Abstract- This paper presents an algorithm for the sequential solution of the ac/dc power flow, which is proposed for the analysis of multi-terminal HVDC systems (MTDC). This sequential power flow algorithm can be implemented easily in an existing ac power flow package and is very flexible when it compared with unified methods. Gauss-Siedel algorithm is used to solve dc power balance equations, 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 results compared with those obtained from DIgSILENT ® PowerFactoryTM demonstrating the validity of the proposed algorithm. As 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.

Abstract—Multi-terminal HVDC (MTDC) transmission system using Voltage Source Converter (VSC) provides an increased transmission network capacity and generally enhanced system reliability, security and controllability. The aim of this paper is to evaluate the impact of dc-voltage control strategies on dynamic behaviour of MTDC VSC-Based HVDC after the sudden disconnection of a converter station. Two dc voltage control methods are considered in this paper: (i) voltage margin method and (ii) standard voltage-droop method. The impact is evaluated using time-domain simulations on simple test system using. The sudden disconnection of a converter-station is used as disturbance. Simulation results demonstrate there is a "collaborative scheme " for the dc voltage support when two converters on the MTDC operate with dc voltage droop characteristic. Index Terms—Control system, high voltage direct current, multi-terminal HVDC, voltage source converter. I.