A sustainable electrical power system demands for improved integration of distributed energy units and renewable energy units. Different integration approaches are proposed but the complete picture of the future power system is still under investigation with regard to technical as well as economic aspects. This thesis contributes in four major topics:1. A set of adequate definitions for various concepts concerning the integration of distributed energy units (distributed generators, loads and storage) in the electrical power system is proposed. 2. A comprehensive overview of the technological control capabilities of distributed generators and the resulting possibilities of providing ancillary services is assessed. 3. The economic potential of a participation of distributed generators in frequency control and reactive power supply is investigated with cost-benefit-analyses that are based on newly developed assessment approaches. 4. A control system for distributed energy units in ISET's Design-Centre for Modular Supply Technology is developed. This control system allows demonstrating the technological and economic capabilities of distributed energy units with regard to the provision of ancillary services by hardware experiments.