Modeling and control of inductive power transfer systems with constant voltage load
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In recent decades, wireless power transfer (WPT) systems have been used in more and more fields, such as portable electronic devices, implanted medical devices, power system monitoring devices, underwater vehicles, etc. Among all WPT technologies, inductive power transfer (IPT) is considered the most promising method due to its high-power and high-efficiency transfer capability at relatively low frequency. With the rapid development of transportation electrification, high-power IPT technology has the potential to become a trend because it is safer, more convenient, and more flexible than traditional wired charging. However, there are many challenges that need to be addressed in specific scenarios. In maritime applications, the coupling of the IPT system changes with the floating of ships, which requires the system to have a high tolerance for misalignment and excellent dynamic behavior. Moreover, a diode rectifier is usually applied at the pickup side to reduce system cost and complexity for highpower transportion applications. The battery on the DC output side can be considered equivalent to a constant voltage load (CVL) due to diode rectification characteristics. Therefore, IPT systems with CVL under a wide range of coupling variations are studied in this thesis, including modeling and control, to achieve high efficiency, good dynamic response, and full power range operation. To analyze the dynamic performance and control design of IPT systems, a nonlinear state-space model is derived, which also exhibits differences compared to an equivalent constant resistance load model. The model in this thesis considers the phase angle between the fundamental pickup voltage and the pickup current to improve accuracy. Moreover, different model reduction methods are used to facilitate controller design and reduce computational requirements, especially for model-based control methods. The developed model provides a solid foundation for the design of subsequent control strategies. High-power IPT systems in marine applications are often required to operate under varying coupling and output power. An existing interesting approach combines system design and subresonant frequency control to achieve global zero voltage switching (ZVS) conditions and minimum volt-ampere at rated power over a wide coupling range. However, the output power range in systems with sub-resonant frequency control is limited. To overcome this limitation, the design and implementation of a coordinated voltage-frequency control method based on sub-resonant frequency control are presented. This method allows the system to switch automatically between voltage control mode and sub-resonant frequency control mode based on changes in the coupling coefficient and power reference to achieve a full range of power output. Moreover, both the frequency control mode and the voltage control mode can be implemented independently in IPT systems. The improvements in different operating modes of IPT systems with coordinated voltage-frequency control are mainly as follows: 1. The systems with sub-resonant frequency control discussed in the previous literature are analyzed and improved in this thesis. First, ensuring soft switching during the transient operation of IPT systems with frequency control is discussed in detail. Moreover, a primary-side gain-scheduled controller for systems with sub-resonant frequency control is designed to significantly improve the dynamic response in a wide range of coupling and power. 2. Voltage control mode is introduced into the system to extend the full power range for systems with sub-resonant frequency control. However, achieving the full range of ZVS has always been a challenge for IPT systems with voltage control. In this thesis, pulse skipping modulations, mainly pulse density modulation (PDM), used in IPT systems are discussed in detail. These methods adjust the sending voltage by skipping pulses to achieve output power regulation on the system, which can always achieve ZVS during large variations in output power and coupling conditions. However, IPT systems with a CVL are found to experience poorly damped oscillations when operated with pulse skipping strategies, which are evaluated in detail. Furthermore, corresponding solutions, including methods for increasing system damping and shaping output patterns to avoid exciting oscillations, are proposed to reduce the current/power ripple of systems with PDM. Finally, a hysteresis on-off modulation strategy is proposed and applied to the IPT system, which can suppress the oscillation effectively and ensure excellent dynamic performance. In this thesis, IPT systems with CVL by implementing the coordinated voltage-frequency control are presented to achieve a full range of output power, maintain high efficiency, and improve dynamic response under large variations in coupling conditions. All concepts and techniques presented in this thesis have been verified by simulations and laboratory experiments. It is worth noting that the discussion in this thesis on frequency control mode and voltage control mode is universal and can be used independently in IPT systems.