In the automotive industry a strong trend towards electrification is determined. It offers the possibility of a more flexible actuation of the vehicle systems and can therefore reduce the fuel consumption and CO₂ emissions for modern vehicles. This is not only valid for typical drive train components, e.g., for hybrid or pure electric vehicles, but also for chassis components and auxiliaries like power-steering pump or air-conditioning compressor. However, a further electrification is limited by the 14V power net of conventional passenger cars. The high electric currents required by new/additional electrical components may lead to increased line losses and instability in the vehicle electrical system. With the introduction of a medium voltage level (≺60V) these problems can be circumvented. Nevertheless, the complexity of the vehicles with strong electrification and several voltage levels also raise the effort of design and development due to the multidisciplinary nature of the electrified vehicle systems.To meet the challenge of the problem mentioned above, the presented work proposes a modern co-simulation approach to make the design and evaluation of a vehicle electrical system more efficient. The simulation process becomes more modular and the strength of the domain-specific simulation tools can be optimally utilized. On the example of a hybrid electric vehicle (HEV) concept, simulation models from different fields of expertise (electrics, electronics, mechanics and thermodynamics) are integrated into an overall vehicle simulation model. The specific simulation tools are coupled via the co-simulation platform ICOS (Independent Co-Simulation) developed at the Virtual Vehicle Competence Center (ViF). Following that, the virtual prototype car is used to analyze the stability behavior of various two-voltage vehicle electrical system configurations including an electric double-layer capacitor (EDLC) as energy buffer or active stabilization with an additional DC/DC converter.