Integration of a multi terminal DC grid in an interconnected AC network

Several efforts are being made to install a multi terminal HVDC (MTDC) overlay grid in Europe to transmit bulk power from centralized renewable energy generation (e.g. in offshore windfarms) to centers of load. This paper describes some functions of a MTDC grid management system how to integrate it into an interconnected ac power system. It includes the calculation of converter schedules and its adaption to unpredicted power flow changes. The converter schedules are calculated by a loss minimizing mixed ac/dc optimal power flow and the adaption of schedules is based on voltage angle measurements (angle gradient method). Those unpredicted power flow changes can be caused e.g. by the activation of load frequency control or the trip of any equipment. In order to keep the dc energy equilibrium the converter reference values which are determined by these functions are modified by a p-v-characteristic if the dc voltage differs from its reference value. The functionality is proven by digital and analog simulations.