**4. Technological Discussions**

Provision of UFTMF especially in medium-voltage power networks is the first challenge of this system. The main source of such high magnitude and an ultrafast magnetic field is a coil with the perpendicular axis to the current path. In addition, the coil should present a low value of inductance. In this system, there is a need to generate an ultrafast rising magnetic field, and consequently, there is a need to the ultrafast rising current in the coil. If the inductance of the coil is too high, the rise time of the current will increase, and UFTMF will not be accessible. The energy source to feed the coil may be a capacitor, which is charged to an appropriate voltage, and it can provide a sufficient amount of energy to increase the current in the coil. Capacitance and inductance of this system will satisfy the rise time of the electric current in the coil.

As illustrated in Figure 7, a two-turn coil was placed beside a VI. This coil was considered with two individual single turn coils on both sides of the VI, and the connecting path between two coils consisted of two symmetric parallel paths. These two parallel paths ensured the transverse magnetic field with the minimum axial component.

**Figure 7.** A vacuum interrupter (VI) equipped with an external UFTMF coil.

An electric current of 200 kA with a rise-time of 1 μSec flowing through the coil ensured UFTMF with a rate of rise of 10<sup>6</sup> T/sec.

Distribution of the magnetic flux density and related arrow-lines inside the arcing medium of VI has been depicted in Figure 8 for the mentioned coil current and by the use of a finite element simulation.

**Figure 8.** Distribution of the magnetic flux density amplitude generated by an electric current of 200 kA with 1 μSec rise-time.

A charged capacitor, which is capable of providing pulsed current to the coil, will be an appropriate choice of energy source for this system. The capacitance and charging voltage of the capacitor can be determined by knowing the inductance of the coil. Finite element simulation of the coil and calculation of the magnetic field and magnetic energy distribution expressed that the inductance of the coil was 0.2 μH. The capacitance and charging voltage of the capacitor were calculated by considering an inductance value of 0.2 μH, a peak current of 200 kA, and a rise time of 1 μSec. The calculated capacitance was 2 μF and the charging voltage was 60 kV. These values satisfied the necessary electric current for the coil to reach the mentioned criteria. A schematic diagram of the feeding circuit is depicted in Figure 9.

**Figure 9.** Schematic diagram of an electric current feeding circuit.

As illustrated in Figure 9, a closing switch capable of passing 200 kA is required. Possible candidates could be controlled spark gap closing switches or triggered vacuum switches (TVS). The breakdown voltage of the gap can be controlled by gap distance and gas pressure of the spark gap [21,22].

One of the big challenges in this technology is penetrating the magnetic field into the arc chamber. Electric contacts of VI and the switching gap are surrounded by the vapor shields [23]. According to the electromagnetic theory, it is expected that UFTMF will be damped by the metallic vapor shields because of its high-frequency content and as a consequence of large induced currents inside the shield. Conventional vapor shields act as a perfect electromagnetic shield for such UFTMF and so magnetic field damps dramatically inside the shield. This problem should be addressed by changing the structure of the vapor shield. One possible approach to resolve this problem is the replacement of the arc shield with a similar one with a different material. The use of one-piece ceramic vacuum interrupters as suggested in [24] could be considered in this context. The tolerable amount of a metal coating on an isolating material (e.g., due to the deposition of metal vapor on the ceramic shield) is dependent on the frequency range of UFTMF. Since the highest frequency content of UFTMF is 500 kHz, a coating layer of 10–20 μm will be completely transparent to the UFTMF. In addition, the challenge may be overcome by making specific cut-lines on the vapor shield and using an overlapping structure.
