2.1 Arc plasma

Because arcs are very unstable and have complex properties, clearly defining the arc that occurs at the cutoff contacts is difficult. So far, knowledge of arcs has been developed based on observations of electrical measurements and analytical data from experiments. In modern times, the volt-ampere (V-I) characteristics of electric arcs, which depend on the test parameters, are essential data for defining complex arc phenomena in power systems ⁽⁵⁾.

The arc is one of the most important factors in the circuit breaker, whose purpose is to protect the sensitive power systems from the fault current flowing in the transient state of the system. We can operate a reliable system without errors if we can safely solve the arc generated by the circuit breaker to a minimum. The opening act of the cutoff contact generates a typical arc. cutoff contact is divided into fixed contact and moving contact and is explained. When a fault occurs in the system and a fault current flows, the circuit breaker performs an open operation. As the moving contact moves away from the fixed contact, the arc voltage rapidly increases, and the arc current decreases. At this moment, the excitation and ionization phenomena occur as electrons escape from the mechanical contact. In other words, an inelastic collision between free electrons and gas electrons occurs. When the moving contact moves, the arc length increases. It occurs as the linear arc voltage increases and the arc current decreases. The arc generation region generated here is called arc plasma, which occurs in an unspecified arc shape ⁽⁴⁾. The length of the arc plasma determines the damage and lifetime of the mechanical contact.

그림 1 초전도 아크유도형 DC 차단기의 구성요소

Fig. 1 Components of superconducting arc-induction DC circuit breaker

2.2 Superconducting arc-induction type DC circuit breaker

As shown in Figure 1, the superconducting arc-induction type DC circuit breaker consists of mechanical contact(□), superconducting magnets(○), and an induction needle(▽). The mechanical contact is in the form of a general mechanical DC circuit breaker, and the material is copper and has a round cylindrical shape. The induction needle is designed to have a strong electric field concentration and is an element that induces and absorbs the arc generated at the cutoff contact. The induction needle is connected in series with the ground wire, so the induced arc is sent to the ground to extinguish the fire. The superconducting magnet is an element that blows the arc through Lorentz force by injecting high magnetic force into the arc generated between the mechanical contacts⁽⁴⁾.

In the normal state of the system, the mechanical cutoff contact is closed, and a steady current flows. In the transient state, if a fault current flows through the cutoff contact, the circuit breaker senses the fault current and opens the cutoff contact. A strong arc is generated between the cutoff contacts, and the phenomenon can be explained as follows. As the distance between the fixed and moving contact increases, the arc strength gradually increases. Since the electric charge moves along the equipotential surface of the fixed contact, the point of occurrence of the arc between the two contacts is assumed to be the top of the cross-sectional area for the convenience of analysis. It is because the potential $v_{1}$ of the upper part of the fixed contact and the potential $v_{2}$ of the middle part is $v_{1}=v_{2}$. The arc phenomenon at the equipotential surface generated at each contact can be explained through Equation (1).

( $k$ = constant, $q$ = total quantity of electric charge, $r$ = Radius of curvature)

If Equation (1) is expressed as proportional, then $q_{1}:q_{2}=r_{1}:r_{2}$. Accordingly, the total charge is proportional to the radius of curvature. To analyze density, it is assumed that the thickness is $\sigma$ in a conductor sphere with radius R. By substituting $Q=4\pi R^{2}\sigma$ into Equation (1), the same derivation as Equation (2) can be obtained.

In addition, it is $\sigma_{1}:\sigma_{2}=R_{2}:R_{1}$ when expressed in a proportional expression, and it can be seen that the charge density is larger as the radius of curvature is smaller. Therefore, the arc flow can be analyzed through the difference in the radius of curvature of the induction needle and the cutoff contact. In addition, the high magnetic field of the superconducting magnet generates a Lorentz force that forcibly controls the arc flow through Equation (3) and enhances the arc-induction effect ⁽⁶⁾.

3.1 Test-bed

In this paper, to examine the reliable cutoff operation of a superconducting arc-induction type DC circuit breaker, we try to confirm the experimental results according to the arc length and the presence or absence of superconducting magnets. The arc length gave a variable to the opening speed of the mechanical cutoff contact to analyze the operating characteristics of the circuit breaker. In addition, it was attempted to compare and analyze the circuit breaker's cutoff operation characteristics according to the superconducting magnet's application. The experiment was conducted through an accident occurrence system that simulated the real system. It was produced to analyze the cutoff operation characteristics of the superconducting arc-induction type DC circuit breaker.

그림 2 실험에 사용된 초전도 아크유도형 DC 차단기 및 실험 장비

Fig. 2 Superconducting arc-induction type DC circuit breaker and experimental equipment used in the experiment

Figure 2-⒜ is the DC power supply. It consists of 150 AH-12 V lead-acid batteries, and 63 units are connected in series. This DC power supply can output up to about 800 V. Figure 2-⒝ is the accident simulation system. It consists of the MCCB(Molded Case Circuit Breaker) and a normal and fault load, control system, etc. The MCCB 200 A breaker connected to the DC power supply is the main circuit breaker according to the failure of the cutoff operation of the circuit breaker under test, and it can control the cutoff operation for at least 60 ms through the accident generator controller. The circuit is composed of two lines to form a closed circuit and is divided into a normal circuit and a transient circuit depending on the presence of each load. A thyristor, a semiconductor for power, operates switching. The fault current time constant is about 0.7 ~ 1.0, corresponding to the HVDC time constant. We can set the normal load used in this experiment to about 10 ~ 46.8 Ω, and we can set the accidental load to about 1 ~ 0.1 Ω. The rated voltage and current are 300 V and 30 A, respectively. Figure 2-⒞ is the arc-induction type DC circuit breaker. Figure 2-⒟ is an enlarged picture of the cutting-off part in Figure 2-⒞. The main line is a line with mechanical contact(Anode and Cathode), and the auxiliary line is a line with an induction needle. The distance between the mechanical contact and the induction needle is about 2 mm⁽⁴⁾.

3.2 Variable of the Arc length

Control the driving speed of the actuator of the superconducting arc-induction type DC circuit breaker to give the arc length variable. The breaker actuator uses a servo motor (200/400 W). The actuator has a maximum stroke of about 150 mm, and the repeatability is about 0.02. The maximum driving speed is about 700 mm/sec, and the acceleration is about 0.2 sec. The driving speeds set in the experiment are about 667 mm/sec, about 833 mm/sec, and about 1,000 mm/sec. The arc length is formed according to each driving speed, and the arc operation characteristics are analyzed. In addition, the arc length was calculated and analyzed based on the data of the operating characteristics generated through the experimental results.

3.3 Variable of the applied the superconducting magnet

The superconducting magnet used in the experiment was a YBCO bulk magnet manufactured using the ISMG method⁽⁷⁾. The size of superconducting magnets is about 40 mm in width and length and 10 mm in height. The magnetic force strength at the center of the superconducting bulk magnet can be about 3.0 kilo Gauss or more at about 77 K. The magnetic force strength of the superconducting bulk magnet shown through the experiment is approx.

4.1 Variable of the Arc length

Figure 3 shows the arc characteristics according to the opening operation of an arc-induction type DC circuit breaker that does not apply superconducting magnets. Figure 3-⒜ is an arc characteristic graph when the driving speed of the opening operation is about 667 mm/sec. The primary line is the main circuit and consists of mechanical contacts. The secondary line is an auxiliary circuit and consists of the induction needle and a ground wire. A steady current of about 30 A flows, and the opening operation of the cutoff contact is performed at about 117.14 ms. The opening operation was completed at about 255.57 ms, and the total time was about 138.4 ms. The length of the confirmed arc was calculated from the speed of the opening operation, and the time taken was about 92.31 mm. The current in the auxiliary line was generated about 8.05 ms after the opening operation of the cutoff contact, and the maximum was about 2.39 A. Figure 3-⒝ is an arc characteristic graph when the driving speed of the opening operation is about 833 mm/sec. The time when the normal current and the opening operation of the cutoff contact were performed are the same as in Figure 3-⒜. The opening operation was completed at about 238.74 ms, and the total time was about 121.56 ms. The arc length calculated from the date of the speed and time taken during the opening act was about 101.25 mm. The current generated in the auxiliary line was generated about 10.59 ms after the opening operation of the cutoff contact was performed, and the maximum was about 3.01 A. Figure 3-⒞ is an arc characteristic graph when the driving speed of the opening operation is about 1,000 mm/sec. The time when the normal current and the opening operation of the cutoff contact were performed are the same as in Figure 3-⒜. The opening operation was completed at about 231.21 ms, and the total time was about 114.23 ms. The length of the confirmed arc was calculated from the speed of the opening act, and the time taken was about 114.23 mm. The current generated in the auxiliary line was generated about 7.51 ms after the opening operation of the cutoff contact was performed, and the maximum was about 3.38 A.

4.2 Variable of the applied the superconducting magnet

Figure 4 is a graph of the operation characteristics of an arc-induction type DC circuit breaker to which superconducting magnets are applied. In Figure 4-⒜, the driving speed of the opening operation is about 667 mm/sec. About 30 A of normal current flows, the opening operation of the cutoff contact is about 117.14 ms, and the experimental conditions are the same. The opening operation was completed at about 124.36 ms, and the total time was about 7.22 ms. The length of the confirmed arc was calculated from the speed of the opening act, and the time taken was about 4.81 mm. The current generated in the auxiliary line was up to about 2.17 A. In Figure 4-⒝, the driving speed of the opening operation is about 833 mm/sec. In normal current, the opening operation of the cutoff contact is the same as in the previous experimental conditions. The opening operation was completed at about 122.73 ms, and the total time was about 5.59 ms. The length of the confirmed arc was calculated from the speed of the opening act, and the time taken was about 4.65 mm. The current generated in the auxiliary line was up to about 1.98 A. In Figure 4-⒞, the driving speed of the opening operation is about 1,000 mm/sec. In normal current, the opening operation of the cutoff contact is the same as in the previous experimental conditions. The opening operation was completed at about 122.02 ms, and the total time was about 4.88 ms. The length of the confirmed arc, calculated from the speed of the opening act and the time taken, was about 4.88 mm. The current generated in the auxiliary line was about 2.01 A maximum.

In this paper, the arc characteristics were compared and analyzed according to the opening operation speed of the arc-induction type DC circuit breaker with or without the application of superconducting magnets. The first is the arc-induction type DC circuit breaker that does not apply super-rolled magnets. The opening operation speed of the arc-induction type DC circuit breaker was controlled using a servo motor. Accordingly, the generated arc length was analyzed through experiments and calculations. When the speed of the opening operation was increased to 667, 833, and 1,000 mm/sec in Figure 1-⒜ to ⒞, the arc length increased to 92.31, 101.25, and 114.23 mm. The current induced by the induction needle of the auxiliary line was 2.39, 3.01, and 3.38 A. Based on this experimental data, it was confirmed that the arc-induction rate of the induction needle increased as the arc length increased. The second is an arc-induction type DC circuit breaker with superconducting magnets. The arc generation time was shortened about 21 times by applying superconducting magnets. The arc lengths were about 4.81, 4.65, and 4.88 mm, resulting in the same length of about 4 mm. We judge this because the superconducting magnet was blown out before arc growth due to the Lorentz force of the high magnetic field. The current induced by the induction needle was about 2.17, 1.98, 2.01 A, and about 2 A were generated equally.