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As the second part of two series paper, the protection scheme is proposed. First of all theoretical analysis and threshold calculation methods are introduced. Aiming to the problem that relay needs to send alert signal instead of trip signal in non-effectively grounded system when single pole to ground fault occurs, a fault type identification method is introduced based on the ratio of absolute values of two poles voltage derivatives. Then 1 mode voltage based protection scheme is developed and the feasibility of design of criterion based on pole voltages, impact of sampling frequency, fault types, resistance and branch on the proposed criterion are discussed comprehensively. MMC based two-terminal and radial and VSC based four-terminal dc distribution systems are simulated in PSCAD. Relay behaviors are tested in three systems under different locations, resistances and types of faults. Results show that the proposed methods can be applied to both MMC and VSC based dc distribution systems and are immune to the fault type, fault location and fault resistance.

DC distribution network has a lot of advantages over traditional ac distribution network and it is considered as one of the best power supply modes in the future. However, protection relay design for dc distribution network is a major challenge due to the requirement of high speed fault detection for the vulnerability of converters. In this paper, it is proposed that the boundary protection is very suitable for dc distribution network since it can protect the whole line only based on local measurement and does not need to coordinate with neighboring relays. However, it needs to find suitable line boundary for dc distribution network. Impedance features of magnetic ring is tested and difference in steepness and peak time of voltage travelling wave in case of internal and external faults when magnetic ring installed at both ends of dc distribution line is analyzed. Results show that magnetic ring is feasible to be used as line boundary. Besides, a wide band model of magnetic ring is proposed based on vector fitting and circuit synthesis method. It paves the way of analysis on features of internal and external faults at dc distribution line and validation of proposed fault detection method.

The traditional fault location methods on DC lines are facing many challenges such as low reliability and unsatisfied location accuracy. To ensure a more reliable fault recovery of DC line, a single-ended fault location method based on active detection is proposed for hybrid modular multilevel converter based high voltage DC transmission systems. The high controllability of hybrid MMC is utilized to activate the control strategy of injecting a sinusoidal signal. Furthermore, based on parameter identification and compensation methods for the distributed parameter model, the fault location equation with fault impedance parameters is formulated. Finally, the fault location can be identified under different fault scenarios. Simulation results verify the effectiveness and accuracy of the proposed active detection method. The proposed method can pinpoint the fault location with a lower sampling rate, its accuracy is not influenced by increasing of the line length and variation of the fault impedance.