The objective of the proposed research is to develop a complete approach towards the protection of Multi-Terminal Direct Current (MTDC) systems. Firstly, to understand the propagation of faults on the DC transmission network, an in-depth analysis of fault characteristics has been performed. This analysis enables optimal sizing of DC circuit breakers, which reduces the maximum over-current and over-voltage applied to the system during faults. To further improve the performance of such breakers, a new control strategy to sequentially trip breaker modules has been proposed. This strategy expedites fault clearance and reduces the amount of energy that needs to be absorbed. Meanwhile, fast and reliable protection relaying algorithms have been developed. Backed up by a secondary layer, these algorithms detect the DC faults immediately after its occurrence and command the circuit breakers to clear the faults. Finally, a real-time simulation platform accelerated by Graphics Processing Unit (GPU) is developed to test and evaluate the complete protection system on a demonstration MTDC network.