# AC Fault Ride-Through Strategy of a Transformerless Soft Normally Open Point
## 1 Introduction  
The distribution network is used for supplying electricity to power users directly, but it has some problems such as low automation level, weak grid structure, and so on. With the development of photovoltaic power generation, wind power generation, electric vehicles, etc., voltage fluctuation and imbalance become more and more serious. However, traditional measures cannot solve these problems [1, 2]. The soft normally open point (SNOP) is used to replace the traditional loop switch; it can control the active and reactive power flowing through the connected feeder lines, to deal with the stochastic fluctuation caused by new energy and new loads [3, 4, 5]. The application of SNOP can completely change the traditional power supply mode of closed-loop design and open-loop operation, and improve the controllability and flexibility of the distribution network. SNOP can adopt different topologies. For medium voltage, the back-to-back DC distribution system which uses the modular multilevel converter (MMC) is the most feasible one. MMC has been widely used in engineering with low loss and high reliability [6, 7]; it usually connects to the AC system using the transformer to isolate the fault between the AC system and the DC system. However, the transformer will increase the footprint of SNOP, which affects the compact design of SNOP, so the transformerless design should be considered [8]. In the situation without the transformer, the zero-sequence component between the SNOP and the AC distribution system cannot be isolated naturally. The zero-sequence component is transferred through the SNOP from the fault supply area to other supply areas, and the fault influence range is expanded. The fault characteristics of transformerless SNOP are quite different from those with transformers. Scholars have carried out extensiveresearch on AC/DC fault characteristics of flexible DC systems with transformers. Based on the two-level voltage source converter (VSC), reference [9] analyzes the response characteristics of unipolar DC grounding faults and bipolar DC short circuit faults, and reference [10] proposes the fault location strategy and system protection strategy. Reference [11] proposes three topology structures of SNOP, and comprehensively compares their advantages and disadvantages combined with the analysis of AC/DC fault characteristics. Reference [12] analyzes the fault characteristics of the bridge arm fault, and proposes a protection strategy based on closing the sub-module (SM) bypass switch to ensure the reliable action of the AC breaker. Reference [13] studies the relevant characteristics of single-phase grounding faults in AC systems and single-pole grounding faults in DC SNOP systems with a single transformer. Reference [14] analyzes the AC and DC fault characteristics of the transformerless SNOP, and derives the analytical equations of the fault current of the post-fault converter under different working modes. References [15; 16] study MMC topologies with DC fault-clearing capability, and propose a DC fault-clearing strategy for fast fault recovery. However, the fault characteristics and the control and protection of the transformerless SNOP have not been studied in depth. SNOP is usually connected to the end of feeder lines. In this paper, the \(\pm 10\) kV transformerless MMC-based SNOP connected to the 10 kV AC distribution system is taken as the research object. The novelty and contribution of this paper are as follows: (1) A mathematical model containing positive, negative, and zero sequence components under AC fault is established by the circuit analysis and the sequence component decomposition. (2) The voltage fluctuation mechanism at the DC side and the non-fault AC side caused by the zero-sequence voltage at the fault side is analyzed under the case without transformers. (3) The zero-sequence control strategy and the SM improvement strategy of the modular multilevel converter (MMC) are proposed. The structure of this paper is as follows.