S2PC 2023, Kaohsiung, Taiwan

Wireless Power Transfer System for Utility Frequency
Output with Non-Linear Load Compatibility
Masaya Tokuriki1, Keisuke Kusaka1, Toshiyuki Kuroda2
1 Nagaoka University of Technology, 2 Tokyo Electric Power Company Holdings, Inc.

This paper proposes a wireless power transfer (WPT) system suitable for utility-frequency output. Conventional a WPT systems with ACDC-AC conversion have required large electrolytic capacitors for DC conversion. Since DC conversion is unnecessary in the proposedsystem, smaller film capacitors can substitute for the large DC-link capacitors. Therefore, the proposed system holds the potential for reducing
circuit volume and extending its lifetime. Additionally, to address the issue of non-linear loads generating unacceptable harmonic currents inthe system, an active power filter is applied. This enables harmonic current suppression, and the total harmonic distortion on the output current
is improved from 16.9% to 5.28%.
Keywords Wireless power transfer system, High-frequency injection, Single-phase active power filter

1. Introduction

The calculated d-axis current combined both fundamental andharmonic components. To extract only the fundamental component
Dioderectifier

Switching : fs
Duty 0.5

Diode rectifier& LPF

iinvinunfolder

Ldc
Cdc2

4.7
mF vdc1

4.7mF

Cdc1

Proposed a WPT system

Fig. 1 illustrates the system configuration of the proposed WPTtopology. In a conventional a WPT system with AC-AC conversion,
it is necessary to convert low-frequency AC to DC in order totransform high-frequency AC. Large electrolytic capacitors areemployed for DC smoothing, leading to the potential for overall
circuit enlargement and reduced lifetime. In the proposed system,
the full-wave rectified sinusoidal waveform generated by theprimary-side diode rectifier is supplied to the primary-side inverter.
This allows the DC link capacitor to be replaced with a small film
capacitor. High high-frequency components are injected byswitching the inverter at the resonant frequency of the transmission
coil and the resonant capacitor of the power transmission unit. A
diode rectifier and LC filter are used to regenerate the full-wave
rectified sinusoidal waveform on the secondary side. Here, DCsmoothing is also not performed, making it possible to replace theSwitching : fr
Duty 0.5

ioutilvout=

2.

𝑖𝑙𝛼 sin(𝜔𝑡) − 𝑖𝑙𝛽 cos(𝜔𝑡)𝑖𝑙𝑑sin(𝜔𝑡) − cos(𝜔𝑡) 𝑖𝑙𝛼[𝑖 ] = [] ⋅ [𝑖 ] = [] . (2)cos(𝜔𝑡)sin(𝜔𝑡)𝑖𝑙𝛼 cos(𝜔𝑡) + 𝑖𝑙𝛽 sin(𝜔𝑡)𝑙𝛽𝑙𝑞=

The use of mobile devices with built-in batteries, such as
smartphones and laptops, has been on the rise. In recent years, there
has been an increasing interest in WPT, and research as one of thecharging methods for these batteries [1-3]. However, previously
reported systems have been individually designed based on thebattery voltage requirements of specific applications, and they all
produced DC output [4-5]. Consequently, applying the WPT system
to existing systems with AC input was not possible. In order toapply WPT to existing systems, utility frequency output isnecessary [6-7].
In this paper, a WPT system optimized for utility frequency
output is proposed. In the proposed system, the large-capacity
electrolytic capacitor used in converting low-frequency AC to highfrequency AC can be replaced with a small-capacity film capacitor.
As a result, the entire circuit can be downsized and an extension ofoperational lifetime. Additionally, a compensating circuit is applied
to mitigate the introduction of unacceptable current harmonics into
the system when high crest factor loads are connected. Finally,through simulation, the feasibility of the proposed system for powertransmission and the ability to generate sine waves at utility
frequency are verified, and its practical implementation isconsidered.

DC link capacitor with a small capacitor, similar to the primary side.
Finally, the secondary-side inverter operates as an unfolder torestore the system output to a utility frequency sine wave.
Additionally, a compensation circuit is connected to prevent
unacceptable harmonics from flowing into the system when a nonlinear load with a high crest factor is connected to the proposedsystem. In this paper, an active power filter (APF) is utilized as thecompensating circuit to suppress current harmonics.
Fig. 2 presents a block diagram for the generation of the control
signals for the APF [8]. To establish the single-phase load current inthe  coordinate system, shift the phase by  / 2 as follow
𝑖𝑙𝛼𝑖𝑙 (𝜔𝑡 + 𝜙)(1)[𝑖 ] = [].
𝑖𝑙 (𝜔𝑡 + 𝜙 + 𝜋/2)𝑙𝛽Using a transformation matrix, the dq coordinate system of thesingle-phase load current can be determined as follows

vdc2iAF

iDC

Non-linear load

VDC
Single-phase APF

Fig. 1. Proposed system configuration with APF applied asa circuit for compensation.
vout

1f PLLsin(wt)cos(wt)il

i*out×+il

ild+LPFil

×+

V*DCVDC+ild +/2 shift

×i*DCPIFig. 2. Generation of reference current for APF.

i*AF

S2PC 2023, Kaohsiung, Taiwan
using a LPF and add the current command iDC to maintain the directbus voltage of the APF. The calculated signal is then inverse dqtransformed, and the compensation current command is generated
by calculating the difference with the load current as follows
∗
(3)
𝑖𝐴𝐹
= 𝑖𝑙 − (𝑖𝑙𝑑−𝐷𝐶 − 𝑖𝐷𝐶 ) sin(𝜔𝑡) ,where ild-dc is the fundamental component of the load current.
Note that PR controllers are employed to compensate for the AC
current distortion.

(a) Operating waveform with non-linear loads.

3. Simulation results
Fig. 3(a) illustrates the voltage across the capacitors on theprimary and secondary sides of the system, as well as the input andoutput current of the system when non-linear loading is connected.
Fig. 3(a) illustrates the transmission voltages on the primary andsecondary sides. For the load in this system, a capacitor-input dioderectifier with a crest factor of 1.97 is connected. At this point, thecompensation circuit is not in operation.
From these results, it is confirmed that power transmission canbe achieve using the high-frequency injection method even when
non-linear loading is connected. Furthermore, the total harmonic
distortion (THD) of the output current at this time is 16.9%.
Fig. 4(a) illustrates the voltage and current waveforms when thecompensation circuit is activated, while (b) shows the transmission
voltage. From these results, it is observed that the presence ofharmonics on the output current is significantly reduced compared
to Fig. 3. Consequently, it is confirmed that the compensator
effectively mitigates the harmonic currents, which are generated
when non-linear loads are connected, resulting in a system output
current resembling a utility frequency sine wave.
Fig. 5 illustrates the THD analysis results for system input
current. These results indicate that the 3rd harmonic, which is at
16.6% before compensation, has been reduced to 4.72% by the APF.
As a result, THD has been reduced from 16.9% to 5.28%.

4. Conclusion(b) Primary and secondary transmit voltages.
Fig. 3. Operating waveform of the proposed system.

(a) Operating waveform with non-linear loads.

(b) Primary and secondary transmit voltages.
Fig. 4. Proposed system applying APF as acompensation circuit.

In this paper, a WPT system suitable for integration into existing
systems is proposed. This system conducts power transfer byinjecting high-frequency without the need for DC smoothing. As aresult, the feasibility of power transmission was confirmed byreplacing the required large-capacity electrolytic capacitors for DC
smoothing with small film capacitors. Additionally, the operation ofthe system when the APF is used as a compensating circuit wasinvestigated. This confirmed a reduction of 11.6% in harmonic
content within the grid current.
This study has been financially and technically supported byTokyo Electric Power Company Holdings, Inc.
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Fig. 5. Harmonics distortion ratio.
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