## 1 Introduction  
Implementing DC grids having enhanced controllability and lower energy losses could be a solution for integrating renewable energy sources that have an inherently intermittent nature. In particular, voltage source converter (VSC) technology has been identified as the best candidate for realizing DC grids due to its ability to control the AC voltage magnitude, phase angle, and output frequency. With the application of pulse width modulation (PWM) control, VSCs can offer higher response time and lower harmonic content [1; 2; 3]. However, due to the absence of current zero and high di/dt resulting from DC faults, fault handling is a serious issue faced by DC transmission lines and its solution is urgently required to further the implementation of DC grids [4; 5].  
Recently, modular multilevel converters (MMCs) have eliminated the shortcomings of conventional VSC topologies. With the use of a modular structure, it has been possible to achieve very high levels of voltage, which are desirable for bulk power transmission [6; 7]. As for MMCs, the efficient control schemes and operational characteristics of MMCs in a DC grid have been a subject of interest in academia and industry, and many solutions have been presented [8; 9; 10]. However, DC fault and its effective handling have been identified as one of the most serious challenges in the implementation of VSC-type DC transmission systems [11; 12]. The impact of DC fault location on fault interruption time and maximum fault current magnitude has not been considered previously [13; 14]. The search for the best MMC topology, control schemes, protection devices, and the combination of all or some of these factors, considering the nature of the DC fault, is the subject of this paper.  
A half bridge (HB) MMC necessarily requires a DC breaker because of its lack of DC fault blocking capability. Therefore, a hybrid circuit breaker (HCB), which can cope with the expected rapid rise of the fault current in an HVDC grid, was used in conjunction with an HB-MMC transmission line [15; 16]. This combination is referred to as Case-1 in the rest of the paper.  
On the contrary, the reverse voltage phenomenon in a full bridge (FB) MMC can limit the fault effectively within a few milliseconds. Therefore, a DC circuit breaker (DCCB) is not required. A residual circuit breaker (RCB) or an ultra-fast disconnector is used to completely remove the residual fault current and isolate the fault. This combination is referred to as Case-2 in the following sections of the paper [17].  
In this paper, the fault handling performance of two mainstream MMC topologies, including HB- and FB-MMCs in combination with the DC side circuit breaker, were investigated. Contrary to the existing research, in which the fault location is usually fixed (center of the line), this paper considered a variable fault location on the DC line, so as to compare the fault interruption time and maximum fault current magnitude. Since a power system must bear the current and the voltage stress during fault occurrence and recovery, the current and voltage transients on the AC and DC sides of the power system were thoroughly analyzed.  
This paper is organized as follows. The operation characteristics of the MMC and simulation model are explained in Sections 2 and 3. The interruption performance of the HB- and FB-MMC DC grids is presented in Section 4. The transient performance of the system in DC faults is presented in Section 5. The strengths and weaknesses of the two cases and the suitable MMC grid topology from the point of view of fault handling performance are proposed. The conclusion is presented in Section 6.