2.1 Principles and mechanism of DC circuit breaker

Figure 1 shows the principle and mechanism of the AI type DCCB, where ⓐ is a positive anode, ⓑ is a negative cathode, ⓒ is an induction needle, ⓓ is a ground wire, ⓔ is the spacing between the mechanical contacts and the induction needle, and ⓕ is the gap between the anode and the cathode in the interval between the poles. Figure 1-(a) shows the position of the AI type DCCB when the system is in a normal state. Normal current flows through the anode and cathode. The induction needles set apart at a certain distance by the induction interval of ⓔ do not affect the normal current. Figure 1-(b) shows the point where the contact starts to open. System transients cause an arc between the electric contacts. Figure 1-(c) shows that the anode and cathode are fully open, thereby ensuring a sufficient gap between them. In this case, the arc flows between the induction needle and the anode rather than between the poles. This arc is absorbed and induced by an induction needle before being extinguished along the ground line to the ground. The flow of the arc in this moment was analysed using Coulomb’s law (1) ⁽⁸⁾.

Equation (1) is Coulomb’s law, which represents the relationship between the stimulus distance and the stimulus intensity, where $m_{1}$ and $m_{2}$ represent the pole strengths and $r$ represents the distance between the poles. $F$ is the force acting between the poles. As the distance between the poles increases, as shown in Equation (1), the force generated between the two poles decreases. This is because when the arc occurs between two poles due to the increase in pole distance, the force generated by the arc decreases, thereby facilitating easy induction of the force to another pole.

2.2 Lorentz force for the superonducting magnet

Figure 2 shows force F [N] generated by current I [A] and magnetic field B [H] in the arc blow method (The Lorentz force based on Fleming’s left-hand law). Equation (2) is a force that affects the direction of motion under the influence of the magnetic flux density of the magnets, which are generated from the electric field triggered by the opening of the mechanical contacts by the Lorentz force. A represents the charged particle, B is the electric field, C is the magnetic field, and D is the movement speed of the charged particles⁽⁹⁾.

In the AI type DCCB, the arc is blown in the direction of the induction needle by the fault current I [A] occurring between the anode and the mechanical breaking contact of the cathode and the magnetic flux density B [H] of the magnet located near the contacts. The cone-shaped induction needle has a high electric-field concentration, thereby facilitating the easy induction of blown arcs.

2.3 Design of the simulation model

Figure 3 shows an AI type DCCB simulation model by superconducting types: ⓐ superconducting bulk-type magnets and ⓑ superconducting wire-type electromagnet. The AI type DCCB contact consists of an anode and a cathode. It measures 12 mm in diameter and 150 mm in height, with its main consisting material being copper. The cone-shaped induction needle measures 4 mm in base diameter, with its round sphere at the top measuring 0.2 mm in diameter. This unique design increases the concentration of the electric field and aids in arc absorption. Figure 3-(a) shows a DCCB with a superconducting bulk magnet while Figure 3-(b) shows a DCCB with a superconducting wire-type electromagnet. In addition, Figure 3-(a) shows a superconducting bulk magnet on both sides of the mechanical breaker perpendicular to the contact while Figure 3-(b) shows that the superconducting wire was designed with an AI type DCCB and a wire-type electromagnet with the bulk-type magnet position. The distance between the break contacts and the magnets of the two models is approximately 42.5 mm.

그림 3 각 타입에 따른 AI 타입의 DC 차단기 모델 (a) 벌크형 자석, (b) 선재형 전자석

Fig. 3 AI type DCCB model by superconducting types: (a) The bulk-type magnets, (b) THe wire-type electromagnet

2.4 The design of the model by superconducting bulk-type magnet and wire-type electromagnet

Figure 3-(a) shows a superconducting bulk-type magnet measuring 40 mm in diameter and 15 mm in height, with its material reflecting the YBaCuO superconductivity. It is a high temperature oxide superconductor with a high critical temperature of 92 K. It also has strong magnetic-field characteristics, with a maximum magnetic force of about 63.89 N⁽¹⁰⁾.

Figure 3-(b), on the other hand, shows a wire-type electromagnet using the superconducting wire. Measuring 42 mm in diameter and 40 mm in height, its main material is 8602 of AMSC, and the stabilization layer is made of stainless steel with about 1.00 × 〖10〗^(-7) [Ωm, 20℃] $\rho$ (resistivity). The critical current flows through the superconductor, which causes the phase conduction to change, and the heat generated in the process can be represented by the energy relation Equation (3). Equation (4) lists Equation (3) as the applied voltage per unit length of superconducting wire. $v$ is the voltage across the superconductor, $R$ is the superconductor generating resistance, $C_{V}$ is the heat capacity of the stabilizing layer constituent material, $\triangle t$ is the temperature change of the superconductor, $E$ is the applied voltage per unit length (V/m) of the superconducting wire, $\rho$ is the resistivity of the stabilization layer material, and $\triangle t$ is the superconductor operating time.

그림 4 초전도 벌크형 자석의 자속밀도 분포도

Fig. 4 Magnetic flux density distribution of superconducting bulk-type magnets

그림 5 초전도 선재형 전자석의 자속밀도 분포도

Fig. 5 Magnetic flux density distribution of superconducting wire-type electromagnet

2.5 Simulation results

Figure 4 shows the magnetic flux density distribution of the superconducting bulk magnet, and Figure 5 shows the magnetic flux density distribution of the wire-type electromagnet. Figure 4 shows that the superconducting bulk magnet has a high residual magnetic field. An about 269.3 [A/m] magnetic field was generated at the center of the bulk magnet while a high magnetic field of about 512 [A/m] was generated outside.

Figure 5 shows a model designed with a superconducting wire-type electromagnet. The magnetic force is generated by the turn ratio of 5. The magnetic field was generated by superconducting wire. The highest magnetic field of 610 [A/m] was recorded in the center of the superconducting wire.