  Voltage Source Converters (VSC) are becoming more common in modern High
Voltage DC (HVDC) transmission systems. One of the major challenges in a
multi-terminal VSCHVDC is protection against DC side faults. Two major designs,
namely, the hybrid DC breaker and the assembly HVDC breaker, are compared for
operational behavior, speed of operation and current carrying capability. The
Dual Modular Redundancy (DMR) technique is utilized for decision making of a
fault scenario. This uses a dual voting system when one result contradicts the
other. This helps in the design of a fail-safe mechanism for the operation of
both types of breakers. Current threshold combined with directional change is
considered for the breaker operation. A three-terminal bipolar VSC HVDC system
is designed in PSCAD/EMTDC and simulation results are utilized to draw a
comparison of the two different designs of DC breakers.
