2.1 Operating characteristics of the mechanical DC circuit breaker

An experiment was conducted on the operating characteristics of a mechanical DC circuit breaker applied to a solar power system. The mechanical DC circuit breaker model is BK63H DC, and it has a terminal structure of the lug type and a method of the thermal-magnetic overcurrent trip. A rated voltage is 500 V and a rated current is 50 A. The rated voltage and current were set and tested in the DC simulation system. We simulated a short circuit by quickly switching between a normal line and a transient line using the switch of the SCR semiconductor type. The normal load connected to the normal line is about 10 Ω and the fault load connected to the transient line is about 1 Ω. Figure 1 is a graph of the cut-off operation characteristics shown the experiment data of a mechanical DC cut-off switch through a short-circuit fault. As the test system, a rated voltage and current are about 500 V and 50 A was set. Figure 1-(a) is about 19.4 ms when the fault occurred. The initial fault current generated through the experiment was generated from about 47 A to about 501.6 A for about 0.3 ms. This is the rate of increase of the HVDC fault current and the time constant is about 0.1.

Figure 1-(b) shows the time when about 14.85 ms passes after the fault current occurs and the opening operation of the mechanical DC circuit breaker occurs. From this point on, an arc voltage is generated by the opening operation of the cut-off contact, and the fault current is reduced. Figure 1-(c) is point of the cut-off operation completed about 54.16 ms. When the fault current was completed, a transient recovery voltage occurred, which occurred from about 54.25 ms to about 639.5 V. This occurred at about 54.95 ms with a rated voltage of about 500.7 V. The total time of the shutdown was about 35.55 ms due to the single shutdown operation of the mechanical DC circuit breaker.

그림 1 초전도 소자와 LC발산진동회로가 없는 기계식 차단기의 동작 특성 실험 그래프

Fig. 1 Graph of experimental results of mechanical DC circuit breaker without the superconducting element and LC divergence oscillation circuit

2.2 The operating characteristics of a mechanical DC circuit breaker that combines the LC diver- gent oscillation circuit and SPD

Figure 2 is an experimental graph in which an LC divergent oscillation circuit is applied to the mechanical DC circuit breaker. The applied voltage and loads (normal and transient) are the same as experimental condition of the mechanical DC circuit breaker in Figure 1. Figure 2 shows a model which is the mechanical DC circuit breaker with the LC divergent oscillation circuit. The LC divergent oscillation circuit is composed of L and C connected in series. L and C are set as follows to match the resonance frequency according to the generated the fault current. L is about 15 uH and C is about 750 uF. This is the time point of the accident in Figure 2-(a), and the fault occurred in about 799.8 ms. As for the fault current, an initial fault current of about 541.9 A occurred in about 803.3 ms. From the time the fault occurred, the flow of current was confirmed by the reactor of the LC divergence oscillation circuit. The initial fault current on the LC circuit is about 67.3 A occurred. Figure 2-(b) shows the point of the time when the cut-off contact of the mechanical DC circuit breaker is open and it is about 812.4 ms. In the LC divergent oscillation circuit, current was generated in the form of insufficient braking. Accordingly, it affected the fault current flowing through the mechanical DC circuit breaker and affected the amplification of the initial fault current. Figure 2-(c) shows the point at which the fault current flowing through the mechanical DC circuit breaker reaches zero, which is about 824.1 ms. After that, the fault current all flowed through the LC divergent oscillation circuit, and a current of about 294.7 A was generated. Figure 2-(d) shows the point of the time when the fault current cut-off is completed, which is about 837.1 ms.

그림 2 초전도 소자가 없는 기계식 LC 발산진동회로가 적용된 기계식 차단기 동작 특성 실험 결과 그래프

Fig. 2 Graph of experimental results of mechanical DC circuit breaker applied LC divergence oscillation circuit without the superconducting element

2.3 Simulation modeling of DC circuit breaker using superconducting fault current limiter

Figure 3 is a circuit diagram of a superconducting DC breaker. We modeled the DC system with the same experi- mental conditions as the mechanical DC circuit breaker by simulation through the PSCAD/EMTDC program. The rated voltage and current are 500 V, 50 A. The configuration is designed with a solar power with DC power, L is the line impedance, and the Superconducting module is a superconducting element. We made a superconducting fault-current-limiter using the GdBCO material. We confirmed the parameters the operating characteristics input to the superconducting module of the simulation through the experiments. We obtained data on the rate of change from the superconducting state to the normal state and the data of the generated resistance through experiments. The data of the superconducting module were formulated and applied to the PSCAD/EMTDC program ⁽⁶⁾. Arc model is a model input by formulating arc characteristics that can be realized in simulation. In particular, the superconducting fault current limiter model was designed based on Equation (1) and the Arc model based on Equation (2). $R_{SC}$ is quenching resistance of the superconductor. $R_{m}$ is the maximum quenching resistance of the superconductor. $T_{SC}$ is the time constant for the operating characteristics of the fault current. ${g}_{m}$ is the arc conductance. $\tau_{m}$ is the arc time constant. $U_{}arc$ is the arc voltage. $I_{}arc$ is the arc current. $P_{o}$ is the cooling power constant.

Mechanical switch is a mechanical DC circuit breaker, and an LC divergent oscillation circuit in which a reactor and a capacitor are connected in series are connected in parallel. In addition, SPD is connected in parallel to extinguish residual current. In the simulation, we designed the SPD with a smaller blocking capacity than SA. Normal load and fault load were configured. The simulation was compared and analyzed with two models.

그림 3 초전도 DC 차단기의 회로 구성

Fig. 3 Configuration circuit of superconducting DC circuit breaker

2.4 Simulation results

Figure 4 is a graph of the operation characteristics of the first model, and the superconducting element is not combined. Figure 4-(a) is about 0.1 sec at the time of the accident. The time constant of DC power was designed to be 0.1. Fault current was generated about 500 A from the start of the accident, and the opening operation of the mechanical DC circuit breaker was performed in Figure 4-(b), and it was about 0.1126 s. Based on the preceding experimental data, the opening point of the mechanical DC circuit breaker was designed. After the opening operation started, an arc voltage was generated between the cut-off contacts. This arc charac- teristic was designed by applying Mayr's arc model. The resonance of the LC divergence oscillation circuit occurred, affecting the flow of the fault current. The fault current in Figure 4-(c) is about 0.1162 s when the fault current reaches the artificial zero-point. Figure 4-(d) is about 0.1175 s when cut-off is completed. Figure 5 is a graph of the operation characteristics of the second model, which is a model combined with a superconducting element. The simulation conditions are the same as before. Figure 5-(a) shows the time of the accident, which is about 0.1 s. This model is a model to which a superconducting element is applied, and the quenching of the superconductor is designed at about 0.1015 s after the fault, and the superconducting threshold resistance is designed to be about 0.8 Ω. Due to the quenching of the superconducting element, the fault current was limited to about 279 A. Figure 5-(b) is the point of the time that is opening operation of the mechanical DC circuit breaker and is about 0.1126 s. Figure 5-(c) shows the point of the time when the artificial zero-point of the fault current occurred and was about 0.1129 s. This was about 3.3 ms faster than when the superconducting fault current limiter was not applied. Figure 5-(d) is about 0.1135 s at the time of cut-off completion. This was about 4 ms faster than the first model.

그림 4 실험 결과를 바탕으로 설계된 시뮬레이션 모델의 동작 특성 그래프

Fig. 4 Graph of the operating characteristics of the simulation model designed based on the experimental results

Table 1 shows the results of experiments and simulations. ① is a model composed of only mechanical DC circuit breaker. ② is a model composed of the mechanical DC circuit breaker and LC divergent oscillation circuit. ③ is a model composed of the mechanical DC circuit breaker, LC divergent oscillation circuit and superconducting module. The Figure 1 is the experiment ①. The Figure 2 is the experiment ①+②. The Figure 3 is the simulation ①+②. The Figure 4 is the simulation ①+②+③. In each case model, the time when the artificial zero point occurred and the time when the fault current was completely cut-off were summarized.

그림 5 실험 결과를 바탕으로 설계된 두 번째 시뮬레이션 모델의 동작 특성 그래프

Fig. 5 Graph of the operating characteristics of the second simulation model designed based on the experimental results

그림 6 초전도 소자의 유무에 따른 기계식 DC 차단기의 전력 부담 비교 그래프

Fig. 6 Graph of the power burden of the mechanical DC circuit breaker Depending on the presence or absence of a superconducting element

2.5 Power burden

Figure 6 shows the comparison data of the power burden according to the application of superconducting elements in a graph based on the simulation data of Figure 4 and 5. Figure 6-(a) shows the point of the time when the fault occurred, and it was about 0.1 s. Figure 6-(b) is the point of the time when the circuit breaker is opened, and it is set identically to compare the data on the power burden. Its value was about 0.1126 s. The time of the mechanical contacts opening depended on the opening characteristic of the BK63H DC circuit breaker. Figure 6-(c) is the point of the time when the blocking when there is a superconducting element is completed. Its value was about 0.1135 s. The power burden at this time was about 0.068 W. Figure 6-(d) is the point of the time when blocking is completed when there is no superconducting element, and the value is about 0.1181 s. The power burden at this time was about 0.092 W. Depending on the presence or absence of a superconducting element, the following operating characteristics were positively generated. First, the growth rate of DC fault current was suppressed by about 55.8 %. Second, the level of the cut-off current from the DC circuit breaker's position was formed to be low. Third, the voltage level of the TRV(transient recovery voltage) was formed lower by 64.6 %. Fourth, it was confirmed that the power burden of the mechanical DC circuit breaker was reduced by about 73.9 %.