Control strategy for a bidirectional wireless power transfer system with Vehicle to Home functionality

The advancements in the charging strategy of electric vehicles will have inevitable effects on the electrical grid in the coming future. Electric vehicle battery chargers are able to perform the bidirectional power transfer according to the vehicle-to-grid concept and will offer valuable services to the distribution grid or to the domestic grid of the vehicle owner. The wireless power transfer battery chargers offer the opportunity for a more safe and user-friendly approach to electric vehicles for people who are not confident with technological apparatuses. Bidirectional wireless power transfer chargers capable of vehicle to grid services are the natural evolution of the above-mentioned concepts. This paper faces the topic of developing the control algorithms for such a battery charger, focusing on the needs of the power conversion stages involved in the functioning of a charger enabled for vehicle to home operation. At first, the separation of the control algorithms into two levels is explained, and then the interaction between the algorithms of the internal and external level is introduced. It was decided to design controllers as simple as possible in implementing the control algorithms. This has made it possible to adopt techniques well known in the scientific community for their design and to contain the computational resources necessary for their implementation. Despite the simplicity of the controllers, the introduction and management of the interactions between the various algorithms has made it possible to develop an overall control strategy that at the same time respects the voltage and current limits set by the grid and the battery and also avoids exceeding the maximum operating conditions of the static converters that constitute the system. The algorithms and the relevant controllers are developed one by one in the continuous time domain, using techniques based on the analysis of Bode diagrams of the transfer functions involved in the operation of the system. In designing the controllers, the effects of their subsequent implementation in a discrete time domain are considered together with the effect of the transmission delay originated by the data exchange between the two sections of the system. The discretization of the controllers has been performed using the Tustin method. The performance of all algorithms has been separately verified in the discrete time domain using simulations developed in the Matlab/Simulink environment. Finally, the functioning of the complete control strategy has been successfully checked in the same environment.