DC fault protection structures at a DC-link node in a radial multi-terminal high-voltage direct current system

In a multi-terminal high-voltage direct current (HVDC) system, DC circuit breakers (DCCBs) are conventionally connected in a star configuration to enable isolation of a DC fault from the healthy system parts. However, a star connection of DCCBs has disadvantages in terms of loss, capacity, reliability and so on. By rearranging the star connection, a novel delta configuration of DCCBs is proposed. The total loss of the proposed delta configuration is only 33.3% of that of star configuration, yielding a high efficiency. Moreover, any DC fault current is shared between two DCCBs instead of one DCCB in the faulty branch suffering the fault current. Thus, DCCB capacities in the proposed delta configuration are only half of those in a star arrangement. Additionally, in the case of one or two DCCBs out of order, the power can still be transferred among three or two terminals, thereby affording high supply security of all HVDC links. On the basis of the DCCB delta configuration, two novel DC fault protection structures with external and internal DC inductances are proposed. Their characteristics are discussed and it is shown a DC fault can be isolated using slow DCCBs. The results demonstrate DC fault tolerant operation is achieved by using the proposed DC fault protection structures with delta configuration.