MODELING AND ANALYSIS OF AC OUTPUT POWER FACTOR FOR WIRELESS CHARGERS IN ELECTRIC VEHICLES
Abstract
This paper presents a general mathematical expression and characteristic analysis of the output power factor before rectification on the receiver side for wireless chargers in electric vehicles. This power factor is usually regarded as unity (i.e., the AC output voltage is in phase with the current), based on fundamental harmonic approximation (FHA). However, the default unity power factor assumption is not accurate for output power derivation even at resonance frequency. This study explores not only output power factor characteristics for different frequencies or power levels, but also the phase relationships of the input and output AC voltages. The continuous conduction mode (CCM) and discontinuous conduction mode (DCM) are both analyzed. An integrated LCC compensation topology is selected as the research object, and its analysis process can be readily extended to other common topologies. Furthermore, this study is beneficial for the implementation of some control strategies requiring precise power computation/estimation, e.g. feedforward control or model prediction control. Finally, a comparison of numerical and experimental results with various misalignment cases validates correctness of the proposed theoretical derivation and analysis methodology.
EXISTING SYSTEM:
A typical WPT system includes several stages, such as a rectifier with power factor correction (PFC), an inverter, a compensation network on the transmitter side, a magnetic coupler (including transmitter and receiver coils), a compensation network on the receiver side and a rectifier for charging the DC battery. A DC-DC converter may be added between the rectifier and inverter on the transmitter side for input DC voltage adjustment. Four basic compensation topologies are labeled as series-series (SS), series-parallel (SP), parallel-series (PS) and parallel-parallel (PP), according to the way the capacitors are connected to the transmitter and receiver coils. Some other novel compensation topologies have been proposed recently. In, a series-parallel-series (SPS) compensation topology is presented. In this new design, one capacitor is connected in series while the other is connected in parallel with the transmitter coil. On the receiver side, one capacitor is connected in series with the coil. Thus both SS and PS characteristics appear in the topology. A LCL network is proposed in, where the transmitter is featured as a constant current source. In, a series-parallel LCC compensation is used for better performance, despite its tricky parameter design needed for control stability and soft-switching realization. An integrated LCC compensation is proposed to reduce the size and weight of additional inductors in introduces a CLCL network where bidirectional power transfer can be achieved.
PROPOSED SYSTEM:
Reliable acquisition of output power factor via thorough theoretical derivation is beneficial for the implementation of several control algorithms (e.g., feedforward control, model prediction control, etc.), which require real-time precise estimation of output power, supposing the input and output voltages and switching frequency are known. On the other hand, the exploration of voltage/current phase relationships and power analysis at various frequencies could make contributions to the design of a novel compensation topology. Therefore, this work is meaningful to the development of effective control strategies in WPT systems and circuit design as well. A LCL converter can be formed by adding an LC compensation network on the primary side or on both primary (transmitter) and secondary (receiver) sides. The advantage for the LCL converter at the resonant frequency is that the current in the primary side coil can be independent of the load condition, or in other words, the LCL network performs like a current source. However, the design of an LCL converter usually requires additional inductors. To reduce the additional inductor size and cost, usually a capacitor is put in series with the primary side coil, which forms an LCC compensation network. By utilizing an LCC compensation network, a zero current switching (ZCS) condition could be achieved for higher efficiency by tuning the compensation network parameters. Also, when the LCC compensation network is adopted at the secondary side, the reactive power at the secondary side could be somehow compensated and the current distortion might be reduced. Consequently, in order to verify the proposed theoretical derivation, an integrated LCC compensation topology is selected as a specific research object. Extension of the presented analysis to other topologies is based on simple transformation rules.
CONCLUSION
In this paper, the exploration of the AC power factor characteristics and voltage phase relationships in wireless chargers of EVs is proposed, in order to correct a common misunderstanding that the AC output power factor of a WPT system is always unity. Continuous conduction mode (CCM) and discontinuous conduction mode (DCM) with various frequencies are discussed, covering expected operation conditions. An equivalent output voltage curve is introduced to decrease the calculation complexity in DCM. With simple transformation, the presented methodology for an integrated LCC compensation topology can be readily extended to other WPT systems. It also contributes the new topology design and realization of some control strategies with precise power calculation/estimation required. The comparison of experimental and calculated results proves the correctness and validity of the proposed strategy.
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