The electric power system plays a central role in the transition to a sustainable energy system: Electric power based on renwable sources permits an increase in electricity consumption to eliminate fossil fuels. This brings large challenges to the electric power system in terms of adaptation to the variable production from wind power and solar electricity. These electricity sources are grid connected through power electronic converters, that behave quite differentely compared to the synchronous generations in the nuclear units and other power plants that they replace. It is not enough that the renewable generation delivers energy. It must also contribute with the system services that are vital to the operation of the power system. Among the system services, balancing of the variable generation is well-known, but they also include frequency control, inertia, voltage control with reactive power, fault currents, power system restoration and island operation. Wind power and solar electricity change the power flows in the grid significantly - not only through its variability, but also through its geografic location. The same applies to e-mobility and other types of new electricity consumption. As grid expansion is associated with decade-long lead times, it is important to fully use existing grid capacity. Increased monitoring of the grid combined with automatic control makes this possible. All this poses considerable challenges to planning and operation of the power system, where safe and reliable operation must still be maintained. Reviewing grid company practices and methods is therefore essential to the future. Finally, electrification requires not only research and development, but also education of the engineers that will implement the transition. 

Key words: power system analysis, grid capacity, stability, renewable electricity generation, wind power, solar electricity, e-mobility