This thesis develops a non-pilot protection system for the next generation power transmission system, the High-Voltage Direct Current (HVDC) grid. The HVDC grid protection system is required to be (i) adequately fast to prevent damages and/or converter blocking and (ii) reliable to minimize the impacts of faults. This study is mainly focused on the Modular Multilevel Converter (MMC) -based HVDC grid since the MMC is considered as the building block of the future HVDC systems.

The studies reported in this thesis include (i) developing an enhanced equivalent model of the MMC to enable accurate representation of its DC-side fault response, (ii) developing a realistic HVDC-AC test system that includes a five-terminal MMC-based HVDC grid embedded in a large interconnected AC network, (iii) investigating the transient response of the developed test system to AC-side and DC-side disturbances in order to determine the HVDC grid protection requirements, (iv) investigating the fault surge propagation in the HVDC grid to determine the impacts of the DC-side fault location on the measured signals at each relay location, (v) designing a protection algorithm that detects and locates DC-side faults reliably and sufficiently fast to prevent relay malfunction and unnecessary blocking of the converters, and (vi) performing hardware-in-the-loop tests on the designed relay to verify its potential to be implemented in hardware.

The results of the off-line time domain transients studies in the PSCAD software platform and the real-time hardware-in-the-loop tests using an enhanced version of the RTDS platform indicate that the developed HVDC grid relay meets all technical requirements including speed, dependability, security, selectivity, and robustness. Moreover, the developed protection algorithm does not impose considerable computational burden on the hardware.