# A Novel Overcurrent Suppression Strategy during Reclosing Process of MMC-HVDC
## 1 Introduction  
The modular multilevel converter (MMC), a new type of voltage source converter (VSC), has many advantages over the traditional two or three level topology, such as better harmonic performance, low switching frequency, etc. This advanced technology is envisioned as the most promising option for the integration of renewable energy sources and has been widely investigated in recent years [1; 2; 3]. In China, an MMC-HVDC grid usually uses overhead lines because large-scale renewable energy bases are generally far from load centers, as it makes the fault ride-through of MMC-HVDC grid become more thorny [4].  
Currently, the half-bridge sub-module (HBSM) is the main topology of a practical MMC-HVDC grid because of better economy [5]. When a short-circuit fault occurs on the DC line, very large fault currents will be caused in the DC lines and converters [6]. Since temporary faults often occur on overhead lines, MMC-HVDC needs reclosing to improve the reliability and continuity of the power supply [7]. Consequently, the vulnerable power electronic elements in the converter will be exposed to overcurrent again when the system attempts to reclose a permanent fault.  
The suppression of overcurrent mainly depends on an effective fault current limiter (FCL). Reference [8] limited the overcurrent in HVDC lines by a current limiting inductor (CLI) and studied the impact of different sizes on current rise. Reference [9; 10] studied the performance of a superconducting fault current limiter (SFCL) and concluded that the appropriated SFCL can reduce the fault current peak, the size of the CLI and also the fault identification time. Reference [11] presented a solid state FCL in a VSC-HVDC system with MMC converters, but the detailed structure and the configuration method were not clear. Reference [12; 13] both proposed a resistive DC fault current limiter. The limiterwas installed in each arm or at the outlet of the converter to increase the damping performance of the DC fault current loop and effectively limit the dc-side current, and then interrupt the fault circuit by a breaker. Reference [14] proposed a RL-FCL which is composed of a reactor, a resistor, a reversed diode and an IGBT connected in parallel to limit the overcurrent and the overvoltage. The aforementioned references mainly focused on the overcurrent limiting when the fault occurred but did not conduct further research on the post-fault and the reclosing process.  
There has been little research on reclosing process, especially for MMC-HVDC. Reference [15] used two thyristors in parallel with the sub-modules to realize fault clearance and automatic recovery, but the converter must be blocked for a long time when reclosing a permanent fault. Reference [16] used series braking resistance (SBR) cooperating with a high-temperature superconducting FCL to reduce the fault currents during reclosing, but it did not illustrate how to select the parameter of the SBR. A soft reclosing model (SRM) is proposed in Reference [17] to limit the reclosing over-current of VSC-MTDC. However, the current calculation in this reference is made on the assumption that the voltage discharge voltage remains constant and neglects the restriction of the safety of the equipment. Overcurrent suppression during the reclosing process of MMC-HVDC needs further research.  
This paper focuses on the suppression of overcurrent for overhead lines based MMC-HVDC grid during the reclosing process. First, mesh current method (MCM) based short-circuit current calculation is introduced to a loop MMC-HVDC grid. Then a novel FCL, which consists of a CLI, a reclosing current limiting resistance (RCLR) and a bypass switch, is proposed. Compared with traditional FCL, the proposed circuit can limit the maximum arm currents to a specified value during the reclosing process, and the calculation method of RCLR is also given in details.