**2. Experimental Setup**

Figure 1 shows an experimental circuit, including a capacitor bank *C*s, a double-break VCB, a capacitive-resistance voltage divider, and an auxiliary electrical circuit. Figure 2 shows a picture of the experimental setup. During the experimental tests, the capacitor bank was initially charged to a certain voltage *U*s, which was applied to the double-break VCB as soon as the switchgear SWDC was closed. The VCB was then triggered to close the circuit. During the closing operation, the moving contact was approaching the fixed contact leading to a decrement of the dielectric strength of the vacuum gap between the contacts. When the pre-charged voltage exceeded the breakdown voltage of the vacuum gap, prestriking occurred before the mechanical touch of the contacts.

The double-break VCB under test included two commercial 12 kV vacuum interrupters in series, which were denoted VI\_A and VI\_B as shown in Figure 1. Cup-type AMF contacts were adopted. The contact material was CuCr30 (30% weight of Cr). The surfaces of the contacts were initially well-conditioned. A capacitive-resistance voltage divider, which can reduce the influence of the stray capacitance to ground, was used to measure the voltage *U*m across the VCB during the transient prestrike process. When the first prestrike occurred, a stepdown in the measured voltage waveform of *U*m was then captured. Moreover, with the help of an auxiliary circuit, which included a battery and two resistors in series that were connected to the auxiliary contact (SWau) of the VCB, the time instant of the mechanical touch of the contacts in VIs can be detected from the waveform of voltage *U*a. The displacement curve of the moving contact, denoted by *<sup>L</sup>*disp, was measured by using a high-precision linear displacement transducer. Figure 3 shows an example of the waveforms of the measured voltages and contact displacement curve, where *<sup>t</sup>*pre is the instant when the first prestrike occurs and *t*aux is the instant when the moving contact is mechanically mated to the fixed contact. Finally, the prestrike gap *d*, i.e., the vacuum gap length between the contacts when the prestrike occurs during the closing operation, can be obtained.

**Figure 1.** Schematic diagram of the experimental circuit.

**Figure 2.** Picture of the experimental setup.

**Figure 3.** Example of the waveforms of voltages *U*m and *U*a and the moving.

Three groups of tests were carried out under the experimental conditions that are summarized in Table 1, where the applied DC voltage *U*s were set as four levels of 10, 20, 30, 40 kV. In *Test* 1 and *Test* 2 the prestrike characteristics of vacuum interrupters (VI\_A and VI\_B) under DC voltages *U*s were investigated separately. In this case, only one vacuum interrupter (VI\_A or VI\_B) was connected into the experimental circuit while the other one was shorted to ground by a busbar. In *Test* 3, the prestrike characteristics of the double-break VCB with two vacuum interrupters (VI\_A together with VI\_B) in series were analyzed, where VI\_A was installed in the high-voltage side of the VCB and VI\_B the low-voltage side (Figure 1). In this case, the total prestrike gap *d* was defined as the sum of the prestrike gaps in VI\_A and in VI\_B. The current-making operations were repeated 30 times in each test for all the test groups.



The prestrike arc current during each current-making operation was only tens to hundreds of amperes. Considering all the contact surfaces had already been well conditioned, the erosion effect on the contact surfaces was low and could be neglected. Moreover, because of the asynchronous property of the mechanical actuators in the double-break VCB, the contacts in VI\_A and VI\_B were not mechanically closed at the same time. The average time delay of VI\_B compared with VI\_A was 0.1 ms (computed from 50 closing operations with no load), which was used as a compensation when calculating the time instant *t*aux.
