IEEE Electrification Magazine - September 2017 - 27
have been thoroughly verified by finite element method
(FEM) analysis. In general, a power transfer capability of
approximately 500 kW/m2 has been found to be reasonable with the available materials and cooling requirements of the coils, with an operating frequency in the
range of 2-8 kHz. The selected range of operating frequency is also influenced by a preference for using
available H-bridge modules derived from standard
690-V VSC drives based on 1.7-kV insulated-gate-bipolartransistor (IGBT).
An example of FEM analysis showing the energy density
(by color scale) and the power flow (by white lines) in the
cross section between the two coils is shown in Figure 5 for
two different operating conditions. Both cases shown in
the figure correspond to a rated power transfer of 1 MW.
Figure 5(a) shows the operation when the two coils are perfectly aligned at the maximum airgap distance of 50 cm
between the windings. Alternatively, Figure 5(b) shows the
operation with the same power flow but at a misaligned
position and slightly reduced airgap distance, corresponding to expected worst-case misalignments in the forwardbackward direction.
Similar FEM analyses have been used to verify the electrical parameters of the system in various positions and to
design the required shielding for complying with standards for magnetic fields. In general, the area behind the
coils can be completely shielded from the magnetic fields
by the magnetic backplate and the conductive shielding.
Thus, passengers onboard the vessel will not be exposed
to any magnetic field due to the operation of the inductive
charger on the outside of the metallic hull. However, on
the shore side, a certain safety distance should be enforced
around the sending-side coil for ensuring compliance
with the regulations, depending on the amount of shielding installed on the outside of the hull of the vessel and in
the onshore area around the charger.
(a)
System Design and Minimization
of Rating Requirements
For SS-compensated inductive charging systems, the
required current to transfer a specific power will generally
increase for longer transmission distances. The reduced
coupling resulting from the enlarging airgap distance will
also result in lower equivalent magnetizing inductance,
which implies that the voltage required for driving the
current needed for transferring the power will be reduced.
Ignoring losses in the system and assuming operation in
resonance with the voltage and current in phase on the
sending side as well as on the receiving side, the product
of current and voltage must remain constant to transfer a
constant power independently of the magnetic coupling
conditions. As a result, if the airgap distance is increased
so that the coupling coefficient is halved, the required current will be doubled while the required voltage will be
halved. For conventional operation with resonancefrequency tracking, all components of the system must be
designed for the required voltage at the minimum airgap
distance and the maximum currents occurring at the lowest expected coupling conditions (i.e., the maximum airgap distance). The variations in airgap distance imposed
as a requirement for the design are for the selected concept corresponding to a range of coupling coefficients,
where the highest value is approximately three times the
lowest value. With conventional operation in resonance,
this would imply that all coils, capacitors, wires, and semiconductor devices would have to be rated for three times
the nominal current at the rated voltage. Such levels of
overrating would significantly influence the cost and
material requirements for the system.
One important property of capacitor-compensated circuits for inductive power transfer is that there exists a
particular set of operating conditions under which the frequency characteristics of the input impedance, i.e., the
2014
2
450
2
2169
450
400
400
350
350
300
300
250
250
200
200
150
150
100
100
50
50
0
7
7.15
× 10-10
(b)
0
1.2 × 10-5
1
Figure 5. An FEM analysis showing the energy density [color scale from 0 (blue) to 450 (red) J/m3] and power flow field (power density is proportional to the distance between the white lines) in (a) aligned and (b) misaligned conditions.
IEEE Elec trific ation Magazine / S EP T EM BE R 2 0 1 7
27
Table of Contents for the Digital Edition of IEEE Electrification Magazine - September 2017
IEEE Electrification Magazine - September 2017 - Cover1
IEEE Electrification Magazine - September 2017 - Cover2
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