Abstract. The integration of offshore wind energy into the existing power system is continuously growing. With the increasing distance of the offshore wind farms (OWF) to the onshore AC transmission systems, HVDC systems are emerging as a preferable solution for the connection of OWF due to their techno-economic advantages in comparison to AC subsea connections. Integrating HVDC systems into the existing AC systems poses various planning and technological challenges. To be able to overcome these challenges a variety of studies has to be conducted, e.g. the HVDC system behaviour under faults. Simulations using electromagnetic transient (EMT) tools represent a generally accepted method to conduct the relevant studies. To increase the trust in the developed concepts subsequent hardware demonstrations would be preferable. However, performing these investigations with full-scale components is often not an option due to unavailability and high costs. As an alternative way, Power-Hardware-in-the-Loop (PHiL) approaches are considered. In this context, a new and worldwide unique laboratory demonstrator - the MMC Test Bench - is set up at RWTH Aachen University as part of the Horizon2020 project PROMOTioN. Here, laboratory-scaled Modular Multilevel Converters (MMCs) are used, which are connected on the DC side by cascaded Pi-line segments. The adjacent AC grids (i.e. offshore wind farms, AC transmission networks) are represented by real-time simulators (RTS) and connected to the MMCs via high bandwidth linear power amplifiers (PA).

In this work, the MMC Test Bench is initially described. Afterwards, the PHiL set-up to demonstrate the implemented controls for an OWF connected to shore via an HVDC link is explained. To allow the joint operation of the hardware set-up and the RTS in a stable manner, adequate PHIL interfaces algorithms have to be designed and the scaling between the RTS, the power amplifiers and the hardware is explained. The connection of the OWF represents a special challenge for PHiL demonstrations as the OWF represents a weak AC system with the MMC in grid forming mode. In a next step, the results of the successful demonstration of the interconnection of the OWF via an HVDC link with the MMC Test Bench are presented. The system behaviour in stationary and transient operation is analysed based on the wind farm start-up sequence as well as different cases of wind infeed fluctuations. The results are compared to a simulated full-scale model and deviations are discussed.