*2.1. Fundamentals of MMC-HVDC System*

Limited by the compactness, lightness, and capital cost requirements of the converter station, the symmetrical monopole MMC-HVDC system has a good application prospect. Both domestic and many foreign MMC-HVDC projects have adopted symmetrical monopole converter stations [34,35]. The research in this paper is based on a symmetrical monopole MMC-HVDC system, as shown in Figure 1. The converter stations serve as an interface between DC and AC systems. The half-bridge SM is commonly used in recent MMC projects [36,37]. The work of this paper is based on half-bridge MMC, and the number of SMs of each bridge arm is *n.* The DC side of the converter is grounded through a large resistor *Rg* and the neutral point is grounded. The bridge arm reactor *L*0 can inhibit the interphase circulation current, reduce the bridge arm current's harmonic distortion rate, and suppress the fault current to protect equipment. *Lm* is a current-limiting reactor, which can effectively reduce the rate of change of the fault current after the DC line fault and leave more time for the protection of the DC line and the action of the DC circuit breaker. The DC current *Idc* in the transmission line is measured at the point between the transmission line and the current-limiting reactor. In this paper, the transient states to be considered for protection include PPFs located at F1, PGFs, lightning disturbances (LDs), and lightning faults (LFs) located at F2, SMs short circuit faults (SMFs) on the arm of a phase located at F3, and single-phase grounding AC faults (AG-ACs) located at F4.

**Figure 1.** Diagram of a modular multilevel converter-based high-voltage direct current (MMC-HVDC).

Figure 2 shows the topology of a typical MMC converter, which has three identical phase units with six arms in total. Each arm consists of a reactor *L*0 and *n* SMs in series.

**Figure 2.** Topological structure of an MMC.
