Modelling and analysis of onboard bidirectional charger for vehicle-to-grid and vehicle-to-load in electromobility energy systems

The growing integration of electric vehicles (EVs) into modern energy systems presents an urgent need for innovative charging solutions that enable bidirectional energy flow and enhance grid stability, energy management, and the role of vehicles as distributed energy resources. This paper presents the design and analysis of an onboard bidirectional charging (OBD) system for vehicle-to-grid (V2G) and Vehicle-to-Load (V2L) applications. The system, rated at 50 kW, facilitates dynamic energy exchange between the EVs, grid, and local loads. A series-connected 43-cell lithium-ion battery with a nominal voltage of 3.7 V and 120 Ah capacity, a 230 V, 50 Hz grid supply, power electronics such as buck-boost converters, an Inductor-Capacitor-Inductor (LCL) filter, and a mode selector controller, designed in MATLAB/Simulink using dual Stateflow, governs battery state-of-charge (SoC) based operations. The simulation results indicate robust V2G and V2L mode performance when the SoC exceeds 40%. The system maintained a stable 400 V DC bus (\pm5 V) and provided consistent 12 V and 24 V outputs. The grid interaction demonstrated high power quality with sinusoidal voltage and current peaks of 325 V and 20 A, respectively. The battery voltage stabilised at 158.6 V with minimal oscillations, and smooth transitions between the V2G, V2L, and Grid-to-Vehicle (G2V) modes were observed. The proposed design facilitates efficient bidirectional power transfer by utilising a four-quadrant full-bridge inverter to enable both V2L and V2V energy exchange, with the mode selection governed by an SoC threshold of 40%. Key findings include rapid transient responses in voltage stabilisation with the V2L mode supplying power linearly with increasing load current, reaching 2100 W at 10 A, and low ripple in DC voltages, ensuring reliable performance under variable load conditions. In this study, the designed onboard bidirectional converter in the V2G mode has a power transfer efficiency of 84%-93% and a stabilisation voltage within ±4.7% of the nominal grid voltage, whereas in the V2L mode, it has a wider voltage fluctuation range of ±9% and a slightly lower efficiency of 78%-88%. The findings of this study reveal the viability of the system in enabling the integration of EVs as distributed energy resources in smart grid applications for energy management and grid support services, and provide important insights into the design of bidirectional charging systems with potential implications for improving control strategies and power quality in future grid scenarios.