The pole-to-ground fault is one of the most typical faults in modular multilevel converters (MMCs) based DC grid. Analyzing the transient characteristics of fault voltages and currents is of great significance for fault identification, configuration of protection, and optimization of system parameters. This paper proposes an equivalent transient characteristics calculation method for pole-to-ground faults for symmetrical monopolar MMC based DC grid. Firstly, according to the mechanism of the pole-to-ground faults, the fault network and the steady-state network are formed. Then, the star-delta transformation is performed on modelling the MMCs in the fault network, which overcomes the problem that the number of the inserted sub-modules (SMs) in each arm is time-variant. Based on the discrete adjoint model, the pre-fault and faulted matrices describing the voltages and currents are established, and a general method for solving the transient voltages and currents is proposed. Finally, the proposed method is validated in a four-terminal symmetrical monopolar MMC based DC grid. The results show that the proposed method is of high accuracy, and is much more time efficient than time-domain simulation.

For the modular multilevel converter (MMC) based dc grid, dc fault current suppression is a key issue. The existing fault current limiting methods by controlling half-bridge MMCs will result in valve-side overcurrent and arm overcurrent. Thus, limiting the fault current with minimizing the impacts on the ac side is necessary. Generally speaking, arm dc reference voltage determines the contributions of the sub-module capacitors to the fault current, whereas arm ac reference voltage influences the ac-side currents. Therefore, the idea of reducing arm dc reference voltage but keeping arm ac reference voltage unaffected is born. However, arm dc reference voltage is only composed of the dc component and cannot be reduced lower than the peak value of arm ac reference voltage. This constraint limits the suppression effects of the above idea on the fault current. To overcome this obstacle, this article tries to further suppress the fault current through arm voltage reconstruction. First, the principle of suppressing the dc fault current without affecting the ac side by only reducing arm dc reference voltage is investigated. Then, a way to reconstruct arm dc reference voltage is proposed. Through reconstruction, arm dc reference voltage can be further reduced to a value lower than the peak value of arm ac reference voltage. Thus the fault current can be deeply suppressed. Finally, simulation verification and comparisons with the existing methods are conducted. The results show that the proposed method can deeply suppress the dc fault current with little effects on the ac side.

The modeling of the Ultra-High Voltage (UHV) tower plays an important role in lightning protection analysis of transmission lines because the model used will directly affect the reliability of the results. Moreover, the higher the voltage level is, the more prominent the impact becomes. This paper first analyzes the inapplicability of the Hara multi-segment multi-surge impedance model for the ±1100 kV UHV towers, and then builds a non-uniform transmission line model of the tower. Secondly, the multi-segment multi-surge impedance model is used to study the influence of the tower’s spatial structure changes on its electromagnetic transient characteristics. It is concluded that the more accurately the nominal height of the tower is modeled, the more accurately its electromagnetic transient response is reflected. Finally, the lightning electromagnetic transient responses of the tower with the non-uniform transmission line model and with the multi-segment multi-surge impedance model are compared and analyzed, which shows that the non-uniform transmission line model is more in line with the actual situation under the lightning strikes.

DC circuit breaker (DCCB) is one of the most promising solutions for handling DC fault in half‐bridge modular multilevel converter (MMC)‐based DC grid. Generally after fault isolation, DCCBs are required to have the ability to quickly reclose so as to restore power transmission. However, the traditional simultaneous reclosing scheme may make the whole system suffer from secondary strikes such as overvoltage and overcurrent in the event of permanent faults. To solve this problem, many adaptive reclosing schemes have been proposed. Among them, the sequential reclosing scheme can achieve rapid recovery without laying any burden on the sampling and protection system. Unfortunately, secondary strikes under a reclosing failure still exist though they can be suppressed. Due to this, this paper illustrates that the overcurrent protection‐based fault identification method is not able to identify the fault in time, and thus the secondary strikes are generated during the second tripping. Optimal configuration of DCCBs' parameters can reduce these adverse impacts but cannot avoid them. Based on this, an adaptive reclosing scheme is proposed. Permanent and temporary faults are recognised according to the voltage characteristics at the beginning of the fault line as soon as the arresters are conducted. Extensive simulations on a four‐terminal DC grid in PSCAD/EMTDC show that the proposed method can eliminate the potential adverse impacts and is robust to fault resistance.

Hybrid circuit breakers (CBs) are the most promising technology to isolate DC faults in modular multilevel converter (MMC)‐based DC grids. However, they consist of expensive power electronic components that are sensitive to high overvoltage and overcurrent. This study proposes a hybrid high‐voltage DC circuit breaker with an energy absorption branch of a parallel arrester structure, and investigates the possibility of reducing the fault current and the switching overvoltage. First, the basic principle of an energy absorption branch with a parallel arrester structure is presented. Then, the simultaneous and sequential insertion strategies are illustrated. Second, each strategy and each structure are combined separately to analyse their respective characteristics in reducing the overvoltage, the fault current, the energy absorption and the fault clearance time. The sequential insertion strategy of the proposed energy absorption branch is proved to have the best performance. Finally, the trade‐offs between these four metrics are achieved through the non‐dominated sorting genetic algorithm II (NSGA‐II) . A general method to determine the parameters of the proposed energy absorption branch from the Pareto front based on different preferences is provided. Simulations on PSCAD show that sequential insertion of the proposed energy absorption branch with the optimal parameters is able to suppress the switching overvoltage and limit the fault current to a relatively low extent simultaneously.

Fast detection and isolation of direct current (DC) faults are key issues for DC grids. Therefore, it is very necessary to study the fault protection principle for DC grids. This paper firstly presents the main difficulties in DC fault protection. Then, a local protection and local action strategy for isolating the DC faults is proposed. To illustrate the performance of the proposed protection strategy, a four-terminal DC grid with the hybrid high voltage direct current (HVDC) circuit breakers (HVDC CBs) is constructed in the time-domain simulation software PSCAD/EMTDC as the test system. The systematical comparison between the ordinary protection strategy and the proposed strategy is carried out. The protection selectivity of the proposed local detection and local action strategy is thoroughly studied through complete DC line fault scanning of the test system. The simulation results show that the proposed strategy is of high protection selectivity and speed. In addition, the current rating and the voltage of HVDC CB could be greatly reduced with the proposed strategy, which proves the economic benefits of the proposed strategy.