전력계통 보호용 변압기형 초전도 한류기의 과도특성

Abstract

The transformer-type superconducting fault current limiter (SFCL) that was proposed in this study is a resistive-type SFCL to which a transformer is connected. The transformer has primary and secondary windings. The power system line is connected to the primary winding of the transformer in series, and the superconducting unit is connected to the secondary winding. The connection of the superconducting units in series and in parallel to each other should be ensured to reduce the fault current. Unbalanced quench occurs in the superconducting units that are connected in series and in parallel to each other, however, due to the difference in their critical behaviors. In this study, the unbalanced quench problem of the serially connected superconducting units was solved by connecting the secondary winding to the units using a neutral line. In addition, different forms of SFCLs were made to devise a structure that is suitable for increasing the capacity of the SFCL. Based on the experimental results, line-to-ground and line-to-line faults in the three-phase power system were simulated to analyze the fault-current-limiting and transient characteristics of the three-phase transformer-type SFCL. The SFCL simulation was conducted using the EMTP/ATP power system analysis program, because the three-phase transformer-type SFCL cannot be applied to the actual power system at present. The design of a two-circuit distribution system enabled the analysis of the operating characteristics in the sections of the faulty feeder according to the existence of the SFCL, and the reliability of the SFCL was verified via symmetrical component calculus. The results of this study are as follows.

1. There is a problem of unbalanced quench in SFCLs, due to the inevitable difference in the critical behavior of the superconducting units. The quench characteristics of the superconducting units can be made regular by connecting the secondary winding of the transformer to the superconducting units that is connected to it in series using a neutral line. The neutral line connection can overcome the difference in the critical behavior of the superconducting units by helping the secondary winding of the transformer to induce the current so that the balanced quench of the SFCL elements can be realized. Such connection led to a balanced quench of the superconducting units and a uniform voltage on the superconducting units, which indicated that the power burden was uniform.

2. To develop a plan to increase the transformer-type SFCL capacity, four superconducting units were connected to the secondary transformer winding in series in three ways. In the case of the general series connection without using the neutral line, only one superconducting unit was quenched because of its unbalanced quench behavior, and its power burden was high. When the neutral line was used, however, the secondary winding individually applied the quench current to the superconducting units, and the quench was uniform regardless of the difference in the critical behaviors. Accordingly, all four superconducting units were quenched, which reduced the power consumption to 1/3. The four quenched superconducting units also quickly recovered their superconducting state within the opening cycle of the circuit breaker.

3. The fault current limiting and transient characteristics of the SFCL were analyzed according to the line-to-ground fault types in the power system. The test condition was given so that three line-to-ground faults occurred consecutively according to the reclosing duty cycle, which is a power system protection method. In the three-phase transformer-type SFCL, one iron core links the primary winding with the secondary winding for each of the three phases. Therefore, a fault on a phase affects other sound or faulty phases according to the mutual induction. It was verified in the experiment that the inductive current induced the quench in the superconducting units in the sound phase and reduced the power burden in the faulty phase, but interfered with the fault current limiting operation in the other faulty phases. In the case of the three faults, the SFCL limited at least 70% of the fault current according to the reclosing duty cycle, and the superconducting units also recovered their superconducting state within a constant time and within the opening cycle of the circuit breaker.

4. The transient analysis of the power system to which the transformer-type SFCL was applied was conducted using the symmetrical component calculus. A smaller transformer winding ratio led to a better fault current limiting characteristic and thus, to a smaller zero sequence current. This indicates that the use of the transformer-type SFCL can reduce the electromagnetic inductive disturbance of adjacent communication lines. In the normal state, only the positive sequence current flows to generate the rotating torque for the motor load. A fault increases the difference between the positive and negative sequence currents, and increases the rotating torque of the motor load. The use of the SFCL reduced the difference between the positive and negative sequence currents. This made the phase angle difference between the motor and the generator constant, and maintained the transient stability of the power system, by ensuring that the rotating torque of the motor load did not significantly differ from that in the normal state.

5. Using the EMTP/ATP power system analysis program, the fault simulation was conducted according to the fault types by applying the transformer-type SFCL to the 22.9kV class distribution power system. About 70% of the fault current decreased in the faulty feeder, and the sound feeder maintained 90% or more of the current before the fault. In the case of the triple line-to-line fault, which created the highest fault current, the transformer-type SFCL reduced about 80% or more of the fault current, and maintained 90% or more of the power in the sound feeder. This indicates that the breaking capacity of circuit breaker can be increased.

6. The symmetrical component calculus was used for the transient analysis of the faulty power system. The results indicated that the difference between the positive and negative sequence currents in the faulted feeder can be reduced. The sound feeder with no SFCL had the same positive and negative sequence currents, which did not create the rotating torque for the motor load. The use of the SFCL made the difference almost the same as before the fault occurrence, and maintained the rotating torque for the motor load that was connected to the sound feeder. The SFCL is expected to be used to maintain the phase angle difference between the generator and the motor, and to ensure the transient stability in the power system.