Magnetizing Characteristics of Bridge Type Superconducting Fault Current Limiter (SFCL) with Simultaneous Quench Using Flux-Coupling

Abstract

A bridge type superconducting fault current limiter (SFCL) with simultaneous quench using two high-temperature superconducting (HTSC) elements and two coils was fabricated to analyze the fault current limiting characteristics. Before and after the fault occurrence, the current limiting operation and the voltage waveforms of each device were compared according to the change of the input voltage. We also analyzed flux linkages and instantaneous powers of the bridge type SFCL with simultaneous quench using flux-coupling composed of HTSC elements with different critical currents. During the fault period, the magnetization power area and the flux linkage’s operating range variation due to the magnetizing current were compared with each other.

1. Introduction

Superconducting fault current limiters (SFCLs) are fundamental components of modern electrical systems at both medium and high voltages. Many research efforts have been carried out to reduce fault currents in alternating current (AC) or direct current (DC) systems [1,2,3,4,5,6,7]. Among other SFCLs, bridge type SFCL using reactors do not require quenching and can significantly reduce AC losses. This bridge type SFCL is generally composed of a diode or thyristor and a superconducting coil, and has the disadvantage of being larger than other types of SFCL due to the superconducting coil. In addition, this SFCL is expensive and requires controllers and circuit breakers to protect the superconducting coils from accidents [8,9,10,11]. In order to overcome this drawback, DC double reactor type SFCL using switching operation of a high-temperature superconducting (HTSC) device has been proposed and fault current limiting characteristics have been reported [12]. However, this DC double reactor type SFCL has not been investigated for the magnetization characteristics and instantaneous power burden characteristics according to the input voltage change at the time of failure. In this paper, we proposed a bridge type SFCL with sim ultaneous quench using flux-coupling as a preliminary step of DC system. The fault current limiting operation, voltage waveforms, instantaneous power and magnetic flux, and magnetization of each device are analyzed for this bridge type SFCL. When the input AC voltage source was changed, we tried to analyze the range of voltage induced by the magnetizing current, the change of energy consumption, the range of the magnetization power, and the operating range of the magnetic flux linkage. A lab scale prototype was built and failure short-circuit experiment was performed. We tried to verify its usefulness by analyzing fault current limiting operation and magnetization characteristics of bridge type SFCL with simultaneous quench.

2. Structure and Operating Principle

2.1. Sturcture and Principle

Figure 1 shows the structure of a bridge type SFCL with simultaneous quench using flux-coupling. This bridge type SFCL consists of one iron core, two windings, two high-temperature superconducting (HTSC) elements and four diodes. The wiring direction between the primary and secondary windings (N₁, N₂) is an additive polarity winding and is connected in parallel with each other. In addition, the AC power supply voltage (E_in) is designed to be variable. The DC section is a full-wave bridge circuit consisting of four diodes. The HTSC elements were Y₁Ba₂Cu₃O_7-x (YBCO) thin films deposited with a 200 nm thick platinum layer and used a product of Theva, with a critical temperature of 87 K.

As a basic principle of operation, under conditions before failure occurs, the resistance of the HTSC elements becomes zero because no quench occurs. In addition, because the current flowing through the windings N₁ and N₂ is DC, these two coils are bypassed and no magnetic flux occurs. Eventually, the impedance of this SFCL becomes zero. However, after a fault has occurred, the transient fault currents exceed the critical currents of the HTSC elements connected in series with the N₁ and N₂ windings, causing resistance in the HTSC elements and quenching them. The DC current flowing through the N₁ and N₂ windings is mixed with the AC ripple component to generate magnetic flux. As a result, the non-inductive coupling breaks, limiting the fault current.

2.2. Equivalent Circuit

Figure 2 shows the electrical equivalent circuit of a bridge type SFCL with simultaneous quench using flux-coupling. This equivalent circuit can be derived from the magnetic equivalent circuit using the topology principle of duality [13]. The resistance and leakage inductance of each winding is omitted for simplicity. L₁ and L_Th represent the self-inductance wound on the core and the equivalent inductance for both windings, respectively. In the case of the additive polarity winding between the N₁ and N₂ windings, the magnetizing current ($i_m$) and the limiting impedance (Z_SFCL) of the bridge type SFCL with simultaneous quench using flux-coupling can be expressed by Equations (1) and (2).

$$i_m=i_1-i_1^{\prime }=i_1+\frac{N_2}{N_1}{i}_2$$

(1)

Here, $i_1$ and $i_2$ denote instantaneous values of currents flowing through the primary and secondary windings, respectively.

$$Z_{SFCL}=\raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{$\left[-\frac{1}{V_1}\left(\frac{N_2}{N_1}\frac{V_{sc2}}{R_{sc2}}\right)+\frac{1}{j\omega L_{eq}}\right]$}\right.+R_{sc1}$$

(2)

Here, V₁ and V_SC2 expressed in phasor form represent voltages induced by the primary winding and the HTSC element 2, respectively. N₁ and N₂ represent the number of turns of each winding, and L_eq is equal to L_Th//L₁. R_SC1 and R_SC2 represent the resistances of two HTSC elements, respectively, and ω is angular frequency.

3. Experimental Results

3.1. Preparation of Experiment

Table 1. Specifications of bridge type SFCL with simultaneous quench using flux-coupling. HTSC = high-temperature superconductor.

Windings (Turn Number)	Value	Unit
Primary Winding (N1)	150	Turns
Secondary Winding (N2)	150	Turns
Two HTSC Elements (${\mathit{R}}_{\mathit{s}\mathit{c}\mathbf{1}}$, ${\mathit{R}}_{\mathit{s}\mathit{c}\mathbf{2}}$)	Value	Unit
Material	YBCO	Thin Film
Critical Current ($I_{c1}$) of HTSC element 1	18.15	A
Critical Current ($I_{c2}$) of HTSC element 2	19.04	A
Total Meander Line Length	420	mm
Thin Film Thickness	2	mm
Line Width	0.3	μm
Gold Layer Thickness	0.2	μm

shows the design parameters of a bridge type SFCL with simultaneous quench using flux-coupling. The critical currents (I_c1, I_c2) of HTSC elements 1 and 2 were used by patterning YBCO thin films with 18.15 and 19.04 A, respectively. The fabrication process of the HTSC elements used in this experiment is described in detail in [14].

Figure 3 shows the experimental setup for simulating the various characteristics of bridge type SFCL with simultaneous quench using flux-coupling when the input voltage is changed differently. The fault test was performed at 80 V_rms and 100 V_rms AC input voltage (E_in) at 60 Hz and fault angle 0 °. The test equipment consisted of a full-wave bridge for DC, a line reactance of 1.59 mH (L_line), a line resistance of 1 Ω (R_line), a load resistance of 50 Ω (R_load), two windings on one iron core, two HTSC elements. After SW₁ is closed, SW₂ is designed to close at the fault angle 0 ° of the AC power supply and reopen after the fault period.

3.2. Experimental Results

Figure 4a,b show the fault current limiting characteristics of the bridge type SFCL when the input voltage source is changed from 80 V_rms to 100 V_rms, respectively. When the input voltage source was 100 V_rms as shown in Figure 4b, the fault current and the magnetizing current increased much faster after a fault, and the HTSC elements 1 and 2 were quenched at about the same time in half cycle. It can be seen that the quench occurs first and the fault current is limited when the input voltage source is 100 V_rms (See Figure 4b) rather than 80 V_rms (See Figure 4a).

Figure 5a,b show the voltage waveforms of each winding and HTSC element when the input voltage source is changed from 80 V_rms to 100 V_rms, respectively. When the input voltage source was 100 V_rms as shown in Figure 5b, the voltage waveforms of the HTSC elements, the full-wave bridge voltage waveform of the rectifier stage, and the voltage waveforms of two windings increased. However, as shown in Figure 5a,b it can be seen that the voltage ($v_2$) across N₂ is minutely generated before the quench occurs due to the leakage current of the winding.

Figure 6 shows the magnetic flux of each winding and instantaneous power burden of each device as the input voltage source changes to 80 V_rms and 100 V_rms, respectively, during the fault period. As the input voltage source was increased, the magnetic flux (ϕ₁) of the primary winding hardly changed, but the magnetic flux (ϕ₂) of the secondary winding decreased significantly. On the other hand, it can be observed that the instantaneous power consumed by the primary and secondary windings and the HTSC elements 1 and 2 are much larger when the input voltage source is increased immediately after the failure.

Figure 7 shows the characteristics of the energy dissipated by the magnetization power and the range of fluctuation in the voltage induced in each winding for the magnetizing current during the fault cycle when the input voltage source changes to 80 V_rms and 100 V_rms, respectively. Figure 7 shows the x-axis as the magnetizing current ($i_m$) and the y-axis as the voltage ($v_1$ and $v_2$) across both windings during the fault period. Moreover, the energy ($J_m$) consumed by the magnetization power can be obtained by integrating the product of the time (t) and the magnetization power ($p_m$) during the fault period. Therefore, the relationship curve between the voltage of each winding and the energy consumed by the magnetizing current can be obtained as shown in Figure 7. When the input voltage source increases during the fault cycle, the range of voltage induced in the primary winding is large, but the range of voltage induced in the secondary winding appears small and converges to nearly zero. In addition, it can be seen that during the fault period, the energy dissipated by the magnetization power increases with the increase and decrease of the magnetizing current and then decreases to converge to the magnetizing current 10 A point. It can be observed that as the input voltage source increases, the range of increase and decrease of magnetizing current is wide, but energy consumption is low.

Figure 8 shows the operating range of the magnetization power ($p_m$) and the magnetic flux linkage (λ) according to the magnetizing current during the fault period when the input voltage source changes to 80 V_rms and 100 V_rms, respectively. Referring to the equivalent circuit in Figure 2, flux linkages (λ₁, λ₂) of the primary and secondary windings and magnetization power ($p_m$) can be redrawn on the axis by the magnetizing current as shown in Figure 8. During the fault period, flux linkages 1 and 2 can be calculated as ${\mathsf{\lambda }}_1={{\displaystyle \int }}^{\text{}}{v}_{1}dt$, ${\mathsf{\lambda }}_2={{\displaystyle \int }}^{\text{}}{v}_{2}dt$, and the magnetization power can be obtained by calculating $p_m=v_1{i}_m$. Therefore, as shown in Figure 8, it is possible to obtain a relationship curve between magnetization power and flux linkage according to the magnetizing current. As the input voltage source increases, the maximum magnetization power is 0.17 kW higher, and it can be seen that the change of the magnetization power area according to the magnetizing current is much larger. As the magnetizing current increases and decreases, the operating range of the flux linkages 1 and 2 gradually increases and then decreases again, showing that the magnetization current converges at about 10 A. When the input voltage source was increased from 80 V_rms to 100 V_rms, the maximum flux linkage of λ₁ and λ₂ during the fault period differed by less than 0.02 and 0.09 Wb, respectively. However, it can be seen that the operating range of λ₁ and λ₂ is slightly larger when the input voltage source is changed to 100 V_rms.

4. Conclusions

In this paper, we compared and analyzed the energy consumption change, magnetization power area change, and flux linkage operating range due to magnetizing current when the turns ratio of N₁ and N₂ were kept 1:1 and the input voltage source was changed from 80 V_rms to 100 V_rms, respectively. Immediately after the fault, the fault current and magnetizing current increased even more rapidly, and it was observed that HTSC elements 1 and 2 were simultaneously quenched in half cycle. When the turns ratio of N₁ and N₂ are the same and the input voltage source is increased from 80 V_rms to 100 V_rms, the following conclusions can be drawn.

(1)

The fault current can be limited faster.

(2)

The magnetic flux (ϕ₁) of the primary winding had little change, but the magnetic flux (ϕ₂) of the secondary winding decreased significantly.

(3)

Immediately after the failure, the power consumption of HTSC elements acted more.

(4)

During the fault period, the range of voltage fluctuation according to the magnetizing current was wider, but the energy ($J_m$) consumed by the magnetization power was small.

(5)

During the fault period, both the maximum magnetization power and the magnetization power’s area change were high according to the magnetizing current.

(6)

The maximum flux linkages (λ₁, λ₂) of the primary and secondary windings during the fault period were small, but their operating range was slightly wider.

In conclusion, the larger the input voltage source, the faster the fault current can be limited, and has the advantage of increasing the maximum magnetization power and the change in the area of magnetization power according to the magnetizing current. However, HTSC elements have a disadvantage in that power consumption is greater and flux linkages have a wider operating range.