In a multi-terminal 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, etc. By rearranging the star-connection DCCBs, a novel delta-configuration of DCCBs is proposed in this paper. As each terminal is connected to each of the other terminals through only one DCCB, the current flows through only one DCCB when transferring power between any two terminals compared with two DCCBs in the current path for the conventional star-arrangement. The total loss of the proposed delta-configuration is only 33.3% of that of star-configuration, yielding a high efficiency. Also, any DC fault current is shared between two DCCBs instead of one DCCB in the faulty branch suffering the fault current. As a result, DCCB capacities in the proposed delta-configuration are only half 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. Based on 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 without exposing any converter to significant over-current. The results demonstrate DC fault tolerant operation is achieved by using the proposed DC fault protection structures with delta-configuration.
- DC fault protection
- DC inductance
- delta-configuration of DC circuit breakers
- multi-terminal HVDC system
- modular multilevel converter (MMC)