This paper presents a transient and non-unit-based protection scheme for consideration in DC grids of the future. The technique utilises the rate-of-change (R-o-C) of the associated travelling wave components following the occurrence of a fault to determine whether the fault is internal or external. For an internal fault, the product of the magnitude of the R-o-C of the fault induced voltage and current travelling wave following fault inception must exceed a predetermined setting, otherwise the fault is external. The DC inductor located at the cable ends provides attenuation for the high frequency contents of the fault generated components resulting from an external fault. The ratio between the forward voltage travelling wave and the backward voltage travelling wave provides directional discrimination. This ratio is less than unity for a forward directional fault and greater than unity for reverse directional faults. The protection algorithm has been validated using PSCAD/EMTDC simulations based on full scale modular multilevel converter (MMC)-based HVDC grid. The simulation results presented, including the performances indices compared to existing and proposed methods available in literature utilising the derivative of the fault induced components show the suitability and reliability of the proposed technique in distinguishing between internal and external faults. Key advantages of the proposed technique is that it simple, easily implemented, and does not rely on complex signal processing technique; and therefore it can easily be implemented to provide autonomous tripping for all relays located on the DC grid.</p
