Research Of Voltage Compensation Type Active Superconducting Fault Current Limiter

With the continual development of power system installed capacity, and the expansion of interconnected network, the level of short-circuit current increases constantly. Once the short-circuit current is greater than the interrupting capacities of existing circuit breakers, the fault cannot be cleared effectively, and further the safeties of power equipments and whole power system may be threatened seriously. In order to enhance the security, stability and reliability of power network, and to reduce the burden induced by the fault current, it's urgent to suppress the fault current to a certain level. Among the proposed curren-limiting technologies, superconducting fault current limiter (SFCL), due to its some special advantages, has been one of the current-limiting technical field's main research directions. In this thesis, a voltage compensation type active SFCL is proposed. This dissertation systematically studies the proposed voltage compensation type active SFCL, including the issues of theoretical analysis, strategy frame, simulation verification, performance comparison, coordinated operation, prototype development and short-circuit test in a dynamic simulation power system. To drive the application of this type SFCL to the medium and high voltage power network, the basic parameter design about a 10kV voltage compensation type active SFCL is carried out and some corresponding engineering problems are discussed briefly.In chapter 2, the structure and principle of the single-phase voltage compensation type active SFCL are firstly introduced. The single-phase active SFCL is composed of an air-core superconducting transformer and a pulse-width modulation (PWM) converter. The superconducting transformer's primary winding is in series with AC main circuit, and its second winding is connected with the PWM converter. In normal state, the injected current in the secondary winding will be adjusted to ensure that the primary voltage of the transformer is compensated to zero. As a result, the SFCL has no influence on the main circuit. When the fault happens, the current-limiting impedance in series with main circuit can be changed by adjusting the injected current in the amplitude and phase angle, and further the current through the fault line can be limited. According to the difference in the regulating objectives of the converter's output current, there are three operation modes. Based on the simulation analysis and the short-circuit test for a small-scale protype, the correctness of the current-limiting principle and the practicability of the air-core superconducting transformer can be proved.In chapter 3, on the basis of the single-phase structure, the integrated three-phase topology structure is proposed, and its operational characteristics under the different types of faults are investigated. In order to realize the three-phase active SFCL's current-limiting characteristics, it is needed to control its component, namely the three-phase four-wire PWM converter with split-capacitors, flexibly and reasonably. The double closed loop control, consisting of capacitor-voltage balance control, is proposed. According to the simulation results, the validity of presented control strategy is fully affirmed. In additon, the influence of the air-core superconducting transformer's parameter variation on the integrated three-phase active SFCL's performance is also investaged, and conclusions are shown as follows:raising the self-inductance of primary winding is beneficial to the current-limiting capacity; enhancing the transformation ratio can lead to the increase of initial compensation current; increasing the coupling coefficient is helpful to reduce the reactive power output of converter.In chapter 4, taking the power system transient stability, distance protection and voltage sag for examples, the influences of the voltage compensation type active SFCL on the existing equipments and operating characteristics of power system are analyzed. Firstly, based on the power-angle characteristic of the single-machine to infinite bus power system, the effect of the active SFCL on the generator's electromagnetic power is researched, and then its influence on the transient stability is studied by adopting the equal area rule. According to the simulation platform, the power-angle swing curves are simulated under the different current-limiting modes, fault types and fault clearance times. It can be found that, the installation of the active SFCL can consume the electromagnetic power, reduce the energy acceleration area, and then enhance the transient stability. From the point of view of revising the distance relay's measured impedance, the effect of the active SFCL on the distance relay can be eliminated. Based on the three different operation modes of the active SFCL, this chapter presents the corresponding modified formulas. The model of the dual-source power system with the active SFCL is built to evaluate the validities of the modified formulas, and the results are positive. In the final part of this chapter, according to the simulation analyses under the different current-limiting parameters and fault locations, the impact of the active SFCL on the synchronous machine's terminal voltage sag due to short circuit is researched. It is observed that the active SFCL can help to reduce the voltage sag and improve the dynamic power quality.The author participated in the research on the 220V/30A three-phase voltage compensation type active SFCL supported by the 863 program, and took charge of the system analysis, parameter design, dynamic experiments and experimental data processing. In chapter 5, the configuration of the 220V/30A three-phase active SFCL and the part of important test results are presented. In the dynamic simulated experiment of power system, the SFCL prototype can not only effectively suppress the fault current under the three-phase short-circuit, but also adjust its steady level. In addition, there is no overvoltage during the current-limiting process.In chapter 6, a practical lOkV distribution network is chosen, and the voltage compensation type active SFCL is applied to a feeder where the fault often occurs. For the air-core superconducting transformer and the PWM converter of the 10kV active SFCL, the basic parameters are designed. In view of the 10kV active SFCL's engineering application, the AC loss of superconducting coil, the low temperature insulating technique, and the converter's switching element are selected to be investigated briefly, and the conclusions can provide the technical foundation for the future engineering protype fabrication.