Search

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. [ABSTRACT FROM AUTHOR]

This paper proposes an adaptive fault current limiting control (AFCLC) for modular multilevel converters (MMC). Without introducing extra current limiting devices, this control scheme enables fast fault current suppression during DC faults. The AFCLC will be triggered automatically once DC faults occur. By adaptively reducing the output DC voltages of MMCs, the fault current can be suppressed. Compared with the existing current limiting methods, the proposed AFCLC has a better performance on fault current limiting, since it only depends on the real-time operating condition and no fault detection delay is imposed. Firstly, the principle of the proposed AFCLC together with the mathematical analysis is disclosed. Then, the sensitivity analysis of the impact of key control parameters on the current limiting effect is investigated. Finally, the effectiveness of AFCLC is demonstrated in a four-terminal HVDC grid test model. The simulation results show that the proposed AFCLC can reduce the interrupted current and energy absorption of a DCCB from 10.39 kA and 38.24 MJ to 4.62 kA and 8.32 MJ, respectively. The simulation results also prove that the AFCLC will not affect the accuracy of the DC fault detection algorithms under DC faults. [ABSTRACT FROM AUTHOR]

• The DCCB is able to automatically re-close after DC faults with minimum cost. • The operating principle and the dynamics during DCCB operation are analysed. • The general parameters design method of the proposed mechanical DCCB is disclosed. • The feasibility of the mechanical DCCB is verified in a four-terminal meshed DC grid. For the modular multilevel converter (MMC) based DC grids using overhead line transmission, to restore the power transmission quickly, the DC circuit breakers (DCCB) are required to be able to re-close after DC line faults. To meet this requirement, an improved mechanical DCCB topology with re-closure capability is proposed in this paper. It consists of a pre-charged capacitor, an auxiliary capacitor, an oscillation inductor, and anti-parallel thyristors. Firstly, the topology and operating principle of the mechanical DCCB are presented in detail. Then, the general sizing methods associated with the impact of parameters on interruption capability are disclosed. The feasibility of the proposed mechanical DCCB is validated in a stiff DC system. Finally, the transient performance of the DC grid embedding the proposed mechanical DCCBs is investigated under various operating scenarios. The results verify the DC fault interruption and fast automatic re-closure capabilities of the proposed DCCB. It is shown that the mechanical DCCBs can be well applied to the overhead MMC based HVDC grids systems. [ABSTRACT FROM AUTHOR]