Fast and reliable DC fault detection is one of the main challenges for modular multilevel converter (MMC) based DC grid with DC circuit breakers (DCCBs). This paper extracts the high-frequency components in transient voltages by wavelet transform and proposes a fault identification method based on the difference of square of transient voltages to identify the faulted lines for DC grids using overhead lines. Meanwhile, a faulted pole discrimination method based on the difference between the change of positive and negative pole voltages is presented. A line protection scheme including detection activation, fault identification, faulted pole discrimination and post-fault re-closing is designed. Using only the local measurements, the scheme can realize the protection of the whole line without communication and has the capability of fault resistance endurance and anti-disturbance. The proposed method is tested with a four-terminal MMC based DC grid in PSCAD/EMTDC. The selection methods of threshold values are presented and the impact of DCCB operation on the reliability of DC fault protection is analysed. Simulation results verify the fast detection and reliability of the designed DC line protection scheme.

To protect the converters and minimize the power transmission interruption during dc line faults, it is necessary to detect the faults at ultra high speed for the modular multilevel converter (MMC)-based HVdc grids. This article reviews the state-of-the-art of dc fault protection methods of MMC-HVdc grids, and summarizes the underlying principles of each method. On this basis, the dc fault characteristics analysis in terms of modal-domain, time-domain, and frequency-domain are analyzed, which direct the protection design. Typical boundary protection and nonboundary protection schemes are reviewed. The advantages and disadvantages of existing fault protection methods are compared. A two-terminal MMC-HVdc prototype is developed to test the effectiveness of three fault protection methods. Comprehensive quantitative assessments of the protection methods discussed above are carried out in a four-terminal MMC-HVdc grid. Finally, the future trends of the protection schemes are discussed and the findings are concluded.

This paper presents an enhanced independent pole control scheme for hybrid modular multilevel converter based on a full-bridge submodule and half-bridge submodule. A detailed analysis of the power distribution between upper and lower arms under asymmetrical dc pole voltages is presented. It is found that the fundamental ac currents in the upper and lower arms are asymmetrical. To enable operation under asymmetrical dc pole voltages, an enhanced independent pole control scheme is proposed. The controller is composed of two dc control loops, two ac control loops, and circulating current suppression control based on current injection. Six modulation indices are presented to independently control the upper and lower arms. With this controller, the dc voltage operating region is significantly extended. To ride through a pole-to-ground dc fault without bringing dc bias at the neutral point of the interface transformer, a pole-to-ground dc fault ride through strategy is proposed. The feasibility and effectiveness of the proposed control scheme are verified by simulation results using PSCAD/EMTDC.

To integrate weak onshore grids, some grid forming (GFM) control schemes taking advantage of DC voltage synchronization have been proposed for modular multilevel converter-based high-voltage direct current (MMC-HVDC) systems. In these control schemes, the sub-module (SM) capacitor energy is only used for grid synchronization. Since the SM capacitor is an energy storage device, it has the potential to actively regulate the stored energy. To fully exploit the energy management capability of MMC, a dual grid forming control scheme with energy regulation capability is presented in this paper. The proposed control scheme enables grid forming for both onshore and offshore MMC stations. Meanwhile, the capacitor energy of MMCs can be flexibly regulated to provide frequency support for the onshore grid. To verify the effectiveness of the proposed control scheme, case studies are first carried out in a point-to-point offshore wind farms (OWFs) integrated HVDC system. In addition, the frequency support performance is evaluated in a 3-area 4-machine grid integrated with two OWFs-HVDC systems. Finally, a single-terminal three-phase MMC prototype is developed to test the voltage decoupling and energy management capabilities of the proposed control strategy.

Modular multilevel converter (MMC) based high voltage direct current transmission (HVDC) is an effective solution for large-scale renewable power integration over long-distance. In the overhead MMC-HVDC systems, the high voltage DC circuit breakers (DCCB) are implemented to interrupt the DC fault current. Subsequent to fault isolation, the DCCBs are required to automatically re-close to restore power transmission quickly. However, when the DCCBs are re-closed to permanent faults, they will be tripped again, resulting in a high requirement of interruption capacity for DCCBs and second overcurrent strikes on the HVDC systems. To overcome the drawbacks of the conventional auto-reclosing scheme, this paper proposes an adaptive reclosing scheme based on the active pulse injection from the converter associated with the coordination control of hybrid DCCBs. The single-end adaptive reclosing scheme as well as two ends adaptive reclosing scheme dedicated to two-terminal HVDC systems and meshed DC grids are presented respectively. By applying this method, the location of faults can also be achieved in the case of permanent faults. In order to verify the effectiveness of the proposed adaptive reclosing schemes, extensive simulations have been conducted under PSCAD/EMTDC.

To detect the DC faults for MMC based DC grids using overhead line transmission, many protection methods in phase-domain have been proposed. These existing protection methods suffer from incomplete function, weak theoretical basis and sensitivity to fault resistance and noise disturbance. To overcome these shortcomings, this paper proposes an improved DC fault protection algorithm using the modal-domain approach for the MMC based overhead DC grids, which decouples interaction between positive and negative poles and mitigates the strong frequency-dependency of the characteristic impedance in phase-domain. The DC fault equivalent circuits are established in modal-domain and the fault characteristics during the initial stage are analysed. Based on the modal-domain analysis, the line-mode reactor voltage which combines fault characteristics of negative and positive reactor voltages, is employed to identify the internal faults. The zero-mode reactor voltage which enlarges the differences between faulty and healthy poles, is employed to select the faulted pole. This method is robust to fault resistance and noise with high detection speed. In addition, it is not affected by power reversal, AC faults and DCCB operation, which are validated and evaluated by simulations in PSCAD/EMTDC.