IEEE Electrification Magazine - June 2020 - 56
The compensation
circuit, consisting
of inductors and
capacitors, works
with the capacitive
coupler and forms
the resonant circuit.
20
15
10
10 MHz
5 MHz
2 MHz
1 MHz
5
0
0
1
2
3
4
5
6
Plate Voltage V1 (kV)
7
capacitance from vehicle to ground is
10 nF. This estimation shows that for a
15-kW system, the chassis voltage is
maintained below the safety requirement when the frequency is higher
than 2 MHz.
High-power, long-distance, and
high-efficiency CPT technology has
been realized and implemented. For
example, a grounded CPT prototype
has been built in the laboratory, as
displayed in Figure 8. The top plate
acts as the chassis, and the bottom
plate is connected to the ground. The
stray capacitance from the chassis to ground provides the
current-returning path. Experiments demonstrate that
high power is achieved at high efficiency and at a safe
chassis voltage.
The following are advantages of CPT technology:
xx
Low cost and low weight: The CPT system only requires
metal materials to generate electric fields. Its shape,
size, and thickness have no significant influence on
the transferred power and efficiency. It provides flexibility in the installation.
xx
High reliability and long lifetime: Considering the vehicle's weight on pavement, metal plates are much
more reliable than coils made by Litz wire and ferrite.
Chassis Voltage (V)
System Power PM (kW)
system power when the coupling
capacitance is 50 pF. It demonstrates
that the power increases with the
voltage and frequency. When the
voltage is 5 kV and the frequency is
2 MHz, the system power can reach
15 kW, which is sufficient to charge
an electric vehicle in moving status.
The transfer efficiency of a CPT
system is provided in Figure 6, indicating the relationship with the coupling coefficient kC and the component
quality factor (Q). Increasing both kC
and Q contribute to improving the
efficiency. For example, as long as kC is larger than 0.1, the
transfer efficiency can easily achieve 90%.
Meanwhile, the chassis voltage on the vehicle side is
also an important concern for practical applications.
Based on IEEE Standard C95.1, the root mean square value
of the current flowing through a human body should be
limited to below 16.7 mA at the frequency range of
100 kHz-110 MHz. If the contact resistance from the
human body to ground is 500 Ω, the chassis voltage should
be lower than 8.35 V for safety concerns. Therefore, the
chassis voltage is estimated, as illustrated in Figure 7. In
this analysis, considering the physical size of an electric
vehicle, the mutual capacitance is 50 pF and the stray
12
10
8
6
4
2
0
0
System Efficiency (%)
100
95
90
kC = 0.3
kC = 0.2
kC = 0.1
kC = 0.05
200
300
Quality Factor (Q )
15
L2
Rectifier
80
75
12
conditions. When the mutual capacitance C M = 50 pF, the chassis
stray capacitance is 10 nF.
Load
85
6
9
Power (kW)
Figure 7. The chassis voltage at different power and frequency
tions when the coupling capacitance C M = 50 pF.
56
3
8
Figure 5. System power PM at different voltage and frequency condi-
70
100
1 MHz
2 MHz
5 MHz
10 MHz
400
Inverter
L1
The Top Plate Acts as the Chassis
500
The Bottom Plate Is
Connected
to the Earth Ground
Two Inner Plates Are
Placed Between the Chassis
and the Ground
Figure 6. The efficiency of a CPT system at different coupling coeffi-
Figure 8. A lab-based prototype of a grounded CPT system used for
cient kC and component Q.
high-power electric vehicle-charging applications.
I E E E E l e c t r i f i cati o n M agaz ine / J UN E 2020
IEEE Electrification Magazine - June 2020
Table of Contents for the Digital Edition of IEEE Electrification Magazine - June 2020
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