Fault Current Limiting Characteristics of a Small-Scale Bridge Type SFCL with Single HTSC Element Using Flux-Coupling

: In this paper, a bridge type superconducting fault current limiter (SFCL) with a single high-temperature superconducting (HTSC) element is proposed to allow fault current limiting operation in direct current (DC) conditions. First, the principle of operation of the bridge type SFCL with a single HTSC element using ﬂux-coupling was presented. After the fault occurrence, the fault current limiting operation and voltage characteristics, the power load characteristics of each device, and the energy consumption of the two coils and the HTSC element were analyzed in the proposed SFCL. As a result, it is conﬁrmed that in the case of the additive polarity winding, the power consumption and the energy consumption of the HTSC element were lower than those in the subtractive polarity winding, and the fault current limiting characteristics were excellent.


Introduction
Today, power distribution facilities in low-voltage systems utilize a switching arc function to control fault current. Power utilities in high voltage systems typically operate with special switchgear, and their components are designed to withstand fault currents before the circuit opens. Power distribution facilities should be equipped with a protection system in case of a short circuit in the grid voltage. The protection system has a small internal resistance during normal operation, but in the event of an accident, it must generate a high impedance within a very short time to limit the short-circuit current and return to normal operation mode immediately after the fault is resolved. A device that can have this function is a superconducting fault current limiter (SFCL) [1,2]. This is because such a function can be provided by changing from a zero resistance to a high normal resistance when the critical current, critical temperature, and critical magnetic field are exceeded. For this reason, numerous SFCLs have been proposed in the past few years [3][4][5][6]. SFCL technology with high-efficiency and high-compression design was introduced almost 50 years ago [7].
SFCLs can be classified into resistance type, bridge type, self-shielding type, and saturated core type according to the structure and operating principle [8][9][10][11]. Among them, the bridge type SFCL uses a diode or thyristor as a Graetz bridge connected to a superconducting coil [12,13]. When normal power is applied to the power system, the voltage across the diode is determined by the diode's internal resistance, and its value is very small. For this reason, the polarity does not change as determined by the direct current (DC) power supply. Since the four diodes are forward biased, each circuit operates independently of each other.
When a short circuit accident occurs in the power system, the voltage across each diode is increased tens of times compared to the normal state. Therefore, in the normal state, two of the forward biases applied to the diode are subjected to the reverse bias, whereby the two circuits that were previously independent affect each other. The alternating current (AC) affects the DC power supply and a fault current flows through the coil. Here, the fault current does not increase rapidly due to the inductance of the coil, but gradually increases, so that even if a short circuit accident occurs, it does not significantly affect the system at the beginning. In this way, the system can be protected when the circuit break (CB) is operated in a situation where the current is not increased to disconnect the circuit from the power supply.
When limiting the fault current, the inductor may be normal, but there is always some current flowing and significant continuous loss. This can be avoided by using a superconducting coil. Fault current can be limited by using an iron core inductor, but a large amount of iron is required to prevent saturation. The Graetz bridge must have a power capacity to accommodate the losses, so the losses are much less when using superconducting inductors. Recently, a bridge type SFCL applied with a thyristor or an insulated gate bipolar transistor (IGBT) switch instead of a diode, and a DC resistance type in which a superconducting coil capable of being quenched is added to the diode bridge has been proposed [14][15][16][17][18][19][20][21][22][23][24][25]. However, this type of SFCL has the disadvantage of being larger than other types of SFCL due to the superconducting coil. The SFCL also requires an additional controller and CB to protect the superconducting coil from accidents.
In order to overcome these drawbacks, we would like to propose a bridge type SFCL with a single high-temperature superconducting (HTSC) element using flux-coupling that can utilize an AC or DC system. The fault current limiting operation characteristics of the bridge type SFCL were analyzed. When the connection direction between the two coils was different during the fault cycle, the fault current limiting operation due to the quench of a single HTSC element, the voltage characteristics of each element, the magnetic flux and instantaneous power of two windings, and the energy consumption were compared with each other. Figure 1 shows the structure of a bridge type SFCL with a single HTSC element using flux-coupling between two windings. The SFCL consists of four diodes connected in the form of full-wave bridges for operation at DC, two coils wound around one iron core, and a HTSC element connected to only one coil in series. The winding direction between the primary and secondary windings (N 1 , N 2 ) can be changed to either subtractive polarity winding or additive polarity winding. The HTSC element was Y 1 Ba 2 Cu 3 O 7−x (YBCO) thin films deposited with a 200 nm thick layer of platinum and was a product of Theva, Germany.

Sturcture and Principle
The basic operation principle is that the resistance of the HTSC element becomes zero because no quenching occurs under conditions prior to the failure. In addition, since the current flowing through the N 1 and N 2 windings is DC, these two coils are bypassed so that no magnetic flux occurs. Of course, even if AC ripple occurs due to the loss of the diode, the magnetic fluxes cancel each other out and become zero.
However, after a fault has occurred, the transient fault current exceeds the critical current of the HTSC element in series with the N 2 winding, and the HTSC element has a resistance that causes a quench. The DC current flowing through the N 1 and N 2 windings causes a mixture of AC ripple components to generate magnetic flux. As a result, non-inductive coupling breaks and limits the fault current.  Figure 2 shows the electrical equivalent circuit of a bridge type SFCL with a single HTSC element using flux-coupling. The resistance and leakage inductance of each winding are omitted for simplicity. The inductance of the two windings is shown as L1 and L2, and the resistance of the HTSC element is represented as Rsc when the HTSC element is quenched and resistance occurs. Depending on the wiring direction between N1 and N2 windings, it is divided into a subtractive polarity winding and an additive polarity winding, indicated by • and ○, respectively. In the case of a subtractive polarity winding and an additive polarity winding from the equivalent circuit of Figure 2, the fault current limiting operating current (Iop) can be represented by Equations (1) and (2), respectively. where Ic means the critical current.   Figure 2 shows the electrical equivalent circuit of a bridge type SFCL with a single HTSC element using flux-coupling. The resistance and leakage inductance of each winding are omitted for simplicity. The inductance of the two windings is shown as L 1 and L 2 , and the resistance of the HTSC element is represented as R sc when the HTSC element is quenched and resistance occurs. Depending on the wiring direction between N 1 and N 2 windings, it is divided into a subtractive polarity winding and an additive polarity winding, indicated by • and , respectively. In the case of a subtractive polarity winding and an additive polarity winding from the equivalent circuit of Figure 2, the fault current limiting operating current (I op ) can be represented by Equations (1) and (2), respectively.

Equivalent Circuit
where I c means the critical current.
Electronics 2020, 9, x FOR PEER REVIEW 3 of 12  Figure 2 shows the electrical equivalent circuit of a bridge type SFCL with a single HTSC element using flux-coupling. The resistance and leakage inductance of each winding are omitted for simplicity. The inductance of the two windings is shown as L1 and L2, and the resistance of the HTSC element is represented as Rsc when the HTSC element is quenched and resistance occurs. Depending on the wiring direction between N1 and N2 windings, it is divided into a subtractive polarity winding and an additive polarity winding, indicated by • and ○, respectively. In the case of a subtractive polarity winding and an additive polarity winding from the equivalent circuit of Figure 2, the fault current limiting operating current (Iop) can be represented by Equations (1) and (2), respectively. where Ic means the critical current.

Preparation of Experiment
The design parameters of a bridge type SFCL with a single HTSC element using flux-coupling are shown in Table 1. The HTSC element having a critical current of 18.15 A was used by patterning the YBCO thin film. The fabrication process of the HTSC element used in this experiment is reported in other previous papers [26][27][28].  Figure 3 shows the schematic diagram of the experimental device for analyzing the fault current limiting operation characteristics, voltage waveforms, instantaneous power and magnetic flux change of a bridge type SFCL with a single HTSC element. As shown in the experimental device of Figure 3, the proposed bridge type SFCL consisted of a bridge diode, an iron core, and a superconducting element. There was no need for a controller and circuit breaker to protect against short-circuit accidents, such as the bridge type SFCL with the conventional thyristor or IGBT switch presented in the introduction. A fault short-circuit experiment was performed at 40 V rms AC input voltage (E in ) at 60 Hz and a fault angle of 0 • . The test equipment consisted of a bridge circuit to obtain full-wave rectification, a line reactance (X line ) of 0.6 Ω, a line resistance of 1 Ω (R line ), a load resistance of 50 Ω (R load ), two windings on an iron core, and one HTSC element. This short-circuit tester was designed to supply AC power (E in ) using short-circuit SW 2 after SW 1 closed and to open the SW 1 and SW 2 after the fault cycle to cut off the power supply.   (6). The fault current (iin) gradually increased immediately after the fault, but it was observed that the fault current suddenly increased before the quench occurred in the HTSC element. This phenomenon is due to the saturation of the iron core, and the fault current increases rapidly, causing quenching of the HTSC element. This proves that the fault current is limited by increasing the impedance due to the quench of the HTSC element connected in series to the secondary winding.

Experimental Results
The experimental results were compared and analyzed according to the subtractive polarity winding and the additive polarity winding, which are the wiring directions between the primary and secondary windings. It can be seen that the voltage generated due to the quenching of the HTSC element is smaller in the case of the additive polarity winding than that of the subtractive polarity winding. In addition, it can be seen that the current (i2) flowing through the secondary winding (N2) is larger in the case of the additive polarity winding than that of the subtractive polarity winding from 2.5 cycles.  Figure 4 shows the fault current limiting characteristics and voltage waveforms of a bridge type SFCL with a single HTSC element using flux-coupling when the two windings are connected to the subtractive polarity winding and the additive polarity winding at the input voltage source of 40 V rms and the fault angle of 0 • . During the fault period, each fault section was subdivided into (1), (2), (3), (4), (5), and (6). The fault current (i in ) gradually increased immediately after the fault, but it was observed that the fault current suddenly increased before the quench occurred in the HTSC element. This phenomenon is due to the saturation of the iron core, and the fault current increases rapidly, causing quenching of the HTSC element. This proves that the fault current is limited by increasing the impedance due to the quench of the HTSC element connected in series to the secondary winding.

Experimental Results
The experimental results were compared and analyzed according to the subtractive polarity winding and the additive polarity winding, which are the wiring directions between the primary and secondary windings. It can be seen that the voltage generated due to the quenching of the HTSC element is smaller in the case of the additive polarity winding than that of the subtractive polarity winding. In addition, it can be seen that the current (i 2 ) flowing through the secondary winding (N 2 ) is larger in the case of the additive polarity winding than that of the subtractive polarity winding from 2.5 cycles. Electronics 2020, 9, Figure 5 shows the instantaneous power burden characteristics of the devices and the magnetic fluxes (φ 1 , φ 2 ) of the two windings in a bridge type SFCL with a single HTSC element using flux-coupling when the two windings are connected to the subtractive polarity winding and the additive polarity winding during the fault period. The magnetic fluxes of the primary and secondary windings were similar in magnitude, regardless of the wiring direction between the two coils. On the other hand, it can be observed that the instantaneous power consumed in the HTSC element was much larger in the case of the subtractive polarity winding than that of the additive polarity winding immediately after the failure. The reason for this is that the HTSC element connected in series to the secondary winding is quenched, and its impedance value is greater. Also, it can be seen that the instantaneous power consumed in the secondary winding is higher in the case of the additive polarity winding than that of the subtractive polarity winding immediately after the failure.
Electronics 2020, 9, x FOR PEER REVIEW 7 of 12 Figure 5 shows the instantaneous power burden characteristics of the devices and the magnetic fluxes (ϕ1, ϕ2) of the two windings in a bridge type SFCL with a single HTSC element using fluxcoupling when the two windings are connected to the subtractive polarity winding and the additive polarity winding during the fault period. The magnetic fluxes of the primary and secondary windings were similar in magnitude, regardless of the wiring direction between the two coils. On the other hand, it can be observed that the instantaneous power consumed in the HTSC element was much larger in the case of the subtractive polarity winding than that of the additive polarity winding immediately after the failure. The reason for this is that the HTSC element connected in series to the secondary winding is quenched, and its impedance value is greater. Also, it can be seen that the instantaneous power consumed in the secondary winding is higher in the case of the additive polarity winding than that of the subtractive polarity winding immediately after the failure.  Figure 6 shows the power consumption and energy consumption of the HTSC element with respect to the current of a bridge type SFCL with a single HTSC element when the primary and secondary windings are connected as the subtractive polarity winding and the additive polarity winding during the fault period. In Figure 4, the fault sections (1), (2), (3), , (6) displayed to subdivide the fault section are shown in the power consumption and energy consumption characteristics curve of each device. At this time, the power consumption characteristics of each device are displayed only on the maximum value of power consumption of each device because it is complicated to display the detailed failure section on each curve. When a fault occurs, the maximum power consumption of the HTSC element is 0.27 kW higher in the case of the subtractive polarity winding than that of the additive polarity winding, but the maximum power consumption of the primary and secondary windings is 0.1 and 0.07 kW less, respectively. During the fault cycle, the maximum energy consumption of the HTSC element is found to be 3.39 J higher in the case of the subtractive polarity winding than that of the additive polarity winding. It can be observed that the maximum energy consumed in the primary winding is as high as 3.39 J, while the maximum energy consumed in the secondary winding is as low as 0.58 J.
In brief, power consumption of each device was > , , < , , , < , , and the energy consumption of each device was > , , > , , , < , , respectively. The patterns of the power consumption and energy consumption of each device except the primary winding were the same. The maximum power consumption in the primary winding was higher in the case of the additive polarity winding than in the case of the subtractive polarity winding. The reason is that in the case of the additive polarity winding, the i1 current flowing in the primary winding suddenly increased significantly at 1.5 cycles due to the voltage drop of . Conversely, the maximum energy consumption in the primary winding was higher in the case of the subtractive polarity winding than in the case of the additive polarity winding. The reason is that the range of power consumption of the primary winding in the case of the subtractive polarity winding during  Figure 6 shows the power consumption and energy consumption of the HTSC element with respect to the current of a bridge type SFCL with a single HTSC element when the primary and secondary windings are connected as the subtractive polarity winding and the additive polarity winding during the fault period. In Figure 4, the fault sections (1), (2), (3), . . . , (6) displayed to subdivide the fault section are shown in the power consumption and energy consumption characteristics curve of each device. At this time, the power consumption characteristics of each device are displayed only on the maximum value of power consumption of each device because it is complicated to display the detailed failure section on each curve. When a fault occurs, the maximum power consumption of the HTSC element is 0.27 kW higher in the case of the subtractive polarity winding than that of the additive polarity winding, but the maximum power consumption of the primary and secondary windings is 0.1 and 0.07 kW less, respectively. During the fault cycle, the maximum energy consumption of the HTSC element is found to be 3.39 J higher in the case of the subtractive polarity winding than that of the additive polarity winding. It can be observed that the maximum energy consumed in the primary winding is as high as 3.39 J, while the maximum energy consumed in the secondary winding is as low as 0.58 J.
In brief, power consumption of each device was P sub sc > P add sc , P sub 1,max < P add 1,max , P sub 2,max < P add 2,max , and the energy consumption of each device was J sub sc > J add sc , J sub 1,max > J add 1,max , J sub 2,max < J add 2,max , respectively. The patterns of the power consumption and energy consumption of each device except the primary winding were the same. The maximum power consumption in the primary winding was higher in the case of the additive polarity winding than in the case of the subtractive polarity winding. The reason is that in the case of the additive polarity winding, the i 1 current flowing in the primary winding suddenly increased significantly at 1.5 cycles due to the voltage drop of v 1 . Conversely, the maximum energy consumption in the primary winding was higher in the case of the subtractive polarity winding than in the case of the additive polarity winding. The reason is that the range of power consumption of the primary winding in the case of the subtractive polarity winding during 5.5 cycles of fault is more intensively concentrated than that in the case of the additive polarity winding.
Electronics 2020, 9, x FOR PEER REVIEW 9 of 12 5.5 cycles of fault is more intensively concentrated than that in the case of the additive polarity winding.

Conclusions
In this paper, the fault current limiting characteristics, instantaneous power, and energy consumption of a bridge type SFCL with a single HTSC element were compared according to the wiring direction between the two coils during the fault period. Since the HTSC element operated under DC conditions, the fault current rapidly increased due to saturation of the iron core immediately after the fault occurred. However, it can be seen that the fault current is limited by the quenching of the HTSC element. It can be seen that the quench voltage of the HTSC element was much larger in the case of the subtractive polarity winding than that of the additive polarity winding. The instantaneous power dissipated in the single HTSC element was much larger in the case of the subtractive polarity winding than that of the additive polarity winding immediately after the fault. However, it could be confirmed that the instantaneous power consumed in the secondary winding was larger in the case of the additive polarity winding. In addition, the maximum energy consumed by the HTSC element during the fault period was higher in the case of the subtractive polarity winding than that of the additive polarity winding. At this time, the maximum energy consumed in the primary winding was higher in the case of the subtractive polarity winding, but the maximum energy consumed in the secondary winding was less. In conclusion, it can be confirmed that the modified bridge type SFCL with a single HTSC element had a fault current limiting operation function. Also, it can be confirmed that the power consumption and energy consumption of the HTSC element was lower in the case of the additive polarity winding than that of the subtractive polarity winding, and the fault current limiting characteristics were excellent. In the future, a basic study will be conducted on whether this modified bridge type SFCL model can be applied to a DC power distribution system.