Supervisory Control and Data Acquisition (SCADA) systems monitor and control electric power distribution. Recent history has shown that cyber-attacks pose a tremendous risk for the economy and safety of modern countries. This paper introduces an architecture and a prototype implementation for a process-aware, network-based Intrusion Detection System (IDS) to secure control networks in the domain of power distribution. Based on a recently proposed process model, the system continuously assesses the local physical process and all control commands with regard to consistency and safety of the underlying physical process. Its detection capabilities focus on process-based attacks like manipulated control commands, which appear legitimate to traditional IDS but might nevertheless have devastating effects on the power distribution. The architecture separates the evaluation part from the traffic processing, which ensures extensibility and scalability. The developed implementation has been successfully deployed at a Dutch power distribution substation. Its detection performance is characterized by a very low miss rate and high precision.