|Basic research in wireless inductive power transfer
|Wong, S. C. (EIE)
|Hong Kong Polytechnic University -- Dissertations
Wireless power transmission
Electric vehicles -- Power supply
|Department of Electronic and Information Engineering
|xxxi, 166 pages : color illustrations
|This thesis focuses on power control and efficiency optimization of inductive power transfer (IPT) systems, including the design of a single-stage IPT converter for electric vehicle (EV) battery charging, a maximized efficiency control of a three-stage IPT system and a development of a stability criteria of power distribution systems. Power is transferred via magnetic field coupling of an IPT system, that it is a kind of commercially available wireless power transfer (WPT) technique. Compared with conventional wired power transfer, IPT can be wireless, convenient, reliable and flexible. IPT has attracted many attentions in a wide spread applications, from low power to high power, from short distance to mid-distance and from stationary to dynamic. However, there are still some challenges in practical IPT applications due to low and varying coupling coefficient of the loosely-coupled transformer. Therefore, the study in this thesis will focus on power control and efficiency optimization in some specific applications. The applications are explained as follows. Maximum power efficiency and load-independent output are widely studied for the IPT converters with basic compensation topologies (i.e., series-series, series-parallel, parallel-series, parallel-parallel). The operating frequencies of the IPT converters to achieve maximum power efficiency and load-independent output depend on the design of the compensation networks. In this thesis, we study the feasibility to achieve maximum power efficiency and load-independent output simultaneously with appropriate design of the compensation networks. Taking the variation of coupling coefficient k into consideration, secondary series compensation topologies (i.e., series-series, parallel-series) are more suitable in dynamic applications, because the design of the compensation network is irrelevant to k.
In stationary EV battery charging application, a typical charging profle uses constant current (CC) charging followed by constant voltage (CV) charging, with power varying from the maximum rated power down to a minimum of 3%. An IPT system should be designed with minimum number of converter stages to achieve high efficiency. However, high efficiency for such a wide load range is difficult to achieve. Moreover, the efficiency-to-load relationship is distinctly different for CC and CV charging, posing difficulties for single-stage IPT converter design. In this thesis, a single-stage IPT converter is designed, complied with the battery charging profle. Soft switching is ensured and overall efficiency is optimized for the whole process of CC charging and CV charging. Due to the variation of the coupling coefficient, it is hard to control the output power together with maintaining maximum power efficiency for a single-stage IPT converter. Therefore, in practical IPT applications with varying coupling coefficient, it is common to cascade the IPT converter with front-side and load-side DC/DC converters. The two DC/DC converters are normally controlled cooperatively for the requirements of output regulation and maximum efficiency tracking using a control technique based on perturbation and observation, which is inevitably slow in response. In this thesis, a decoupled control technique is developed. the load-side DC/DC converter is solely responsible for output regulation, while the front-side converter is responsible for impedance-matching of the IPT converter by controlling its input-to-output voltage ratio. DC and small-signal transfer functions are derived for designing the parameters of fast linear controller. It is common that a DC power distribution system consists of a single source and multiple load converters sharing a common DC voltage bus. Same confguration can also be adopted in an IPT power distribution system. Without a voltage regulator, the series-series compensated IPT (SSIPT) converter provides a constant current output with high output impedance where operating at its power efficient point. Multiple load-side converters can be connected to the SSIPT converter in parallel. Such current source DC power distribution system is relatively unexplored. In this thesis, a more general set of criteria based on power balance is proposed. To ensure the stability of the system, the load-side converters are distinguished between voltage-driven and current-driven converters. All the applications proposed are developed with detailed analysis, verified with simulation and experimental measurements from some appropriate prototype converters.
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