A Study on Improving Electrical Characteristics of 2G HTS Field Coils by Metal Insulation Winding Technology for 10 MW Class Wind Generator

Abstract

Second generation (2G) high temperature superconducting (HTS) coils have recently attracted increasing attention of many researchers in developing high performance superconducting application because they possess high current density under high magnetic field and high stability margin compared with their low temperature counterparts. This technology could be considered as an effective solution to develop large–scale offshore wind turbine generators. The 2G HTS magnets for wind turbine generators, in form of racetrack type coils, can achieve higher magnetic field than permanent or resistive magnets. However, protection of the 2G HTS coil is well known to be one of the challenging problems for reliability of operation of HTS magnet applications because of the low normal zone propagation velocities that is incurred by the low index numbers and large specific heat. The no–insulation (NI) winding technique has been introduced to develop 2G HTS magnets with significantly enhanced thermal stability resulting from self–protection characteristic. The main idea behind this technique is based on the elimination of the turn–to–turn insulation layers in the HTS coil. During quenching, excessive current can be automatically diverted through neighboring turns; thus, a 2G HTS magnet can remain stable at a higher current density. However, the decay time constant can be a major challenge for the NI coil due to the bypass current phenomenon, which is caused by the absence of insulation resistance. The bypass current flows through the turn–to–turn contact under time–varying condition, leading to a slow charging delay time (τd) of the NI coil. As a result, the application of the NI winding technique is limited to devices that require fast electromagnetic responses, such as superconducting rotating machines and superconducting magnetic energy storage systems. Recently, the metal insulation (MI) winding technique, which employs metal insulation between the turn–to–turn layers, has been suggested as a promising solution to overcome the drawback in slow τd of NI winding technique. Several research groups have been studied MI coil in the steady and transient states to investigate the electromagnetic response and thermal stability, respectively. The results demonstrated that the MI coil achieved both fast τd under time–varying condition and high thermal stability during the overcurrent test. However, these studies have been performed mostly in pancake type coils. Therefore, to utilize the MI winding technique for the field coils of superconducting rotating machines, it is necessary to estimate the τd as well as thermal stability behaviors of the MI winding technique which is wound in form of racetrack type coils. In this dissertation, the electrical characteristics of MI–SS racetrack coil under the steady and transient states were estimated in both experiment and simulation. In addition, the effects of stainless steel thickness, winding tension, cooling approach, and rotating magnetic field on the τd and thermal stability were also investigated. The results of this dissertation can be used as a technical reference for development the potential 2G HTS field coils, i.e., enhancing τd as well as thermal stability, of a 10 MW class HTS generator used in offshore wind turbine environment. First, an electrical equivalent circuit model was proposed to numerically analyze the characteristics of the NI 2G HTS coil in steady and transient states. A steady state was performed under rated operating current shooting to estimate the magnetic field response performance of the NI HTS coil. The transient state was conducted under an overcurrent shooting to analyze the thermal quench behavior. The simulation results of the NI HTS coil were discussed and compared with the experimental ones to validate the simulation approach based on the proposed equivalent circuit model. Then, the NI and MI–SS racetrack coils were performed in both experiment and simulation to estimate the magnetic field response in the steady state and thermal stability in the transient state. The results demonstrated that the MI–SS racetrack coil can be charged considerably faster than that of NI racetrack coil. During the overcurrent, the MI–SS racetrack coil operated successfully to protect the test coil from burn–out because a portion of excessive current can automatically diverted through the stainless steel tape layers. The stainless steel thickness and winding tension are identified as the factors affecting the τd and thermal stability of MI–SS coils. To verify these characteristics, three types of MI–SS racetrack coils were fabricated with various SS thicknesses and winding tensions: 100–μm SS thickness and 10–kgf winding tension; 100–μm SS thickness and 5–kgf winding tension; and 50–μm SS thickness and 10–kgf winding tension. Three MI–SS racetrack coils were characterized by sudden discharging, charging, and overcurrent tests. The sudden discharging and charging tests were performed in a steady state to investigate the decay time constant of the center magnetic field discharging the coils and the delay time of the magnetic field charging the test coils, respectively. To estimate the thermal stability of the three MI–SS coils, overcurrent tests were conducted in a transient state. In addition, the MI–SS racetrack coil was tested under various cryogenic cooling conditions and rotating magnetic fields to analyze the electrical and thermal characteristics because the 2G HTS field coils can be exposed to rotating magnetic fields in practical wind generator application which caused by unsynchronized armature windings during electrical or mechanical load fluctuations. The cooling and vacuum tests were conducted to setup operating temperature for the conduction cooling system at 35 and 77 K. Then, the charging and discharging tests were carried out to investigate the effect of cooling approaches, i.e, LN2 bath and conduction cooling conditions, to the characteristic resistance and magnetic field response performance. To evaluate the effect of external magnetic field on Ic and thermal stability characteristics of the MI–SS racetrack coil, the transient test was performed under rotating magnetic field, which was generated by armature windings of 75 kW class induction motor, and conduction cooling system of 35 K. Furthermore, based on the experimental results of small–scale MI–SS racetrack coils, the total contact resistance between turn–to–turn layers of the test coils was calculated and applied to design the field windings using 2G HTS tape for the 10 MW class HTS generator used in offshore wind turbine environment. Subsequently, a proposed equivalent circuit model was utilized to analyze the electromagnetic response and thermal stability characteristics of MI–SS 2G HTS field coils for the 10 MW class HTS generator. Finally, the τd and thermal stability of these MI–SS 2G HTS field coils were compared and analyzed in detail.