The time evolution of power systems is modeled by systems of differential and algebraic equations (DAEs) . The variables involved in these DAEs may exhibit different time scales. Some of the variables can be highly active while other variables can stay constant during the entire time integration period. In standard numerical time integration methods for DAEs the most active variables impose the time step for the whole system. We present a strategy, which allows the use of different, local time steps over the variables. The partitioning of the components of the system in different classes of activity is performed automatically based on the topology of the power system. The performance of the multirate approach for two case studies is presented.