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Design of a High-Frequency CLLC Resonant Converter for Maximizing Overall Efficiency in the Bidirectional Operation of an Electric Vehicle Charger

Chahkandinezhad, Ali | 2024

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  1. Type of Document: M.Sc. Thesis
  2. Language: Farsi
  3. Document No: 57652 (05)
  4. University: Sharif University of Technology
  5. Department: Electrical Engineering
  6. Advisor(s): Tahami, Farzad
  7. Abstract:
  8. With the increasing share of renewable energy, the issue of grid stability will become more significant. Energy storage is an essential part of the new power system architecture to address this problem. The most cost-effective form of energy storage will be the use of electric vehicle (EV) batteries, as it does not require additional investment. Vehicle-to-Grid (V2G) technology enables the stored energy in an EV battery to be returned to the grid. This means that using a bidirectional charger, energy can be stored in the vehicle during off-peak hours and then returned to the grid during peak consumption hours, allowing the vehicle to act as a distributed generation source for the main grid. One of the converters that can be used as an EV charger is the bidirectional CLLC converter. This converter is a suitable choice for EV chargers due to features such as high efficiency and power density, Zero Voltage Switching (ZVS), and bidirectional capability. Typically, the first harmonic approximation is used for analyzing CLLC resonant converters, but this method lacks accuracy and its error increases with changes in load voltage and current or when operating in Discontinuous Conduction Mode (DCM). Additionally, in most optimizations of the CLLC converter, the optimization is directed towards charging and at the converter’s nominal power point. The problem with this approach is that the designed converter does not meet all the defined constraints at other operating points. Considering the charge and discharge profile of a lithium battery, it can be seen that the operating range of the converter includes many voltages and currents. As a result, there are many operating points for the converter. Considering these issues, this research initially implements an optimization algorithm in MATLAB based on the battery’s charge and discharge profile and various constraints, taking into account the losses of different circuit elements. This is done using time-domain analysis, which is much more accurate than the first harmonic approximation. The optimization objective function is to minimize the energy losses while adhering to various constraints and limits for the circuit elements. Additionally, due to the use of Wide Band Gap (WBG) technology and Gallium Nitride (GaN) transistors with their high operating frequency capability, the converter’s transformer is designed to be Planar and optimized, with the resonant inductors integrated into it. This reduces the size and number of optimization elements and increases the power density of the converter. By implementing the proposed algorithm on a Nissan Leaf battery, the converter’s energy efficiency reached 97.3% in grid-to-vehicle mode and 97.63% in vehicle-to-grid mode. Finally, a 3.2 kW electric vehicle charger will be proposed and built using a bidirectional CLLC converter, Planar transformers, and Gallium Nitride (GaN) transistors as primary and secondary switches. The optimization algorithm results will then be validated using the constructed converter
  9. Keywords:
  10. Resonant Converter ; Electric Motor Vehicles ; Bidirectional Electric Vehicle Charger ; Vehicle to Grid (V2G) ; Time Domain Method ; Planar Transformer ; Wide Band Gap (WBG)Switches

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