2.1 Design of the I-SC SCB
Figure 1 shows the structure of a I-DC SCB, in which a superconducting coil and a mechanical
DC circuit breaker are connected in a series. A superconducting coil limits the fault
current rise time and the value of the fault current. The mechanical DC circuit breaker
cut-off the fault line.
그림. 1. I-DC SCB 구조
Fig. 1. I-DC SCB structure
Figure 1 shows the structure of a I-DC SCB, in which a superconducting coil and a mechanical
DC circuit breaker are connected in a series. A superconducting coil limits the fault
current rise time and the value of the fault current. The mechanical DC circuit breaker
cut-off the fault line. Figure 2 shows the superconductor design algorithm. When the value of the current (I1) flowing
through the line is equal to or lesser than the operation start current of the superconductor
(Ioperation start current), the superconductor maintains a superconducting state with
zero impedance. If the value of I1 applied to the superconductor exceeds the Ioperation
start current of the superconductor due to system failure, however, the impedance
of the superconductor increases according to the constant integration time, the limit
value, and transitions to the phase transition state. This was based on Eq.(1) (3).
그림. 2. 초전도체 설계 알고리즘
Fig. 2. Superconductor design algorithm
Figure 3 shows the current characteristics of the superconductor according to the impedance
of the current limitation. The current limiting impedance was increased from 1 Ω to
25 Ω to identify the appropriate current limiting impedance of the superconductor.
The results of the analysis show that the maximum fault current was fixed at 17.25
kA, regardless of the value of the impedance after it exceeded 3 Ω. Therefore, the
maximum quench resistance Rm was set to 3 Ω. Also, Tsc was 0.3 ms as a transition
characteristic time constant of the quench state to obtain 3 Ω within 2 ms, while
the inductance of the superconducting coil is 0.01 H. A mechanical DC circuit breaker
has a structure in which three circuit’s are connected in a parallel structure.
그림. 3. 한류 임피던스 크기에 따른 초전도체의 전류 특성
Fig. 3. The current characteristics of the superconductor according to the impedance
of the current limitation
The main interruption circuit is the main circuit, while the commutation circuit and
the absorption circuit are the auxiliary lines. If a fault occurs, the main circuit
opens the mechanical contact CB to cut-off the fault line. A consideration of the
arc characteristics was essential, since DC has no natural current zero point, unlike
in the AC. A few of the arc characteristics include the Mayr, Cassie, and Schavemaker
arc models. Among them, the Mayr arc model, which analyzes near 8000 K and near current
zero point, was selected. The arc characteristics were applied to the CB by using
the Mayr arc model, as shown in Eq.(2) (2)(4-6).
Eq.(3) represents the breaking current with the breaking capacity of CB at 1 kA(5). The commutation circuit is a circuit in which L and C are connected in a series,
which generates a oscillation current by series resonance at the frequency of Eq.(4), based on the inductance of the superconducting coil. The current zero point was
generated when the impedance of the commutation circuit exceeds the arc impedance
of the main circuit, and the CB was opened quickly from the main circuit. L is 0.2
mH, and C is 49.5 μF, according to Eq.(4).
The absorption circuit is a circuit to which the SA circuit was applied when the SA
operation voltage is applied, the residual voltage and current of the breaker flows
to the ground. The operating voltage of the SA is 125 kV (5-6).
2.2 I-DC SCB mechanism and analysis conditions
When DC power is applied, the superconducting coil (SC coil) senses the current (I1)
flowing through the line to determine whether it is fault state or not, according
to the algorithm shown in Figure 2. The superconducting coil maintains its superconducting state, and the current was
stably conducted during the normal hours. If a fault occurs, the superconducting coil
was quenched from the superconducting state to the normal conducting state within
a few milliseconds. A quenched superconducting coil generates an impedance to reduce
the rise of the fault current, and limits the maximum value of the fault current.
At the same time, an open signal was applied to the mechanical DC circuit breaker,
while the first limited fault current is introduced into the mechanical DC circuit
breaker by the superconducting coil. Next, the mechanical DC circuit breaker completes
the interruption operation by assisting the commutation and absorption circuit to
cut-off the main circuit. The applied voltage is DC 100 kV, while the SC coil, main
circuit, commutation circuit, and absorption circuit are as described above. Since
the normal current flowing in the local HVDC line is about 200~300 A, the load is
set to 500 Ω. If a fault occurs at AC 345 kV, the fault current is up to 68 kA. The
resistance load is set to 1.42 Ω, so that the maximum fault current is kept at 70
kA. The mechanical DC breaker has a delay time of about 10 ms after considering the
relay and operation time.
The applied voltage was then increased to 100, 140, 180 kV, respectively, in order
to analyze the capacity increase characteristics due to superconducting coil. The
simulation conditions remained the same.
2.3 Simulation analysis
Figure 4 shows a graph representing the interruption characteristics of the mechanical DCCB(M-DCCB)
when a voltage of 100 kV. As the fault occurred, the value of the fault current increased
to 41.13 kA. An fault occurred and a current zero point was generated after about
46.5 ms. As a result, the M-DCCB completed the interruption operation after about
58 ms.
그림. 4. 100kV 인가 시 DCCB의 특성 곡선
Fig. 4. Characteristic curves of the DCCB at 100 kV
Figure 5 shows the characteristics of the M-DCCB when a voltage of 110 kV. The 110 kV is about
9% higher than the rated voltage. As shown in Figure 5, The value of fault current increased to 45.71 kA and a current zero point was generated
at 63.8 ms. The interruption operation was completed 78.2 ms after fault.
그림. 5. 110kV 인가 시 DCCB의 특성 곡선
Fig. 5. Characteristic curves of the DCCB at 110 kV
However, the interruption time range of the existing mechanical HVDC CB(30~50 ms).
Therefore, it is judged that the interruption operation has failed that because Figure 5 was out of the existing M-HVDC CB interruption time.
Figure 6 shows a graph representing the interruption characteristics of the I-DC SCB when
a voltage of 100 kV. As described above, the superconducting coil was in the zero
impedance state, and the current was stably conducted before a fault occurs. Then,
a simulated fault occurred at 0.1 sec.
그림. 6. 110kV 인가 시 I-DC SCB의 특성 곡선
Fig. 6. Characteristic curves of the I-DC SCB at 110 kV
About 2 ms after the fault, the superconducting coils were quenched according to the
motion algorithm in Figure 2. The quench of the superconductor limits the value of the fault current to about
17.25 kA. Then, L and C in the commutation circuit of the mechanical DC circuit breaker
were serially resonated to generate the oscillating current. As a result, the current
zero point was generated about 11.9 ms after the fault, and the interruption current
was also generated, as shown in Eq.(3). The interruption operation was completed within about 18.4 ms, when the superconducting
coil was power of burdened with 10.22 MW, according to Eq.(5).
Figure 7 shows the characteristics of the I-DC SCB when a voltage of 140 kV. The 140 kV is
about 28% higher than the rated voltage. As shown in Figure 6, fault occurred, and the superconducting coil was quenched to limit the fault current
value.
그림. 7. 140kV 인가 시 I-DC SCB의 특성 곡선
Fig. 7. Characteristic curves of the I-DC SCB at 140 kV
The commutation and the absorption circuit assisted the main circuit, and the current
zero point was generated after about 16.2 ms, while the interruption operation was
completed within 22.2 ms. At this time, the power burden applied to the superconducting
coil is 27.19 MW, and when calculated into a percentage based on 100 kV, power that
is about 62.4% higher.
Figure 8 shows the voltage at 180 kV, about 45% higher than the rated voltage. The operation,
as shown in Figure 7, was performed. The current zero point was generated, and the interruption operation
was completed after approximately 28.5 ms. The value of the power burdened on the
superconducting coil was 84.7%. The power burden on superconducting coils is an important
factor in determining superconductor capability and capacity, as well as the capacity
and capability of associated circuit breakers. Since the value of the power burden
applied to the super- conducting coil is lower, it is possible to reduce them, while
efficiency remains high in terms of economy.
그림. 8. 180kV 인가 시 I-DC SCB의 특성 곡선
Fig. 8. Characteristic curves of the I-DC SCB at 180 kV
Figure 9 shows the analysis of the power burden and interruption time applied to the superconductor
according to the voltage increase. The results of the above-mentioned analysis were
judged based on the following two criteria.
그림. 9. 전압 증가에 따른 차단시간 및 초전도 코일의 전력 부담
Fig. 9. Power burden of superconducting coil and interruption time according to voltage
increase
Criteria 1) It should be faster than the interruption operation time of the existing
mechanical HVDC CB (30~50 ms) (5)
Criteria 2) The power charge ratio applied to the rated voltage superconductor should
be within 100%.
The M-DCCB can only be used at rated voltage. On the other hand, The I-DC SCB with
a rated voltage of 100 kV can be cut off and applied up to 180 kV. It is because that
the interruption time was faster than the HVDC interruption time(criteria 1).
However, when a voltage higher than 180 kV is applied, the value of the power burden
applied to the superconducting coil exceeds 100% of the rated voltage(criteria 2).