Yuejin Tang

Current Limiting Characteristics of a Novel Flux-Coupling Type Superconducting Fault Current Limiter

A novel flux-coupling type superconducting fault current limiter (SFCL) is proposed in this paper. This type SFCL is composed of two superconducting coupling coils, a controlled switch and two metal oxide arresters (MOAs). The primary coil and the secondary coil, which are wound in the reverse direction and are respectively in parallel with the two MOAs, are connected in parallel through the controlled switch and a main circuit breaker. Under normal condition, due to the non-inductive coupling characteristics between the two superconducting coils, the impedance of the proposed SFCL will be too small to affect the main circuit. After the fault happens, the controlled switch will be opened from its initial closed state, and meanwhile the MOAs will prevent the two coils from being damaged by the overvoltage caused by the switching operation. Since the flux between the coils cannot cancel out each other, the non-inductive coupling will disappear, and then the limiting impedance will be triggered for current-limitation. In order to evaluate the current-limiting performance of the novel flux-coupling type SFCL, a laboratory scale prototype was made. The two superconducting coils were fabricated with Bi2223/Ag, and an anti-parallel IGBT pair was chosen as the controlled switch. According to the test results, it was found that, the fault current could be limited quickly and effectively, and there was no overvoltage during the operation process of the controlled switch.

Control scheme studies of voltage source type superconducting magnetic energy storage (SMES) under asymmetrical voltage

Symmetry of system voltage is very important to the performance of superconducting magnetic energy storage (SMES). Both theoretical analysis and simulation results show that, negative sequence current and 2nd harmonic power will be injected into the system, when conventional symmetrical fire is used in power conditioning system (PCS) of SMES. Two indices are used to evaluate the performance of PCS of SMES: the relative error of average value and 2nd harmonic component of active power supplied by SMES. To improve the performance of SMES, symmetrical vector control is put forward in this paper. According to the objective, there are three kinds of symmetrical vector control: positive sequence voltage control, zero negative sequence current control and constant active power control. Their principles are presented and improvements on the performance of SMES are verified by electromagnetic transient simulation. It shows that all those three methods decrease the relative error of average value of active power supplied by SMES. Zero negative sequence current control and constant active power control can also limit the relative error of 2nd harmonic component of active power supplied by SMES effectively.

Conceptual Design of the Cryogenic System for a 12 MW Superconducting Wind Turbine Generator

A direct drive 12-MW superconducting wind turbine generator has been designed. The rotor of the generator is made of low temperature superconductor (LTS) coils, which are wound from NbTi/Cu cable-in-conduit conductors (CICC) cooled with forced flow supercritical helium at 4 K. To ensure the stability of the generator rotor, it is necessary to design a reliable cryogenic system. With consideration of the radiation heat loads, ac loss, conduction heat leak by residual gas, current leads and torque tube, the heat load of the cryogenic system is analyzed. Aiming at decreasing the heat load of the cryogenic system and enhancing the overall efficiency of the generator, new structures of torque tube and thermal shields are proposed, and the main parameters of the cryogenic system are determined.

Recent Main Events in Applied Superconductivity in China

With strong support from Chinese government bodies such as the National High Technology Research and Development Program of China, National Natural Science Foundation of China and the National Basic Research Program of China, the field of applied superconductivity in China has been developed in different areas. Especially in the fusion application area, the first fully-superconducting tokamak, EAST (Experimental Advanced Superconducting Tokamak), has been successfully constructed and commissioned in the last two years. Based on the requirement from the ITER project, high performance Bi-2223 HTS tape, and Nb3Sn and NbTi strands have been developed as well. In the area of HTS applications for electric power systems, R&D has been focused on superconducting power cable, superconducting magnetic energy storage, superconducting transformer, superconducting fault current limiters. In this paper, the recent progress in applied superconductivity in China is reported.

Reducing the Fault Current and Overvoltage in a Distribution System With Distributed Generation Units Through an Active Type SFCL

For a power distribution system with distributed generation (DG) units, its fault current and induced overvoltage under abnormal conditions should be taken into account seriously. In consideration that applying superconducting fault current limiter (SFCL) may be a feasible solution, in this paper, the effects of a voltage compensation type active SFCL on them are studied through theoretical derivation and simulation. The active SFCL is composed of an air-core superconducting transformer and a PWM converter. The magnetic field in the air-core can be controlled by adjusting the converters output current, and then the active SFCLs equivalent impedance can be regulated for current-limitation and possible overvoltage suppression. During the study process, in view of the changes in the locations of the DG units connected to the system, the DG units injection capacities and the fault positions, the active SFCLs current-limiting and overvoltage-suppressing characteristics are both simulated in MATLAB. The simulation results show that the active SFCL can play an obvious role in restraining the fault current and overvoltage, and it can contribute to avoiding damage on the relevant distribution equipment and improve the systems safety and reliability.

Application of Small-Sized SMES in an EV Charging Station With DC Bus and PV System

As small-sized superconducting magnetic energy storage (SMES) system is commercially available at present, the function and effect of a small-sized SMES in an EV charging station including photovoltaic (PV) generation system is studied in this paper, which provides a practical application of small-sized SMES. The comparison of three quick response energy storage systems including flywheel, capacitor (super-capacitor) and SMES is also presented to clarify the features of SMES. SMES, PV generation system, and EV battery are connected to a common dc bus with corresponding converters respectively. Voltage source converter (VSC) is used for grid-connection. With characteristic of quick power response, SMES is utilized to maintain the dc bus steady. During the long-term operation of EV charging station, an energy management strategy is designed to control the energy transfer among PV units, SMES, EV battery, and power grid. The EV charging station system is modeled in MATLAB/SIMULINK and simulation tests are carried out to verify the function and performance of SMES.

Techno-Economic Evaluation of a Novel Flux-Coupling Type Superconducting Fault Current Limiter

Superconducting fault current limiters (SFCLs) offer a solution to control and suppress fault current levels on utility distribution and transmission networks, and are believed to be one of the most exciting applications of superconductors in power systems. Consequently, the various types of SFCLs have been developed in many countries. But, there is a very long way for SFCLs from prototypes or paper concepts to the practical applications and commercialization because of many unsolved problems. In this paper, a novel flux-coupling type SFCL is presented. This SFCL is composed of two superconducting windings and a controllable switch. In order to validate its fault current limiting performance, a simulation for the operation of this kind of SFCL in a modeling power system has been done. The simulation results show that it is feasible technically. Based on that, the economic feasibility by introducing a flux-coupling SFCL into the high voltage grid is estimated. Finally, the optimal locations for SFCLs are proposed.

100 kJ/50 kW HTS SMES for Micro-Grid

A 150 kJ/50 kW conduction-cooled HTS SMES have been developed. This paper presents the design and configuration of the SMES and parts of important test results. The magnet of the SMES is a solenoid type coil made of Bi2223/Ag tapes, and cooled to about 20 K by conduction-cooled method. To evaluate the performance of the SMES, a series of tests are successfully carried out in an electric power system dynamic simulation laboratory. The current carrying ability of the magnet, the active and reactive power exchange capability between the SMES and power system, and the power compensating effect of the SMES are experimentally investigated and some test results are given. The designed SMES is key equipment for the micro-grid demonstration garden of Yunnan electric power grid of China, and will be installed for field tests.

Design of a 10 MJ HTS Superconducting Magnetic Energy Storage Magnet

This paper outlines a systematic procedure for the design of a toroidal magnet for Superconducting Magnetic Energy Storage System and presents the optimum design for a 10 MJ class high temperature superconductor (HTS) magnet. The main magnetic component which influences the maximum critical current was investigated. Stray field and operating current needed in a toroidal magnet with different number of elemental coils were compared too. Based on above analysis, both electromagnetic and mechanical design objects were considered when optimizing the volume of HTS Superconducting magnet with optimal number of toroidal elements with Finite Element Method. Parameter surveys of the magnetic fields and electromagnetic stress applied to the HTS magnets were carried out and a suitable magnet dimension with low inductance and high current was studied. The results verify that the double pancake type is more favorable for actual design and operation.

Design of a Superconducting Synchronous Generator With LTS Field Windings for 12 MW Offshore Direct-Drive Wind Turbines

Offshore wind generation is an important trend of global wind power generation. To reduce the cost of energy of offshore wind generation, high-power direct-drive wind turbines are being pursued around the world. With the development of superconductors and related technologies, superconducting (SC) generator offers a possible technological approach to produce large-scale direct-drive wind turbines with a modest cost. A design of a SC wind generator (SCWG) for offshore direct-drive wind turbines is presented in this paper. The proposed SCWG is an electrically-excited synchronous generator with stationary field windings made of low temperature superconductor wires. The electromagnetic design considerations on SCWG are studied. The description on design of copper rotor and SC stator are presented. Several key design technologies including rotor cooling and supporting structure design, SC field winding design, and cryogenic refrigeration system for SC magnets are analyzed. The weight and cost of the proposed SCWG are estimated and compared with that of a permanent magnet wind generator (PMWG) design and several existing similar SCWG designs. It is found that the proposed SCWG is 46% lighter than the PMWG with the same specifications. Moreover, a rough estimation of material cost of the SCWG is carried out, and comparison analysis of cost is also performed.