**1. Introduction**

With the development of fully controlled power electronic devices (FPEDs), such as the gate turn-o ff thyristor (GTO), insulated gate bipolar transistor (IGBT), integrated gate commutated thyristor (IGCT), and injection-enhanced gate transistor (IEGT), the hybrid direct current (DC) circuit breaker based on power electronic technology has become an important technical solution and research hotspot for DC breaking [1–4]. This type of DC circuit breaker combines the advantages of solid-state switches (SSs) and mechanical switches (MSs), and shows the characteristics of a strong current carrying capacity, low on-state loss, fast breaking speed, and no need for cooling devices [5,6].

According to the di fference of the current commutating method from MS to SS, hybrid DC circuit breakers based on power electronics technology are divided into forced-commutate hybrid DC circuit breakers (FHCBs) and natural-commutate hybrid DC circuit breakers (NHCBs) [7,8]. The FHCB is mostly used in the field of voltage above 10 kV [9,10], and one of the most representative FHCBs is the 320 kV/2.6 kA hybrid DC circuit breaker developed by ABB in 2011 [11]. NHCB is mostly used in the voltage field of 10kv and below, and there has been a lot of research on the NHCB [12]. Genji et al. [13] proposed the NHCB firstly in 1994, and SS of NHCB is mainly composed of GTO. Polman et al. [14] developed a 600V NHCB by fast MS and IGBTs in 2001, and it is a bi-directional switch. Luca Novello et al. [15] developed a prototype of 1 kV/10 kA NHCB by using IGCT's series and

parallel technology in 2011. Lv gang et al. [16] studied the vacuum arc commutation characteristics of NHCB, and a 10 kV/3.5 kA NHDB prototype was designed and tested.

There have been some studies on the vacuum arc commutation characteristics of NHCB. The vacuum arc commutation model was constructed in [16,17], and the theoretical calculation formula of the rate of the current rise in the commutation branch is given. In [18], a theoretical analysis model is established, and the influence of adding impedance on vacuum arc commutation was studied. The authors of [19] simplified an equivalent treatment of the IGBT module in SS, and the criteria of the vacuum arc commutation were obtained. The authors of [15] studied the vacuum arc characteristics at different breaking distances and different currents. The authors of [20] studied the two current commutation measures for hybrid DC circuit breakers, and proposed a new current commutation measure. The authors of [21] studied the influence of internal and external factors on vacuum arc commutation, and reasonable parameters that can promote vacuum arc commutation were summarized.

The duration of vacuum arc commutation is a very important parameter, as it will directly affect the arcing time and interrupting time of NHCB, and shortening it will facilitate the interruption of NHCB. However, little work has focused on the duration of vacuum arc commutation, and, especially, there is a lack of mathematical formulas about it. In this regard, we conducted research on the vacuum arc commutation characteristics of NHCB, and focused on the influence of the main parameters on the duration of vacuum arc commutation. Section 2 establishes a simplified vacuum arc commutation model of NHCB, and theoretically derives the influence of the main parameters on the vacuum arc commutation characteristics. Sections 3 and 4 establish a vacuum arc commutation test circuit, and experimentally study the influence of the main parameters on the vacuum arc commutation characteristics. Section 5 first discusses the theoretical derivation and experimental results of the previous two sections, proposes the concept of the vacuum arc commutation coefficient, and then discusses the influence of the main parameters on the vacuum arc voltage. Finally, Section 6 concludes this paper.
