무절연계 권선기술을 적용한 고온초전도 마그넷의 비동기 회전자계에 대한 전자기적 특성 해석

Abstract

The high-temperature superconducting (HTS) generator for wind turbine has been generally built with the synchronous type. In structure of partially HTS generator (HTSG), copper wire is used to manufacture three-phase armature winding installed in stationary part and generates the alternating current (AC) magnetic field which is circumferentially rotated around the rotor. Moreover, the rare earth barium copper oxide (REBCO) coated conductors are the so-called second generation HTS tapes, which are used at field winding in rotary part and generates the direct current (DC) magnetic field. An HTS field winding is operated inside static magnetic field because it is always synchronized with AC magnetic field generated from armature winding during the steady-state operation of synchronous generator. However, the HTS field winding of synchronous generator can be occasionally operated under the time-varying magnetic field due to the unsynchronized operation of armature winding during electrical or mechanical load fluctuations. Especially, in application for wind turbine, the driving situations in which the rotational frequency between the armature and field windings is asynchronous, may occur because the wind speed is drastically changed in real time. Thus, the HTS field winding becomes unstable state due to being subjected to a time-varying magnetic field. As the transient conditions mentioned above, conventional HTS magnet, which is insulated with the electrically and thermally non-conductive materials between turn to turn, has been suffered from the local quench, resulting in permanent damage. The novel electrical insulation winding techniques, such as no-insulation (NI) and metal insulation (MI) in which an insulation material is completely removed between turns and the electrically and thermally conductive metals are inserted between turns, respectively, were proposed to improve the stability of HTS magnets by solving the critical drawback of conventional insulation winding technique. However, the practical demonstration of NI or MI magnets for the field winding of HTSG and their behaviors under unsynchronized magnetic field, have not yet been conducted and studied on, respectively. Therefore, the transient operational reliabilities of NI and MI magnets should be examined and investigated under unsynchronized operation environments to confirm their technical feasibility on the HTS field winding of the generator for wind turbine. This study presents the results about analysis and experiment on the electromagnetic characteristics of two kinds of REBCO magnet, such as NI magnet and MI magnet co-wound with stainless steel (STS) tape, under rotating magnetic field. By using two-dimensional finite element analytical approach, the magnetic behaviors of two test magnets, i.e., perpendicular magnetic field effect, were numerically analyzed and compared considering synchronous and asynchronous operations of the HTSG. Then, a characteristic evaluation device to apply asynchronous rotating magnetic field was designed and developed. The system is divided into two parts. One is a cryostat part to test the two magnets at 77K in liquid nitrogen batch, the other is three phase armature winding part to generate the rotating magnetic field with controllable parameters. The electromagnetic characteristics of two test magnets, such as critical current, n-value, terminal voltage, and center magnetic field, are experimentally investigated according to the changes in strengths and frequencies of the asynchronous rotating magnetic field. Finally, experimentally comparative results of two test magnets were discussed in terms of the HTS field winding of the superconducting generator for wind turbine.