15% reduction in AC loss of a 3-phase 1 MVA HTS transformer by exploiting asymmetric conductor critical current

An asymmetric dependence of the critical current on the direction of an applied magnetic field in HTS coated conductors has a non-trivial influence on the AC loss of coil windings. We report the modelled influence of real conductor critical current asymmetry on the AC loss characteristics of a 1 MVA HTS transformer design previously demonstrated by the Robinson Research Institute as well as a stand-alone coil having the same geometrical and electrical parameters as the low voltage (high current) winding of the transformer. We compare two commercial HTS conductors with distinctive differences in their critical current asymmetry and show a maximum variation of 15% and 29% in the calculated AC loss of the transformer and the stand-alone coil winding, respectively, when the conductor orientation is varied in the top and bottom halves of the windings. AC loss simulation giving consideration to asymmetric conductor critical current before winding the transformer could lead to substantial AC loss reduction even using the same amount of conductor and the same transformer design

Optimizing coil configurations for AC loss reduction in REBCO HTS fast-ramping magnets at cryogenic temperatures

AC loss is one of the critical issues for designing REBCO fast-ramping magnets operating at cryogenic temperatures. There are many ways to reduce AC loss for coil windings. However, it is not clear which method is the most effective way to minimize AC loss in the coil windings for a given Ampere-turns. In this work, we numerically studied coil configurations of several small superconducting magnets constructed from 12 mm SuperPower REBCO coated conductors, for fast-ramping application with the same Ampere-turns to identify the lowest AC loss among them. The HTS magnets have a total turn number of 50 and inner diameter of 30 cm, carrying AC current operating in the temperature range of 20 K – 40 K at 25 Hz. We incorporated several existing loss reduction strategies including spacing between the turns for single pancake coils, grading Ic values for the solenoid configuration, and applying flux diverters to shape the magnetic field around the coil windings. The simulation was implemented using a homogenized H-formulation. Across all studied loss reduction methods, the use of flux diverters has the largest impact in AC loss reduction. The AC loss values in the solenoid winding comprising a stack of five single pancake coils with 0.1 mm turn-to-turn gap with the flux diverters agree well with those in the single pancake coil for 2 mm turn-to-turn gap with the flux diverters. Solenoid type coil configurations with flux diverters generate much smaller AC loss than the single pancake type with flux diverters when they generate the same center magnetic field

AC loss in REBCO coil windings wound with various cables: effect of current distribution among the cable strands

To construct large-scale superconducting devices, it is critical to enhance the current carrying capability of superconducting coils. One practical approach is to utilise assembled cables, composed of multiple strands, for the winding. There have only been a few investigations of the dependence of current distribution among the strands on the AC losses of the cables and coils wound with these cables. In this work, we studied three types of cables; i) 8/2 (eight 2 mm-wide strands) Roebel cable, 8/2 Roebel; ii) two parallel stacks (TPS) which have the same geometrical dimensions as the Roebel cable, 8/2 TPS; and iii) an equivalent four-conductor stack (ES) comprising four 4 mm-wide conductors, 4/4 ES. We proposed a new numerical approach that can achieve equal current sharing and free current sharing among the strands. We examined the loss behaviour of all three types of straight cables and two coil assemblies comprising two, and eight, stacks of double pancakes (DPCs) wound with these cables, respectively. 2D FEM analysis was carried out in COMSOL Multiphysics using the H - formulation. The stacks were modelled in parallel connection, with the same electric field applied to all strands, so that current is distributed between the conductors. Simulated transport AC loss results in the straight 8/2 Roebel cable, 8/2 TPS and 4/4 ES were compared with previously measured results as well as each other. The numerical AC loss results in two coil windings 2-DPC and 8-DPC wound with the 8/2 Roebel cables were compared with the results in coil windings wound with the 8/2 TPS and 4/4 ES. No transposition was introduced at the connection between double pancakes, in order that current can be shared among the strands in the 8/2 TPS and 4/4 ES. The results indicate that AC loss in the straight 8/2 TPS and 4/4 ES is larger than that in the 8/2 Roebel cable. Current is more concentrated in the outer strands for the straight 8/2 TPS and 4/4 ES than the 8/2 Roebel cable and causes greater AC loss than the 8/2 Roebel cable. The 8-DPC coil winding wound with the 8/2 Roebel cable has the smallest loss and the coil winding wound with the 8/2 TPS has the greatest loss at two current amplitudes investigated. At 113 A, the AC loss value in the 8-DPC coil winding wound with the 8/2 TPS is 2.2 times of that wound with the 8/2 Roebel cable

Response Time of a Fiber Bragg Grating Based Hydrogen Sensor for Transformer Monitoring

We developed and optimized a new fiber optic sensor using palladium foils attached to optical fiber Bragg gratings (FBG) for hydrogen measurements. Fifteen in parallel processed sensors were characterized and qualified in two custom tailored experimental set ups and their response to a 5% hydrogen/nitrogen gas mixture and the same gas bubbled trough transformer oil was measured. The hydrogen response is similar for both medium and close to the theoretical maximum sensitivity, but the response time was found to be very different, much slower in oil than in gas. A theoretical comparison of hydrogen diffusion trough palladium and hydrogen absorption on the palladium surface as well as a measurement of the hydrogen uptake and diffusion trough the oil to the sensor have been done to investigate the origin of the different response time. They indicate that the response time determining step is the absorption of hydrogen on the palladium surface and that this process is slowed down in oil compared to a pure gas environment

Design, Build, and Evaluation of an AC Loss Measurement Rig for High-Speed Superconducting Bearings

Friction and heat generated in conventional bearings impose a limit on maximum design speed in electrical machines. Superconducting bearings offer the potential for low loss, simplified, and passively stable bearings that can overcome the speed limit and operate at high loads. Although such bearings are contactless and seem to be loss free, AC loss mainly caused by magnetic field inhomogeneity gradually slows down the rotating body. This loss, whose mechanism has not been fully explored, is measured through spin-down tests where the rotational speed of the spinning rotor is measured as a function of time. However, there are some challenges in performing a reliable spin-down test. In this paper, we discuss these challenges as well as the engineering of an experimental test rig that enables us to spin-up, release, and recapture the levitated permanent magnet. We also discuss the specifications of the driving mechanism including the self-aligning coupling, which accommodates permanent magnets of different sizes. Initial test results at 6600 rpm are discussed and further technical improvements to the test rig suggested. This rig will be used as a key tool to explore the AC loss mechanism and inform the design of bearings for high-speed superconducting machines

Design, Build, and Evaluation of an AC Loss Measurement Rig for High-Speed Superconducting Bearings

Friction and heat generated in conventional bearings impose a limit on maximum design speed in electrical machines. Superconducting bearings offer the potential for low loss, simplified, and passively stable bearings that can overcome the speed limit and operate at high loads. Although such bearings are contactless and seem to be loss free, AC loss mainly caused by magnetic field inhomogeneity gradually slows down the rotating body. This loss, whose mechanism has not been fully explored, is measured through spin-down tests where the rotational speed of the spinning rotor is measured as a function of time. However, there are some challenges in performing a reliable spin-down test. In this paper, we discuss these challenges as well as the engineering of an experimental test rig that enables us to spin-up, release, and recapture the levitated permanent magnet. We also discuss the specifications of the driving mechanism including the self-aligning coupling, which accommodates permanent magnets of different sizes. Initial test results at 6600 rpm are discussed and further technical improvements to the test rig suggested. This rig will be used as a key tool to explore the AC loss mechanism and inform the design of bearings for high-speed superconducting machines

Monitoring pre-stressed composites using optical fibre sensors

Residual stresses in fibre reinforced composites can give rise to a number of undesired effects such as loss of dimensional stability and premature fracture. Hence, there is significant merit in developing processing techniques to mitigate the development of residual stresses. However, tracking and quantifying the development of these fabrication-induced stresses in real-time using conventional non-destructive techniques is not straightforward. This article reports on the design and evaluation of a technique for manufacturing pre-stressed composite panels from unidirectional E-glass/epoxy prepregs. Here, the magnitude of the applied pre-stress was monitored using an integrated load-cell. The pre-stressing rig was based on a flat-bed design which enabled autoclave-based processing. A method was developed to end-tab the laminated prepregs prior to pre-stressing. The development of process-induced residual strain was monitored in-situ using embedded optical fibre sensors. Surface-mounted electrical resistance strain gauges were used to measure the strain when the composite was unloaded from the pre-stressing rig at room temperature. Four pre-stress levels were applied prior to processing the laminated preforms in an autoclave. The results showed that the application of a pre-stress of 108 MPa to a unidirectional [0]16 E-glass/913 epoxy preform, reduced the residual strain in the composite from −600 µε (conventional processing without pre-stress) to approximately zero. A good correlation was observed between the data obtained from the surface-mounted electrical resistance strain gauge and the embedded optical fibre sensors. In addition to “neutralising” the residual stresses, superior axial orientation of the reinforcement can be obtained from pre-stressed composites. A subsequent publication will highlight the consequences of pres-stressing on fibre alignment, the tensile, flexural, compressive and fatigue performance of unidirectional E-glass composites

Design Improvisation for Reduced Harmonic Distortion in a Flux Pump-Integrated HTS Generator

This paper presents a design improvisation of a flux pump-integrated 10 kW high-temperature superconducting (HTS) proof of concept generator for reduced harmonic distortion. To carry out the design improvisation, a finite element analysis (FEA) model of the 10 kW HTS generator is developed, and time-stepped magnetic transient simulations are conducted on the 2D model. The effects of stator yoke material, winding pitch factors, and load configurations on total harmonic distortion (THD) are investigated. The results showed that fibre-reinforced polymer (FRP) epoxy (G10) can be used as the stator yoke material to effectively avoid the hysteresis and eddy current losses. In addition, the study results show that for the non-conventional design of the machine, a winding pitch of 2/3 and the armature-load configuration of Star-Delta gives THD values within the standard limit defined by IEEE Standard 519-2014. The THD values indicate that the machine design configuration is suitable for the development of machines for both stand-alone and grid-connected operations, according to IEEE STD 519-2014