode (**Figure 10**)

With segmented rails and centralized power supply, the advantages of this design are as follows:


**Figure 10.** High frequency scheme—segmented rail mode.

However, this design has the following disadvantages:


(4) High frequency and high voltage scheme and low voltage and constant current rail mode (**Figure 11**).

**Figure 11.** High frequency and high voltage scheme—low voltage and constant current rail mode.

(5) Combination scheme (**Figure 12**)

**d.** Lower self‐inductance, less sensitive to variations in parameters, and therefore increasing

**a.** High number converters, difficult to control and high maintenance and construction cost; **b.** High number of components is required and therefore low reliability of the whole system.

With segmented rails and centralized power supply, the advantages of this design are as

**c.** Lower self‐inductance, less sensitive to variations in parameters, increases the system

**a.** When the power supply breaks down, all of the segmented rails will stop functioning,

(4) High frequency and high voltage scheme and low voltage and constant current rail mode

**b.** High loss in the cable connecting the power supply to the segmented rails;

**c.** High capacity power supply and therefore large requirements of the components;

**b.** Different segments can be activated at different time periods, lesser power loss;

However, segmented rails also have the following disadvantages:

(3) High frequency scheme—segmented rail mode (**Figure 10**)

**a.** Lesser power converter units, easier to maintain;

**Figure 10.** High frequency scheme—segmented rail mode.

thus lowering the system reliability;

(**Figure 11**).

However, this design has the following disadvantages:

the system stability.

116 Wireless Power Transfer - Fundamentals and Technologies

follows:

stability.

This type of rails combines the advantages of abovementioned rails; however, the system is complex and only suitable for a large‐scale dynamic charging system.

### **4. Circuit topologies and impedance matching**

In the DWPT system, the gap between the receiver and transmitter is always changing. Different cars have different heights with respect to the ground and the coupling coefficient will varies significantly. Coupling coefficient is an important parameter in WPT. If the value is too low, the efficiency may drop considerably. Contrarily, frequency splitting phenomena may occur if the coupling coefficient is too high, and the system functions in the unstable region. Therefore, the circuit topology should be designed to be insensitive to coupling changes.

In order to achieve a steady power supply with variations in coupling and to increase the system stability in the light‐load region, an LCLC topology can be used. The current at the primary is kept constant and stress on switches is reduced during on‐off. At the receiver side, a parallel‐T configuration can increase the tolerance of the system toward coupling variation. The proposed topology is shown in **Figure 13**.

**Figure 13.** Circuit topology of double LCLC.

The transmitter current is written as follows:

$$\dot{a}\_p = (U\_i - U\_{r0}) / (\alpha\_0 L\_p) \tag{1}$$

With *λ* = *L <sup>s</sup>* / *L* <sup>2</sup> <1 as the load coefficient, the receiver output voltage is as follows:

$$U\_o = U\_{oc} \not\!\!\mathcal{X} = \alpha\_0 k \sqrt{L\_p L\_s} I\_p \not\!\!\chi\tag{2}$$

The output voltage is 1/λ times the receiver voltage. A step‐up voltage converter is used to provide sufficient power when coupling is low, therefore increasing the tolerance of the system against lateral displacement.

The voltage ratio and efficiency are given as follows:

$$\begin{cases} G = MR\lambda \Big/ L\_0 (R\lambda^2 + r\_s) + r\_0 C\_p \left( M^2 \alpha\_0^2 + r\_p (R\lambda^2 + r\_s) \right) \\ \eta = \alpha\_0^2 M^2 R\lambda^2 L\_0 \Big/ (\alpha\_0^2 M^2 + r\_\rho (R\lambda^2 + r\_s)) (L\_0 (R\lambda^2 + r\_s) + C\_p r\_0 (\alpha\_0^2 M^2 + r\_\rho (R\lambda^2 + r\_s))) \end{cases} \tag{3}$$

where *r*<sup>0</sup> is the internal resistance of the inverter circuit, *r*<sup>p</sup> is the resistance of the transmitter, and *r*p is the resistance of the receiver.

The power and efficiency curves are given in **Figure 14**. The efficiency is high at the low‐ coupling region which is particularly important for the DWPT application.

As shown by the curves in **Figure 15**, the efficiency and power are significantly improved for different loads and coupling coefficient compared to series topology.

**Figure 14.** Efficiency and voltage gain vs. coupling coefficient.

region. Therefore, the circuit topology should be designed to be insensitive to coupling

In order to achieve a steady power supply with variations in coupling and to increase the system stability in the light‐load region, an LCLC topology can be used. The current at the primary is kept constant and stress on switches is reduced during on‐off. At the receiver side, a parallel‐T configuration can increase the tolerance of the system toward coupling variation.

0 0 ( )/( ) *p ir p i UU L* = -

With *λ* = *L <sup>s</sup>* / *L* <sup>2</sup> <1 as the load coefficient, the receiver output voltage is as follows:

*U U k LLI o oc* = = lw

w

The output voltage is 1/λ times the receiver voltage. A step‐up voltage converter is used to provide sufficient power when coupling is low, therefore increasing the tolerance of the system

22 2 22 2 2 22 2

 l

where *r*<sup>0</sup> is the internal resistance of the inverter circuit, *r*<sup>p</sup> is the resistance of the transmitter,

The power and efficiency curves are given in **Figure 14**. The efficiency is high at the low‐

As shown by the curves in **Figure 15**, the efficiency and power are significantly improved for

l

( ( ))( ( ) ( ( )))

 l

0 0

++ +

w

*sp p s*

*r M rR r*

l

+

(3)

(1)

<sup>0</sup> *psp* (2)

changes.

The proposed topology is shown in **Figure 13**.

118 Wireless Power Transfer - Fundamentals and Technologies

**Figure 13.** Circuit topology of double LCLC.

against lateral displacement.

*G*

=

ì

ï í ïî

w 0

l wh

and *r*p is the resistance of the receiver.

ll

The transmitter current is written as follows:

The voltage ratio and efficiency are given as follows:

0 0 00

= + +

2 22 2 0 0

*p*

*MR L R r r C M r R r*

( ) ( ( ))

*p s*

l

*MR L M r R r L R r C*

*s p s*

 w

coupling region which is particularly important for the DWPT application.

different loads and coupling coefficient compared to series topology.

++ + +

**Figure 15.** Power and efficiency of the two kinds of structure vs. coupling coefficient.

While designing the circuit of WPT, the compensation is performed under no‐load condition. In normal operating condition, frequency tracking is used to ensure resonance by keeping the same phase between primary voltage and primary current [12]. Besides, to ensure the EMC and system stability, control is used to achieve constant current. The magnetic field from the transmitter is in steady state. For example, in the WPT system developed by KAIST, the input voltage of the inverter is adjusted using a three‐phase thyristor converter shown in **Fig‐ ure 16** to achieve constant current at the transmitter.

**Figure 16.** Diagram of the KAIST IPTS showing a power inverter, a power supply rail, and a pickup.

For the secondary side, in order to realize constant current, constant voltage, or constant power, a DC/DC converter is usually implemented. **Figures 17** and **18** show the DC/DC converters used in the WPT systems of the University of Auckland and KAIST [15, 16].

**Figure 17.** Secondary DC/DC converter.

**Figure 18.** Functional diagram of OLEV power receiver system.

**Figure 19** shows a secondary‐side circuit which consists of both controllable rectifier and DC/ DC converter. SPWM synchronous rectification is employed at the controllable rectifier. The duty cycle of the rectifier is regulated through SPWM; the effective resistance can be adjusted in the range of Rload ~*∞*. While for a boost converter, the effective resistance can be in the range of 0∼*∞*. Therefore, any desired values of the effective resistance can be realized to improve the system overall efficiency.

**Figure 19.** Dynamic impedance adjustment for secondary side pickups.

#### **5. Control strategies**

**Figure 16.** Diagram of the KAIST IPTS showing a power inverter, a power supply rail, and a pickup.

used in the WPT systems of the University of Auckland and KAIST [15, 16].

**Figure 17.** Secondary DC/DC converter.

120 Wireless Power Transfer - Fundamentals and Technologies

**Figure 18.** Functional diagram of OLEV power receiver system.

For the secondary side, in order to realize constant current, constant voltage, or constant power, a DC/DC converter is usually implemented. **Figures 17** and **18** show the DC/DC converters

> Three types of control were proposed for DWPT: primary control, secondary control, and double‐side control. The University of Auckland proposed adjusting the duty cycle of the inverter to control primary resonant current, simplifying the system configuration [17]. KAIST designed constant current control at the primary. A DC/DC converter is added before the inverter, and the DC voltage from the main line is adjusted to achieve constant current for different loads [13]. The main objective of primary control is to produce constant magnetic field, then robust power control can be implemented. The University of Tokyo utilizes secondary control strategy. A buck converter is added after the rectifier [4]. General state space averaging (GSSA) is used to construct the small‐signal model. Constant power or maximum efficiency is then realized using PI pole placement [18]. In addition, controllable rectifier and hysteresis comparator are also proposed for implementation at the secondary side to control the output power or maximum efficiency [19]. Double‐side control can be with or without communication. ORNL combines the control of both sides, using a closed loop control and frequency adjustment with communication to realize wireless charging [3]. The Hong Kong University proposed simultaneous control of both power and maximum efficiency without communication. The smallest input power is searched to realize constant output power of the inverter [20] (**Table 3**).


**Table 3.** Comparison of advantages and disadvantages of various control strategies.

The DWPT system is subject to disturbances such as variation of mutual inductance caused by movement of the vehicles. New robust control strategies, which are more superior to PID controllers [4,18,19] in disturbance suppression, are currently being studied.

#### **6. Electromagnetic interference**

The DWPT uses a high‐frequency, strong magnetic field to transfer power wirelessly. The EMC is an important consideration as the DPWT system is surrounded by many sensitive electronic circuits. The requirements include shielding design, frequency allocation, and grounding design. According to the standard set by the International Commission on Non‐Ionizing Radiation Protection (ICNIRP), the current density exposed to the public is 200 mA/m2 , when the frequency is 100 kHz. The values may affect the nerve system of human body. The limit of specific absorption rate (SAR) is 2 W/kg and power density is 10 W/m2 ; if the exposure to the human body is higher than these limits, heating of the human tissues may occur (**Table 4**).


**Table 4.** Comparison of merit and demerit of various magnetic shielding methods.

The suppression of the leakage field can be divided into active shielding and passive shielding. In passive shielding, a magnetic path is created using magnetic material or canceling field using a low magnetic permeability metallic conductor [21–23]. The self‐inductance and mutual inductance are increased when using magnetic material. The magnetic flux distribution is improved due to higher coupling coefficient, and transfer loss is decreased. However, the shielding effect is limited. Metallic shield is widely used in a high‐frequency magnetic field to suppress electromagnetic interference. Both KAIST and ORNL utilize this kind of shielding method. The advantages include simple design and easy to use. However, metallic shielding cannot cover the transmitter and receiver completely. The exposed conductor is subject to friction and eddy current which will increase the heat loss. KAIST proposed a new active shielding method in 2015. A conventional ferrite plate is embedded in multiple metallic sheets as shown in **Figure 20**. Experimental results show that the magnetic interference is effectively reduced [24].

**Control strategy**

**Shielding method**

Merits Fully enclosed metal

Demerits Eddy loss affecting the system efficiency

conductor housing provide excellent shielding effect

Merits Constant current in

Demerits Unable to control for

transmitter, steady magnetic field, no need to consider reflected impedance

122 Wireless Power Transfer - Fundamentals and Technologies

maximum efficiency, limited control of output load, and constant current charging is not realizable

**6. Electromagnetic interference**

**Primary control Secondary control Both side control**

Constant charging current, constant charging voltage, or maximum efficiency

Adjustable range of the secondary side is limited, and accurate model is required

controllers [4,18,19] in disturbance suppression, are currently being studied.

specific absorption rate (SAR) is 2 W/kg and power density is 10 W/m2

low loss

**Table 4.** Comparison of merit and demerit of various magnetic shielding methods.

The DWPT system is subject to disturbances such as variation of mutual inductance caused by movement of the vehicles. New robust control strategies, which are more superior to PID

The DWPT uses a high‐frequency, strong magnetic field to transfer power wirelessly. The EMC is an important consideration as the DPWT system is surrounded by many sensitive electronic circuits. The requirements include shielding design, frequency allocation, and grounding design. According to the standard set by the International Commission on Non‐Ionizing Radiation Protection (ICNIRP), the current density exposed to the public is 200 mA/m2

the frequency is 100 kHz. The values may affect the nerve system of human body. The limit of

human body is higher than these limits, heating of the human tissues may occur (**Table 4**).

Magnetic field shaping, increasing coupling coefficient and therefore

**Metal conductor Magnetic material Active shielding Resonant reactive shielding**

Limited shielding effect Additional coil lower

Flexible placement, good shielding effect

the system efficiency

**Table 3.** Comparison of advantages and disadvantages of various control strategies.

**With close‐loop communication**

Both desired power and maximum efficiency are achievable simultaneously

Additional wireless communication is required, lower the system reliability and real‐time performance

**Without close‐loop communication**

Both desired power and maximum efficiency are achievable simultaneously

Conflict control between primary side and secondary side

, when

; if the exposure to the

Does not consume power from the system, controllable

Difficult to design, complex

configuration

**Figure 20.** Ferrite shielding structure using an embedded metal sheet.

Regarding active shielding, additional coils with or without power supply are implemented at the WPT system to create a cancelling field as shown in **Figure 21**. Compared to metallic shielding, the space required is smaller.

**Figure 21.** Magnetic field cancellation using a resonant coil.

KAIST published a paper in 2013, proposing an active shielding method using a resonant coil. A switching array is used to change the values of compensated capacitors, thereby controlling the amplitude and phase of the cancelling field. An experiment was performed using green public transportation [25]. In 2015, an improved version using double loop and phase adjust‐ ment to achieve resonance was proposed to achieve an active shielding without power supply. The shielding coils are placed at the side of the coupling mechanism as shown in **Figure 22**. The current induced by leakage field is then sensed. Magnetic field with the same amplitude but opposite polarity with the leakage is then created for field cancellation [26].

**Figure 22.** Resonant reactive power shielding with double coils and four capacitors.

In 2013, ORNL proposed using an aluminum board to reduce electromagnetic interference [27]. As shown in **Figure 23**, a 1‐mm‐thick aluminum shield is placed above the cables. The magnetic field measured at the passenger‐side front tire is reduced from 18.72 μT to 3.22 μT.

**Figure 23.** Suppression of magnetic field after adding aluminum plate and its effect.
