Abstract
Wireless power transfer (WPT) technology, due to its security and convenience, has been widely applied in electric vehicles, mobile communication devices, wireless motor drives, unmanned aerial vehicles, and other products.Advanced modulation and control strategies can ensure the high-efficiency operation of WPT systems. In this thesis, various modulation strategies and control methods, including phase shift modulation, voltage cancellation (AVC), and pulse density modulation (PDM), primary-side control, secondary-side control, dual-side control are summarized. By applying phase shift modulation to both the inverter and the active rectifier, a multi-phase shift control technique can be obtained. Currently, the triple phase shift (TPS) control strategy has received significant attention in WPT systems due to its ability to achieve zero-voltage switching (ZVS) on both sides of the power switches. However, the existing literature on TPS strategy has some limitations, and the optimal operating point cannot be guaranteed under all conditions. In this thesis, the reasons for the non-optimal operating point in existing literature are explained, and the impact of harmonics on output power and system efficiency under the TPS strategy is analyzed in detail. The optimal operating point under complex conditions, such as asymmetric and strongly coupled systems, is also thoroughly analyzed. Furthermore, it is explained that the inter-harmonics of PDM can cause severe current ripples in WPT systems. A hybrid modulation method combining AVC and PDM is proposed, which can be applied to frequency control to reduce the frequency variation range required for achieving ZVS. This method can also be applied to TPS control to effectively improve the system efficiency. For WPT systems with active power devices on both sides, such as secondary-side control and dual-side control, frequency tracking and phase synchronization is a challenge for both sides' safe and reliable operation of converters. A novel synchronization method based on the voltage transient is proposed in the thesis. Its most significant advantage is that it is not affected by system detuning and can be implemented at any operating frequency. An equivalent sensor inductor (magnetic ring with multiple turns of a coil) is connected in series to the resonant tank at the receiving end. The sudden change of the inverter output voltage will cause the voltage transient in the sensor inductor. By detecting and tracking the voltage transient, the frequency and phase synchronization of both sides can be achieved.
The equivalent sensor inductor connected in series in the resonant tank of the WPT systems can not only be used for synchronous operation on both sides. In the thesis, this equivalent sensor inductor was proposed and applied in many fields and is expected to become a basic sensor for WPT systems in the future. The voltage transient values caused by the rising and falling edges of the inverter output voltage and the rectifier input voltage are relevant to the mutual inductance and load voltage. By detecting the voltage transient amplitudes, the mutual inductance and load voltage can be identified directly for the strong coupling case. For the weak coupling case, the current amplitude in the primary resonant tank can be further utilized to handle the sensitivity problem. Sensitivity analysis presents that the identification method can also have satisfactory performance even when the system parameters have some errors. In addition, a novel multi-parameter identification method for WPT systems is proposed. The method utilizes the characteristic of the secondary side being approximately open-circuited when the switching frequency is far away from the resonant frequency to identify the self-inductance of the transmitter coil and the compensated capacitance. By detecting the voltage transients on the sensor inductor connected in series with the resonant tank, the transmitter-side controller can detect the phase angle between the inverter output voltage and the rectifier input voltage. Combining this phase angle, the system's switching frequency, inverter output voltage, and output current, multiple system parameters including the mutual inductance, self-inductance of the receiving coil, or the compensated capacitance at the receiver side and load voltage can be identified.
Mutual inductance and load identification are crucial for tracking load demands, optimizing system efficiency, and enabling communication-free control. Based on the excellent performance of the equivalent sensor inductor in synchronization and parameter identification, a communication-free cooperative control method that combines parameter identification and synchronization is implemented in the TPS control in the final section of this thesis. This method enables optimal efficiency operation under the TPS strategy while tracking load current commands without any communication between both sides.
In conclusion, this thesis conducts an in-depth analysis of the modulation strategy of the WPT system and proposes a hybrid modulation strategy to optimize the operation of the system. In addition, the innovative use of the equivalent sensor inductor connected in series in the WPT system has successfully achieved the synchronous operation of converters on both sides, the identification of multiple parameters and the communication-free cooperative control of the WPT system.
| Date of Award | 9 Jun 2025 |
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| Original language | English |
| Awarding Institution |
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| Supervisor | Chaoqiang JIANG (Supervisor) |
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