The Inner Tracker (ITk) is a new, all-silicon detector under development for the ATLAS Phase-II upgrade and will operate at the High Luminosity LHC. A serial power (SP) scheme will be used for the ITk Pixel Detector. New elements are required to operate and monitor a serially powered detector, including a Detector Control System (DCS), constant current sources and front-end electronics with shunt regulators. A demonstrator for the three outer barrel layers of the ITk Pixel Detector is being built at CERN to verify and qualify the SP concept, and to gain experience operating a SP chain. It includes all required elements for controlling and operating SP chains safely, from an interlock system to in-situ monitoring with a new DCS. A first full electrical prototype with seven quad modules was successfully tested. This prototype includes pixel modules, the DCS elements, realistic services and can be read out by different data acquisition systems. Problems identified during the commissioning and operation of the small electrical prototype went into the requirements for the future front-end (FE) chips. Many institutes worked on the development and integration of the individual parts. Further and longer chains are under construction and will allow tests of multiple SP chains mounted on the same local support in parallel. It is also foreseen to operate a SP chain with 16 modules.