**1. Introduction**

Vacuum circuit breakers (VCBs) are widely used in medium voltage power systems while SF6 gas circuit breakers are still the prevailing technology in high- and extra high-voltage networks. However, SF6 gas has been specified as a strong greenhouse gas since 1997 in the Kyoto Protocol and its emission has been strictly restricted. Therefore, it has been of increasing interest to develop VCBs to higher voltage levels due to their advantages such as being maintenance free, having a long mechanical life, excellent dielectric strength, high interruption capability, and environment-friendly characteristics [1].

The major difficulty of extending the application of VCBs to higher voltage levels arises from the physical disadvantage of a vacuum in terms of the dielectric characteristics, i.e., the non-linear relationship between the dielectric strength and contact gap length [2]. To be precise, the breakdown voltage is nearly linear to the vacuum gap length within about 5 mm [3]. However, for a larger gap, the dielectric strength shows a strong full voltage effect. An appropriate way to develop high voltage

level VCBs is to use the double-or multi-break VCBs consisting of two or more vacuum interrupters (VIs) in series, which takes full advantages of the excellent dielectric strength and high interruption capability of short vacuum gaps [4].

Prestrike in VIs happens when the movable contact is approaching the fixed contact during the current-making operation. Once the electric field strength across the contacts becomes larger than the breakdown strength of the vacuum gap, an electric arc may occur prior to the mechanical touch of the contacts, allowing a high inrush or making currents flow through. The prestrike phenomenon in VCBs can cause electromagnetic transients in electrical systems, which may lead to harmful effects and even damage in electrical devices in the system such as transformers, electrical machines and the circuit breaker itself [5]. The pre-breakdown phenomena in VCBs is complicated. There are mainly two theories about the pre-breakdown in VCBs. One is that field-emission current induces breakdown, the other is that microparticles induce vacuum breakdown [6]. If the field strength at the surface of a clean cathode in a VCB exceeds about 2 × 10<sup>7</sup> V/m, then field emission currents will be observed [7]. If the field strength exceeds about 1 × 10<sup>8</sup> V/m, pre-breakdown will occur [8]. Pre-breakdown induced by microparticles mainly occurs at larger gap spacing [9].

Numerous efforts have been made to understand the prestrike phenomenon in vacuum interrupters. Some researchers mainly focused on the melting effect of the prestrike arc on the contacts of the VCB, and this effect would affect the breaking performance of the VCB. To study this, Slade et al. [10,11] undertook a series of experiments to find that as the prestrike gap of the single-break VCB increased, the prestrike arc lasted longer. The prestrike arc could cause the contacts to weld and the damage caused by the welding depended on the arcing time. In order to determine the correlation of the prestrike process and the breaking process of the VCB which mainly was researched in the field of switching capacitor banks, Dullni et al. [12] found that the restrike phenomenon had some correlation with the prestrike process mainly due to the protrusions caused by the prestrike arc on the contacts in the single-break VCB. Körner et al. [13,14] obtained similar but more detailed results to Dullni. They found that the significant development of local protrusions on the contact surface affected both the microstructure and the macrostructure of contact surface during the capacitive making and breaking process. Some researchers focused on the welding force caused by the prestrike arc on the contacts of the VCB. If the value of the inrush current was high enough, the contacts would be melted and become welded. The welding force was a basic value that could reflect the melting degree of the contacts. Kumichev et al. [15] found that there was a correlation between welding forces caused by capacitive prestrike process and the number of non-sustained disruptive discharges (NSDDs). Yu et al. [16,17] found that both the amplitude and the scattering of the prestrike gaps and the welding force were significantly influenced by the contact materials about the capacitive current prestrike in single-break VCB. The damage caused by the prestrike arc was mainly influenced by the value of the prestrike current (5 kA, 10 kA or 20 kA). The material and structure of the contacts could also influence the prestrike arc behavior. Geng et al. [18] found that axial-magnetic field (AMF) contacts can effectively reduce the erosion of the contacts produced by the prestrike arc when the capacitive current is generated because of the diffusing effect produced by the AMF in single-break VCB, and higher inrush current value would cause larger damage on the contacts. Some researchers have undertaken calculations and simulations in order to gain an in depth understanding of the prestrike process. A.A. Razi-Kazemi et al. [19] established a new realistic transient model for prestrike phenomena in single-break VCB. Non-linear movement of the contacts, probabilistic behavior of the breakdown voltage and the non-linear dielectric strength curve of the VCB were fully considered in this model. Other researchers focused on the prestrike process mainly due to the design or application of the VCB. X. Ma et al. [20] undertook experiments to observe the direct current (DC) pre-breakdown and breakdown characteristics of micrometric gaps varying from 25 to 1000 μm. The results showed that the pre-breakdown conduction was dominated by high field-electron emission and could be valuable for the design of vacuum gaps in field emission displays. Kharin et al. [21,22], in a series of papers, undertook calculations and experiments to explore the complexity of closing single-break vacuum

interrupter contacts, in order to make an electrical circuit. Sima et al. [23] found that phase-controlled VCBs can reduce the transient overvoltage and overcurrent compared with ordinary VCBs when they were used to switch 10 kV shunt capacitor banks. However, previous studies mainly focus on the prestrike characteristics of single-break VCBs. At present, experimental investigation of the prestrike characteristics of a double-break VCBs has not been reported so far.

The objective of this paper is to determine experimentally the probabilistic characteristics of prestrike gaps in a double-break VCB under DC voltages. This work can provide very important information on the prestrike arcing process, which is of significant importance for the study of the dielectric performance in VIs. Moreover, a better knowledge of the scatter in prestrike gaps in VIs can be very helpful to improve the control accuracy of phase-controlled switching.
