1. Introduction

Medium Voltage Direct Current(MVDC) is defined the system having the voltage level and transmission capacity between High Voltage Direct Current(HVDC) and Low Voltage Direct Current(LVDC). The MVDC is being focused for the introduction of the renewable energy sources, increasing DC acceptance, securing reliability and stability of the system⁽¹⁾.

The HVDC and LVDC are being actively researched worldwide. As for domestic DC technology, HVDC was first introduced through long-distance transmission. The stability and reliability of the LVDC system got through the DC island construction project⁽²⁾. However, not only the voltage level of MVDC is not clearly decided, but also that technology development is slower than HVDC and LVDC systems. Therefore, technologies for connection with the MVDC system are being studied^(3,4). And high-reliability protective device is required as one of the important factors for the stable operation of the system. Accordingly, various methods are being proposed to stably extinguish the arc generated in the DC system^(5-7).

Semiconductors are considered the most to prevent DC fault. It separates systems through fast switching but has disadvantages of heat generation and economic feasibility^(8-10). The cut-off time of the existing mechanical circuit breaker is about 30～50ms, which is slower than that of the semiconductors. As a solution to this, we propose a model in which the mechanical circuit breaker and a superconductor are combined. It is a superconducting LC resonant DC circuit breaker. The superconductor compensates for the disadvantage of the mechanical circuit breaker by reducing the fault current within about 2ms. Also, a LC resonant DC circuit breaker creates zero-point to fault current through a LC resonance circuit and supports the opening operation of the mechanical circuit breaker. It was verified through experiment about type of superconducting element and voltages^(11,12).

The transient states of the DC system are typically divided Pole-to-Pole fault(PTP) and Pole-to-Ground fault(PTG). The fault current in the PTP is generally more than 10 times higher than the steady state current. Interrupting the fault current quickly is important because there are some risks of overheating and fire of peripheral devices when the fault current is sustainedly maintained^(13,14).

In this paper, we intended to confirm the operation characteristics of the superconducting LC resonant DC circuit breaker to get the data for types of the DC fault according to typical two fault types in the DC system. 15kV MVDC simulation circuit was designed using the PSCAD/EMTDC program and the superconducting LC resonant DC circuit breaker was applied to the simulation circuit. Also, the initial

Fig. 1. A circuit of the superconducting LC resonant DC circuit breaker

fault current, cut-off time and power burden were compared and analyzed in the operation characteristics of fault type. This is because the above factors are important considerations in analyzing the operation reliability of the circuit breaker.

2. The superconducting LC resonant DC circuit breaker

We propose the fast and highly reliable superconducting LC resonant DC circuit breaker. Fig 1 shows a circuit of the superconducting LC resonant DC circuit breaker. It consists of a current-limiting part and a cut-off part. The current-limiting part is a superconducting element. The cut-off part is the LC resonant DC circuit breaker. It includes a mechanical circuit breaker, the LC resonance circuit, and a Lightning Arrestor(LA).

The superconducting element is not quenched in the steady state and the steady state current flows to the load through a closed mechanical circuit breaker. And the magnitude of current is determined according to Eq (1). The resistance of the circuit on the steady state is shown in Eq (2). The resistance of the mechanical circuit breaker is very low and not included. Also, line reactance is not considered since DC has not frequency.

In the transient state, the resistance generated by the superconducting element should be considered. And ground resistance should also be considered in PTG. The resistance of the superconducting element and the ground reduces the fault current. It is explained through Eq (3)-(4). The fault current is reduced by quench and flows into the mechanical circuit breaker when the fault current exceeds the critical value of the superconducting element. And the fault current flows through the LC resonance circuit connected in parallel due to the arc generated when the mechanical circuit breaker is opened. It assists the opening of the mechanical circuit breaker by creating the zero-point when the oscillation amplitude of the LC circuit becomes lager than the fault current. The current flowing through the LC resonance circuit is shown in Eq (5). The cut-off operation is completed by discharging the residual current in the LA.

$R_{l}$ : Line resistance [Ω],

$R_{L}$ : Road resistance [Ω]

$R_{g}$ : Ground resistance [Ω]

$R_{SC}$ : Resistance of the superconducting element [Ω]

$u_{{arc}}$ : Arc voltage [V],

$i_{s}$ : Total fault current [A]

$L$ : LC circuit inductance [H],

$C$ : LC circuit capacitance [F]

3. Fault types

Transient states of DC system are typically divided PTP and PTG. PTP is fault that occurs when two lines are contacted directly or through conductor contact. PTP affects the switching behavior of DC-DC converters. In addition, the fault current rises rapidly due to the discharge of the capacitor in the Voltage Source Converter(VSC) based system^(15,16). PTG is fault caused by insulation defects between the DC pole and the ground. PTG damages the grid transformer in the AC/DC grid system^(17-18). PTG circuit impedance is higher than that of PTP since the circuit contains the ground resistance as shown in Eq (4). Therefore, it is difficult to detect fault current in the Low Voltage Direct Current(LVDC) system. In this paper, general types of PTP and PTG were simulated. It is assumed that PTP is a case of two lines in contact, and PTG is a case of electrical breakdown.

Arc model must be applied to main circuit because the DC has no zero-point in the current, which the arcing time becomes longer^(19-21). The Mayr model is mainly used for the modelling of the arc in the vicinity of current zero-point when the temperature of the plasma is below 8000K. Therefore, the Mayr model was used as the arc model when the mechanical circuit breaker opened.

4. Simulation design

Fig 2 shows the superconducting LC resonant DC circuit breaker of the simulation circuit. The conditions of the simulation circuit are as follows. Load resistance was set 10 Ω^(11,12). And ground resistance was set to 1 Ω to simulate the DC electrical breakdown situation on PTG^(22-24). L and C of the LC divergence oscillation circuit are 1mH and 25uF and are calculated by Eq (6)^(25,26).

Fig. 2. Simulation circuit

Simulated time of the DC faults is 100ms.

4.1 Current-limiting part

Fig 3 shows the modeling of superconductor properties based on Eq (7) using the PSCAD/EMTDC. It was modeled through precedent research^(11,12). The superconducting element is quenched when the fault current exceeds the critical current. And the resistance of the superconducting element is then applied to the circuit. $T_{SC}$ [s] was set to be 0.75ms. Also, the inductance of the coil-type superconducting element was set to be 2mH in the precedent research. This inductance delays the rise time of the fault current.

(7)

$R_{S C}(t)=\left\{\begin{array}{cc}0 & \left(t<t_{\text {quench }}\right) \\ R_{m} \sqrt{1-\exp \left(-\frac{t}{T_{S C}}\right)} & \left(t_{\text {quench }}<t\right)\end{array}\right\}[\Omega]$

$R_{m}$ : maximum resistance of the superconducting element [Ω],

$T_{SC}$ : time constant for the transition to the superconducting state [s],

$t_{quench}$ : time when the superconducting element is quenched [s]

Fig. 3. The quench system superconducting element

5. Simulation results

5.1 PTP

Fig 4(a) and (b) show graphs of operation characteristics of the LC resonant DC circuit breaker with or without the superconducting element in PTP. The graph was analyzed by dividing them into sections ① ~ ④. ① is the point when the fault was simulated and ② is the starting point of the mechanical circuit breaker opening. ③ is point when the fault current reaches zero-point by the LC resonance circuit. ④ is the point at which cut-off is completed through the LA.

Fig. 4. Operation characteristics in PTP (a) without the superconducting element (b) with the superconducting element

The steady state current was about 1.5kA in Fig 4(a). The fault current rose to about 14.9kA at ①. The zero-point was created at about 147.9ms by the LC resonance circuit after the opening operation of the mechanical circuit breaker at ②. The mechanical circuit breaker was completely opened at ③. The residual current in the circuit was discharged through the parallel connected the LA. The fault current was zero at ④ and it took about 81.1ms from the fault starting time.

In Fig 4(b), the steady state current was the same as in Fig 4(a). The current rose because of the fault at ①. The initial fault current was reduced to about 9.9kA by the quenching of thesuperconducting element. The mechanical circuit breaker was opened at about 110ms. The fault current reached the zero-point by the resonance of L and C at about 119.1ms. After that, LA was operated, and the LC resonant DC circuit breaker voltage rose. The cut-off was completed at ④, and it took total about 47.0ms.

Fig. 5. Operation characteristics in PTG (a) without the superconducting element (b) with the superconducting element

The initial fault current in Fig 4(b) was reduced by about 5.0kA than in Fig 4(a).

5.2 PTG

Fig 5(a) and (b) show graphs of operation characteristics of the LC resonant DC circuit breaker with or without the superconducting element in PTG.

In Fig 5(a) at ①, the intial fault current rose to about 7.4kA. The fault current was lower than in PTP because of the ground resistance in PTG. The zero-point was created by the LC resonance circuit at about 113.2ms after the opening operation of the mechanical circuit breaker at ③. Also, it took about 32.0ms from fault occurs to cut-off.

Fig 5(b) shows that the initial fault current was reduced to about 6.0kA by the superconducting element. It was about 1.3kA lower than the initial fault current of Fig 5(a). The fault current reached

Fig. 6. The power burden of the superconducting element according to PTP, PTG

the zero-point at about 111.3ms after the arc occurs in the mechanical circuit breaker. The cut-off was completed at about 124.5ms.

6. Conclusions

In this paper, we designed a DC simulation circuit and applied the superconducting LC resonant DC circuit breaker to this circuit. We designed the resistance generation equation of the superconductor using the PSCAD/EMTDC program and applied it to the superconducting element in the circuit. And the Mayr model was applied to implement DC arc in the mechanical circuit breaker. Also, we analyzed and compared the operating characteristics of superconducting LC resonant DC circuit breaker according to DC fault type.

As a result, the initial fault current was reduced by about 39.6% in PTP and about 17.8% in PTG by the superconducting element. In addition, the mechanical circuit breaker completed cut-off within about 32ms in PTP and about 24.5ms in PTG. The power burden of the superconducting element was about twice that of the LC resonant circuit breaker in PTP. And in PTG, it was confirmed that it was about 4.5times.

If the superconducting LC resonant DC circuit breaker is applied to the DC grid, it can increase the probability of the fault cut-off. And it is expected that the deterioration of the protective devices and the surrounding equipment can be reduced.