Book excerpt: DC electric power distribution systems have higher efficiency, better current carrying capacity and faster response when compared to conventional AC systems. They also provide a more natural interface with many types of renewable energy sources. Furthermore, there are fewer issues with reactive power flow, power quality and frequency regulation, resulting in a notably less complex control system. All these facts lead to increased applications of DC systems in modern power systems. Still, design and operation of these systems imposes a number of specific challenges, mostly related to lack of mature protection technology and operational experience, as well as very early development stage of standards regarding DC based power infrastructure.