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With the rapid development of flexible DC transmission and multi-terminal high voltage direct current transmission (HVDC) (MTDC) grid, the protection schemes of flexible DC transmission lines are still the traditional methods, which do not achieve the properties of fast and selective operation in the DC grid. In this study, using Zhangbei MTDC grid as the research object, the fast protection of DC transmission lines of MTDC grid has been studied. First, the voltage response characteristics of fault line and non-fault line at the boundary of DC lines are analysed. On the basis of this, using wavelet transform to extract the characteristic value of voltage pole-wave, this study put forward a high-speed non-unit protection criterion to identify the internal and external faults. In addition, two auxiliary criteria are constructed to identify lightning disturbance and faulted pole, respectively. Finally, a simulation model of Zhangbei MTDC grid is constructed, and the effectiveness of the proposed protection scheme is verified by extensive simulation results.

Voltage source converters (VSCs)-based high-voltage direct current grid (HVDC grid) is increasingly being used in high-level and long-distance power transmission. DC breakers could be applied to isolate fault on long overhead DC links, whereas most of these faults would be temporary and can be recovered after some deionisation time. The objective of this article lies in proposing and implementing DC line soft reclosing sequence based on the controllability of hybrid HVDC breaker (HHB). The proposed method has been evaluated in a four-terminal VSC-based HVDC grid with HHBs at each DC lines' end. The analysis and simulations in PSCAD/EMTDC show that the proposed sequential operation of HHB can softly recover the faulty line and identify whether the fault is temporary or permanent, with little impact on the whole DC system.

This work studies the application of short-time Fourier transform (STFT) to extract current harmonics of various fault types to design a protection method for meshed multi-terminal voltage source converter-high-voltage direct current (VSC-HVDC-MTDC) grids. The frequency spectrum of fault current harmonics is used to detect internal and external faults, the faulty pole and fault type. The method does not need the implementation of series inductance on the sides of the DC lines and it is also effective for lines combining overhead lines and cables. The window function and hop size play the primary role in STFT, which are investigated in a sensitivity analysis. A modified version of CIGRE meshed DC grid system is simulated in PSCAD in order to show the robustness of the method, and further signal processing analysis is done in MATLAB. The results show the accurate detection and discrimination between faulty sections using the proposed harmonic-based method and how important the STFT parameters are in order to have a robust protection algorithm.

In order to reduce the cost of energy devices for IGBT drive circuits of the hybrid HVDC breaker, a self-energy device for the HVDC breaker is proposed here, which connects to DC lines and takes power from DC lines directly. The device consists of three operating modes: the start-up charging mode, the steady-state charging mode, and the maintenance restart charging mode. In view of these modes, corresponding control strategies are proposed to ensure the stability of power supply to drive circuits, and the simulation model is built and studied. For multi-terminal HVDC systems or HVDC grids, several self-energy devices in different lines can be connected through a certain combination, which can get the DC current flow regulation function.

Overvoltage caused by faults or switching is one of the decisive factors affecting the insulation level of equipment in UHVDC transmission systems. As for ±1 100 kV transmission projects, there has been no clear conclusion about the overvoltage distribution characteristics along transmission lines. This paper studies characteristics of the overvoltage distribution along ±1 100 kV transmission lines after building the simulation platform of ±1 100 kV UHVDC transmission systems with the use of PSCAD software. The typical fault conditions are simulated such as monopolar line-to-ground fault and line-broken-down fault of transmission lines. The distribution characteristics of overvoltage along the line under different faults are obtained. The simulation results show that the arresters installed along the lines can significantly reduce the switching voltage level of DC lines. The voltage-current characteristics and the installation density of the arresters along the line have an important influence on the overvoltage suppression effect. Through the comparative analysis, a configuration scheme of arresters along the lines is proposed, which has a great effect for overvoltage suppression. The results are instructive for the design of insulation coordination in practical projects.