변압기 결합형 초전도 한류기의 전력계통 적용특성

Abstract

With the rapid industrialization and economical development, the electricity demands of the industrial facilities and densely populated large cities are continuing to increase in Korea. The increase in the power consumption requires the extension of power facilities, but it is difficult to secure spaces for equipment installation in the limited space of urban areas. In addition, the 154 kV or 345 kV transmission systems in Korea has a short transmission distance, and are connected to one another in network structures that ensure the high reliability and stability of power supply. This structure reduces the impedance during the fault in power system, and increases the magnitude of in the short circuit fault current. There are many methods to address the fault current. The breaker whose breaking capacity is insufficient should be replaced by the one with a higher capacity. However, replacing the breaker involves large technical and economical burdens. If the buses of the power system are separated, the resulting overload and voltage fluctuation degrade the power quality and supply reliability. Serial reactors or high impedance power devices can also be installed for the replacement, but they cause voltage drop during normal operation. Thus, there were no existing methods to address the fault current. The superconducting fault current limiter (SFCL) was devised to effectively address these existing problems. The SFCL is a new-concept eco-friendly protective device that ensures fast operation and recovery time for the fault current and does not need additional fault detection devices. Therefore, many studies are being conducted around the world. The SFCL in the power system does not affect the surrounding equipment, without loss during normal operation, but if the fault current exceeds the critical current of the superconductor in the event of a fault in the system, the superconductor is quenched and changed into the normal conduction state. A high impedance is generated from the superconductor in the superconducting state, and the superconductor operates to limit the magnitude of the fault current. Because the SFCL limits the fault current, the existing breaker can stably operate and the system is effectively protected against the fault, without the need for replacing or increasing the capacity of the existing breaker. The introduction of the SFCL applied to the breaker that has an insufficient breaking capacity will be able to increase the breaker capacity and ensure stable system protection. Based on the simple resistor-type SFCL, this study addressed a new-concept SFCL with the SFCL applied to the third winding of the commercial transformer. The SFCLs in the past studies have been independent devices that are installed standalone in the power system. This type requires high cost because it needs additional installation space and should be installed for each breaker. The SFCL in this study, however, is installed on the tertiary winding of the existing transformer. In addition, the volume of the superconductor can be minimized by controlling the fault current limitation rate of the SFCL depending on the number of turns of the tertiary winding. It is economical because expensive superconductors can be reduced, and it can solve the existing problems by reducing the size of the peripheral devices, including the cryostat. In this study, the SFCL was applied to the commercial transformer to analyze its operating characteristics in the event of the fault in the system.

1. The superconducting fault current limiter (SFCL) was applied to the third winding of the commercial transformer. In the normal condition, the transformer conducted its basic operations (step up, step down and voltage compensation). In the fault condition of the power system, the fault current flowed to the third winding, quenching the superconducting element connected to the third winding, and limited 90% of the fault current.

2. The operating characteristics of the single and three phase transformers with the SFCL are represented by the operating time of the SCR switch, which is important in the transformer SFCL combination technology. Contact point of a in the SCR is immediately turned on by the signal to the gate. In the case of contact point of b in the SCR, however, the contact is turned off when the current falls below the holding current or when the inverse voltage is applied to the SCR. The results of the application of the transformer and SFCL to the single-phase system showed that the fault current was limited half a cycle after the current of contact point of be in the SCR reached 0 [A].

In the case of the three phase system, however, the magnetic flux generated by other sound phases affected the current of the SCR-b, and the contact was turned off as soon as the gate signal was applied, due to the fault detection of the CT. The fault current was limited within half a cycle after the fault occurrence In the power system, to which the SFCL will be applied, is a three phase system, and its application to the system was advantageous.

3. The operating characteristic of the SFCL according to the change in the number of windings of the single phase transformer was analyzed. As a result, a greater number of secondary windings delayed the quenching point of the superconducting element, and a greater number of third windings delayed the quenching point of the superconducting element. This was because the impedance and current decreased with the increase in the number of third windings, and the achievement of the critical current was delayed.

4. The difference in the critical characteristic, which is produced during the fabrication of the superconducting element, causes the irregular quenching characteristic. One superconducting element covered the power in the power consumption curve of resistive type SFCL but two elements covered the power in the power consumption curve of the transformer combined with a single phase SFCL. In addition, it is advantageous in increasing the capacity because two superconducting elements can share the power burden.

5. To derive application characteristics in diverse power system situations, the operating characteristics were analyzed according to the use of the transformer (step up, voltage compensation and step down), third winding ratio and fault type. The use of the transformer is determined by the secondary winding ratio. A fewer number of secondary windings led to a faster turn-off operation time of the SCR-b, and a greater number of third windings led to a faster turn-on operation time of the SCR-a. Accordingly, the fault current is limited within half a cycle when the transformer is used for step down with a small number of secondary windings. In the characteristic analysis according to the fault types the fault on phase S, which was positioned at the center core leg, quenched the superconducting element before the turn-off of the SCR-b due to the magnetic flux from other sound and faulty phases. Accordingly, the current on the SCR-b was not 0[A], and SCR-b turned off after half a cycle and limited the fault current. In the case of the triple line to ground faults, the fault current on phase S was also limited after half a cycle, and the fault currents on phases R and T were limited well within half a cycle.

6. The simulation results of the EMTP power system analysis program showed that 90% or more of the fault current was limited after half a cycle depending on the fault types and the peak of the initial fault current was limited by at least 20% more when the SCR operated within half a cycle than when it operated after half a cycle. This indicates that the SCR operation can efficiently limit the fault current when it operates within half a cycle. In addition, this indicates that the power breaker capacity can be increased in the real power system.