2.1 Superconducting coils

In normal superconducting state, there is no impedance in superconductor. By this reason, power could supply without any electric losses or power burden. Upon the occurrence of a fault, however, when the amplitude of fault current exceeds the critical current of the superconductor, it generates a specific high impedance in a few milliseconds according to their unique characteristic. This behavior is named as quenching. Equation (1) represents the quenching characteristics of the superconductor. The value Rm is the maximum resistance of the superconductor in the quenching state. Tsc is the time constant of the superconductor during transition from the superconducting state to the normal state. To is the time to start the quenching⁽³⁾.

In coil type superconductor, the constant for Rm= 8Ω and Tsc=0.75 msec. The reason is the peak resistance value of the superconductor was modeling to be within 2 msec. Fig. 2 represents the coil type superconductor quenching characteristic graph of coil type superconductor⁽⁴⁾.

Fig. 2. Coil type superconductor quenching characteristic graph

2.2 Configuration of circuit

The mechanical DC circuit breaker is comprised of 3 circuits; breaking circuit, commutation circuit and absorber circuit.

First, there are mechanical contact and arc box in the breaking circuit. Arc box is placed for arc characteristics in the circuit breaker. As previously stated, DC interruption is usually accompanied with arc. To implement the arc characteristics in the simulation, arc modeling is indispensable. Mayr arc model had used to apply arc box modeling. Coil type superconductor DCCB is aimed to adapt to the DC links between the distributed power systems. This part is suited for high voltage. Mayr arc model has selected to arc model because it is appropriate in high voltage and low current.

Also Mayr assumed that the arc has fixed across- sectional area losing energy only by radial thermal conduction. The Mayr model is suited for modeling of the arc in the vicinity of current zero when the temperature of the plasm is below 8000 K⁽⁵⁾. Arc box was modeled based on Equation (2).

Second, inductance (L) and capacitance(C) are the elements of the commutation circuit. The momentary mechanical contact is opened, the current flows into commutation circuit and induces oscillation. By the function of the oscillation current, zero current could be induced. Equation (3) is the arc current occurred in between the mechanical contact in the breaking circuit.

The arc time constant is 0.3 uF and arc cooling power is 50 MW. The constants for the commutation circuit were set at L = 300 uH and C = 30 uF⁽⁶⁾.

At this point, $w _ { c } = \sqrt { 1 / L C }$.

Lastly, an arrester was placed in the absorber circuit. The residual current was discharged through the arrester. 120 kV of arrester voltage rating was inserted into the simulation^(7,8).

3.1 Simulation modeling

In this paper, we would like to confirm the protection range of DCCB according to coil type superconductor. Mechanical DCCB were designed by rating voltage of 120 kV. The DC sources that were applied to the simulation were 120 kV, 180 kV and 240 kV in 60 kV interval each. The fault occurred at 1 second after beginning of the simulation, and the fault duration was 0.5 sec. As the circuit breaker had mechanical contacts to operates, accordingly the opening operation would have of 10ms delay time. It works at 1.010 sec. The figures showed that the line resistance was set at 2.5 Ωand load resistance was set at 24.77Ω

3.2 Simulation results

Fig. 3 shows the current characteristics when the applied voltage is 120 kV DC. It shows behaviors of CB with and without coil type superconductor. When the power was applied to the without superconductor, a 4.39kA steady- state current flowed into the circuit. Through the simulated fault at 1 sec, fault current of 12.6 kA was generated. After 10 msec, mechanical contact in the breaking circuit was operated and oscillation current flowed with commutation circuit. At this moment, oscillation current increased maximum 21.2 kA. It took 13.08 sec for breaking operation. In the end, the residual current discharged through the arrester in 4~5ms. On the other hand, when the DCCB had applied superconductor, fault current was limited to 7.3 kA by quenching operation of superconductivity. After 10ms mechanical circuit breaker worked and whole interruption operation ended in 10.08 msec.

Fig. 3. DC current characteristics between with and without superconductor when the input voltage is 120 kV

Fig. 4 represents the current characteristics in 180kV. At first 6.59 kA of steady state current flowed in normal condition. When the fault occurs, 19.2kA of fault current was generated in the DC circuit breaker which is composed without superconductor. After the operation of the contact, oscillation fault current increased until 35.1kA. And the whole interruption was done in 23.40 msec. When the DC circuit breaker applied a superconductor, the fault current that limited by superconductivity was generated about 10.8 kA. As aforementioned mechanism, it took only 12.18 msec to cutoff the fault current.

Fig. 4. DC current characteristics between with and without superconductor when the input voltage is 180 kV

Fig. 5 shows characteristics in input voltage of 240 kV. The figure of 8.79 kA steady state current flowed in normal state. In the without superconductor, 25.22kA of fault current was generated. After it took same mechanism as above-mentioned. In breaking circuit, there is no current flow after oscillation, however, 10.3 kA of current flows through absorber circuit. Indeed interruption had failed.

Fig. 5. DC current characteristics between with and without superconductor when the input voltage is 240 kV

Whereas in the case with superconductor, 13.7kA of limited fault current was generated. The whole interruption operation has done in 14.9 msec.

Results show that when the input voltage was 120 kV, fault current was interrupted stably without superconductor. However, when the input voltage was 180kV, the both fault current and interruption time was increased than before. At last interruption has failed in 240 kV. On the other hand, when the superconductor has applied to DC circuit breaker, the whole three cases were cut-off stably within 16 msec.

3.3 Consideration

Fig. 6 shows the interruption time and power burden graph according to rise of input voltage. The power burden represents energy that generates in the circuit breaker which is calculated by integration of the rated voltage and rated current. This is the case of 120 kV rated DC circuit breaker with the coil type superconductor. It could find that although the input voltage had increased, cut-off operation worked in a very high speed. Also it only took 14.9 msec when the input voltage was 240 kV. Therefore protection range could enlarge at least 2 times when superconductor applied. According to AC circuit breaking standard, fault current should cut-off in 3~5 cycles. But the figure represents the interruption time in the full grid with all electric devices. When we check the operation speed of current limiter only, it should have to operate within 8 msec. However, this is the case of DC circuit breaker with coil type superconductor. Therefore the maximum interruption time was computed at 16 msec in this paper. As follows the standard, the figure in the data represents that interruption time is appropriate.

Fig. 6. Interruption time and power burden of coil type superconductor applied DC circuit breaker according to increase of input voltage

When the massive power burden had occurred, the heat generated within the circuit increases and resulted electric breakdown and deterioration as breaker failure. Thus, a stable cut-off criterion was calculated as a point where it would take twice as much power as the initial power burden. At least 4.35 MVA was imposed on the approval of 120 kV. At 240 kV, approximately twice the 8.3 MVA power burden was generated. Therefore, if superconductor applied to 120 kV rated DC circuit breaker, the maximum protection range was calculated at 240 kV that twice as much as the rated.

At last, depending on the maximum resistance and capacity of coil type superconductor, the protection range can be increased.