Power Flow Solution on Multi-Terminal HVDC Systems: Supergrid Case

High Voltage Direct Current (HVDC) systems offer distinct advantages for the integration of offshore wind farms to inland grid system. HVDC transmission system based on Voltage Source Converter (VSC) enables multi-terminal use HVDC for the integration of large-scale wind power in the North Sea. That network requires a special formulation for power flow analysis as opposed to the conventional method employed on AC networks. This paper presents a sequential AC/DC power flow algorithm, which is proposed for the analysis of multi-terminal VSC HVDC (VSC-MTDC) systems. This sequential power flow method can be implemented easily in an existing AC power flow package and is very flexible when compared with unified methods. Gauss-Siedel is used to solve DC power balance equations, as it offers two keys advantages: very fast and simple computational implementation, and errors do not accumulate during the calculation. The algorithm is tested using the WSCC 3-machine, 9-bus system with a 3-terminal MTDC network and the results are compared with those obtained from DIgSILENT PowerFactory demonstrating the validity of the proposed algorithm. As an aggregate value, a representative test case of the projected scheme for the phase I of the Supergrid project on the North Sea is presented. The proposed approach presented in this paper is used to calculate DC power flows for some scenarios.