Effect of power flow control methods on the DC fault response of multi-terminal DC networks

Fault protection aspects are very crucial for the realization of multi-terminal dc (MTdc) networks. This paper investigates the impact of power flow control strategies on the response of MTdc networks to a dc fault. The successful ride-through of a dc fault is defined by two factors: first, a fast fault isolation and second, a fast post-fault system recovery. According to these criteria, two control methods are compared: the common Single converter Voltage Control (SVC) method and the Distributed Voltage Control (DVC) strategy. The main difference between the two methods is the number of the converter stations used for the control of the dc voltage in the MTdc network. A five-step methodology is proposed to determine the dc lines critical loading and to assess the system current and voltage response to a pole-to-ground dc fault. The simulated network consists of four voltage-source converters (VSC) radially connected using a bipolar topology with metallic return. The study shows that both control strategies have the same effect on the system´s faulty pole operation, provided that the protection design ensures the system post-fault direct voltage controllability. When the DVC strategy is used, the MTdc network has more redundancy, which enables it to resume operation in most dc fault cases, despite having a higher effect on the post-fault operation of the remaining `healthy´ pole converters.