A Single-phase On-board Electric Vehicle (EV) Charger with Enhanced Control System and Vehicle-to-Vehicle (V2V) Charging Assistance Capability
The numbers of plug-in electric vehicles (PEVs) are growing rapidly throughout the world and, accordingly, they have drawn lots of attention because of their various potential functions on the grid. The main functions of EV chargers, grid to vehicle (G2V) and vehicle to grid (V2G), are supported by bidirectional power-electronics converters as the key components of EV chargers. Moreover, EV chargers can be utilised to provide ancillary functions for the grid (particularly residential networks), such as reactive power support, voltage regulation and/or harmonics reduction.
Nowadays, one of the main barriers to EV market growth is the insufficient number of charging stations. Even with satisfactory progress in building charging stations in some countries, there is still a big concern among customers toward an emergency situation when their EV battery becomes unexpectedly flat and they do not have access to a charging station. If the EV owners have a chance of charging their cars from other EVs in the form of charging the battery in grid-isolated mode (vehicle-to-vehicle (V2V)), their concern of having a flat battery will be diminished significantly, paving the way for developing the EV market.
Furthermore, the transient caused by the charging/discharging operation of EV chargers and its impact on the grid is another concern from the increased penetration of EV chargers in household charging stations. This phenomenon partly originates from the filters used in the control system of the EV chargers. For instance, a notch filter (NF) is required in the voltage-control loop of the dc/ac converter to block the double-frequency (2-f) ripple of the dc-bus voltage. In addition, an orthogonal-signal generator (OSG) is used in the algorithm of a single-phase phase-locked loop (PLL), or in a single-phase synchronous-reference frame known as a direct-quadrature (DQ) frame for power regulation of the dc/ac converter.
The research work in this thesis presents solutions for the above-mentioned constraints as follows:
The first contribution of this research is to present a design and implementation of a multifunctional single-phase EV charger which can support the main functions of V2G/G2V and ancillary functions such as reactive power support, harmonic reduction and voltage regulation simultaneously. The proposed design can provide the three ancillary functions without adding any additional components to the EV charger, or being reliant on the EV battery.
The second contribution of this research is to add a novel V2V functionality to the designed EV on-board charger which enables the charger to charge the EV battery in grid-isolated mode. Accordingly, using the proposed design, two EVs can be connected via a low-cost charging cable (so-called V2V cable) without the need for an additional portable charger. Moreover, the V2V operation can be accomplished with the maximum power ratio of the EV charger (i.e. 4 kW in charging level 2), since the same charging infrastructure is used for transmitting the power between EVs.
The third contribution is to design and implement a novel dc-bus voltage-control method which proposes a new way of implementing the notch filter (NF) via integrating its internal variables into the control loop of the dc/ac converter. Using the proposed design, the system exhibits enhanced transient responses at both the dc-bus voltage and the output ac current.
The fourth contribution is to design and implement a new dc-bus voltage-control method which estimates the double-frequency (2-f) ripple of the dc-bus voltage without the need of using a NF in the control loop. This technique has no adverse impact on the original bus-voltage dynamic response. Accordingly, the bus-voltage control can be designed with higher speed and robustness, and the whole system can operate with a reduced transient at both the bus voltage and the output ac current.
The fifth (final) contribution of this research is to develop a novel OSG technique which is called an enhanced adaptive filter (EAF). The proposed EAF has a single input and excludes the drawback of the 90° delay. Unlike some existing methods, the EAF is not dependent upon the system parameters. It exhibits better dynamic response and improved filtering characteristics. The proposed EV charger and its enhanced control approaches are thoroughly analysed and validated through simulations in MATLAB/Simulink and through experimental results.