Rodney A. Badcock

Progress in the Manufacture of Long Length YBCO Roebel Cables

We have manufactured lengths of up to 10 m of Roebel cable from wide strips of second generation YBCO wire. The wide strips are first screened using magnetic scanning to check for superconductor defects to avoid incorporating poor quality wire into the cable. Manufacturing issues such as the long length registration of the strand crossovers and the maximum yield from wide strip is discussed. Differences in cutting and winding due to different tape substrate materials are shown to be minimal. We report electrical measurements on samples of HTS Roebel cable. Insulation which protects the assembled conductors from environmental degradation is reported.

Development of a brushless HTS exciter for a 10 kW HTS synchronous generator

HTS synchronous generators, in which the rotor coils are wound from high-T c superconducting wire, are exciting attention due to their potential to deliver very high torque and power densities. However, injection of the large DC currents required by the HTS rotor coils presents a technical challenge. In this paper we discuss the development of a brushless HTS exciter which operates across the cryostat wall to inject a superconducting DC current into the rotor coil circuit. This approach fundamentally alters the thermal load upon the cryogenic system by removing the need for thermally inefficient normal-conducting current leads. We report results from an experimental laboratory device and show that it operates as a constant voltage source with an effective internal resistance. We then discuss the design of a prototype HTS-PM exciter based on our experimental device, and describe its integration with a demonstration HTS generator. This 200 RPM, 10 kW synchronous generator comprises eight double pancake HTS rotor coils which are operated at 30 K, and are energised to 1.5 T field through the injection of 85 A per pole. We show how this excitation can be achieved using an HTS-PM exciter consisting of 12 stator poles of 12 mm YBCO coated-conductor wire and an external permanent magnet rotor. We demonstrate that such an exciter can excite the rotor windings of this generator without forming a thermal-bridge across the cryostat wall. Finally, we provide estimates of the thermal load imposed by our prototype HTS-PM exciter on the rotor cryostat. We show that duty cycle operation of the device ensures that this heat load can be minimised, and that it is substantially lower than that of equivalently-rated conventional current leads.

Impact of flux gap upon dynamic resistance of a rotating HTS flux pump

HTS flux pumps enable superconducting currents to be directly injected into a magnet coil without the requirement for thermally inefficient current leads. Here, we present results from an experimental mechanically rotating HTS flux pump employing a coated-conductor stator and operated at 77 K. We show the effect of varying the size of the flux gap between the rotor magnets and coated conductor stator from 1 to 7.5 mm. This leads to a corresponding change in the peak applied perpendicular magnetic field at the stator from approximately 350 to 50 mT. We observe that our experimental device ceases to maintain a measurable output at flux gaps above 7.5 mm, which we attribute to the presence of screening currents in the stator wire. We show that our mechanically rotating flux pump is well described by a simple circuit model which enables the output performance to be described using two simple parameters, the open-circuit voltage V oc and the internal resistance, R d. Both of these parameters are found to be directly proportional to magnet-crossing frequency and decrease with increasing flux gap. We show that the trend in R d can be understood by considering the dynamic resistance experienced at the stator due to the oscillating amplitude of the applied rotor field. We adopt a literature model for the dynamic resistance within our coated-conductor stator and show that this gives good agreement with the experimentally measured internal resistance of our flux pump.

Anomalous open-circuit voltage from a high-Tc superconducting dynamo

We report on the behavior of a high-Tcsuperconducting(HTS) homopolar dynamo which outputs a DC open-circuit voltage when the stator is in the superconducting state, but behaves as a conventional AC alternator when the stator is in the normal state. We observe that this time-averaged DC voltage arises from a change in the shape of the AC voltage waveform that is obtained from a normal conducting stator. The measured DC voltage is proportional to frequency, and decreases with increasing flux gap between the rotor magnet and the HTS stator wire. We observe that the DC output voltage decreases to zero at large flux gaps, although small differences between the normal-conducting and superconducting waveforms are still observed, which we attribute to screening currents in the HTS stator wire. Importantly, the normalised pulse shape is found to be a function of the rotor position angle only. Based on these observations, we suggest that the origin of this unexpected DC effect can be explained by a model first proposed by Giaever, which considers the impact of time-varying circulating eddy currents within the HTS stator wire. Such circulating currents form a superconducting shunt path which “short-circuits” the high field region directly beneath the rotor magnet, at those points in the cycle when the rotor magnet partially overlaps the superconducting stator wire. This reduces the output voltage from the device during these periods of the rotor cycle, leading to partial rectification of the output voltage waveform and hence the emergence of a time-averaged DC voltage.

Through-Wall Excitation of a Magnet Coil by an External-Rotor HTS Flux Pump

High-temperature superconducting (HTS) magnet systems conventionally require normal-conducting current leads, which connect between the HTS circuit and an external power supply located at room temperature. These current leads form a thermal bridge across the cryostat wall, and they represent the dominant heat load for many magnet applications. The use of a superconducting flux pump device is an alternative approach to exciting a magnet coil, which can eradicate this parasitic heat load, as such devices do not require direct physical connection to the HTS circuit. However, earlier proposed flux pump designs have required power-dissipating active components to be located within the cryogenic envelope, thus imposing their own parasitic heat load. Here, we report the successful demonstration of a mechanically rotating HTS flux pump, which operates entirely outside of the cryogenic envelope. This prototype device projects flux across a cryostat wall, leading to the injection of a direct current into a thermally isolated closed HTS circuit. This is achieved through the implementation of a flux-concentrating magnetic circuit employing ferromagnetic yoke pieces, which enables flux penetration of the HTS circuit at large flux gaps. We have demonstrated the injection of direct currents of > 30 A into a closed HTS circuit while operating this device across a cryostat wall.

Design and Heat Load Analysis of a 12 MW HTS Wind Power Generator Module Employing a Brushless HTS Exciter

Brushless high-temperature superconducting (HTS) flux pump exciters, which enable large currents to be injected into a superconducting circuit without requiring a power supply, slip ring, and current leads, are promising candidates for HTS rotating machine application. This paper outlines the design and heat load analysis of a 12-MW HTS wind power generator module employing a brushless HTS exciter. The 12-MW HTS generator module and the HTS exciter were simulated using the 3-D finite element method. The module design of the generator was focused on reducing the heat load and inductance per rotor pole for application of an HTS exciter. A highly permeable ferromagnetic material was used to increase the magnetic flux density incident on the HTS stator wire of the exciter, even with a large radial gap between the rotor and the stator, and hence increase the injected current. Based on the electromagnetic simulations, the design of the module was confirmed, and the iron loss of the exciter was calculated. Then, the conduction and radiation heat loads were simulated. The induced dc current value and ramping time of the DC current at the HTS stator wire of the exciter were calculated. The detailed results of the module with the HTS exciter were discussed, and the results obtained in this paper are useful in designing large-scale HTS generators.

The scaling of transport AC losses in Roebel cables with varying strand parameters

A Roebel cable is a good candidate for low-voltage windings in a high-temperature superconductor (HTS) transformer because of its high current-carrying capability and low AC loss. Transport AC loss measurements were carried out in 1.8 m long 15/5 (fifteen 5 mm wide strands) and 15/4 Roebel cables. The results were compared with those in many Roebel cables composed of 2 mm wide Roebel strands. Comparison of the AC losses hinted that the intrinsic difference in normalized transport AC losses is due to differences in the g/w (ratio of the horizontal gap between the Roebel strands over the Roebel strand width) values. The intrinsic difference was confirmed by measuring transport AC loss in a series of horizontally arranged parallel conductor pairs with various g values. A method to scale transport AC losses in Roebel cables with varying strand parameters was developed. The scaling method will be useful for a rough assessment of AC loss in one-layer solenoid winding coils, such as in a HTS transformer.

A Novel Rotating HTS Flux Pump Incorporating a Ferromagnetic Circuit

High-temperature superconductor (HTS) flux pumps enable large currents to be injected into a superconducting coil without requiring normal-conducting current leads. We present results from an experimental axial-type HTS rotating flux pump that employs a ferromagnetic circuit to focus incident flux upon a coated-conductor stator wire. We show that this device can inject currents of > 50 A into an HTS coil at 77 K and is capable of operating at flux gaps greater than 18 mm. Accommodating a cryostat wall within this flux gap will enable future flux pump designs, in which all moving parts are located outside the cryostat.

Design of a 12-MW HTS Wind Power Generator Including a Flux Pump Exciter

A flux pump (FP) exciter injects dc current into the high-temperature superconducting (HTS) field coils of an HTS rotating machine without a slip ring and current leads. When designing a large-scale HTS generator with integrated FP exciter, the coil inductance, field current, and time constant need to be optimized for better performance of the machine. In this paper, a 12-MW HTS wind power generator with integrated FP exciter was designed. The essential parameters of a 12-MW HTS generator were optimized using the Taguchi method, targeting the minimization of weight and volume of the generator, the length of HTS wire, and the inductance. In particular, the FP exciter was adopted for supplying dc current to the HTS field coils without the power supply and the slip ring. The magnetic field distribution was analyzed using the 3-D finite-element method. The induced dc current and charging and discharging times of the FP exciter were compared with the metal current leads, for confirmation of the effectiveness of the FP exciter. The detailed results of the HTS generator design were discussed in detail.

Impact of S tator Wire Width on Output of a Dynamo-Type HTS Flux Pump

Superconducting flux pumps enable dc supercurrents to be injected into a superconducting coil without direct connection to an external current supply. Here, we report experimental data from a dynamo-type flux pump employing a high-Tc superconducting stator. This device employs a mechanical rotor that rotates outside the cryogenic envelope, and excites the superconducting circuit through the cryostat wall via a magnetic circuit formed between the rotor and stator. We show that the width of the stator wire employed in this device has a significant effect on its output performance. At low frequencies, both the short circuit current, Isc, and the open-circuit voltage, Voc increase with stator width, and we obtain a maximum value of Isc > 340 A using a stator wire width of 46 mm. However at higher operating frequencies, we observe that Isc reaches a maximum value before dropping with any further increase in stator width. We attribute this to thermally dissipative effects due to circulating eddy currents in these wide superconducting stators. At frequencies above ~400 Hz, we observe a sharp rise in the internal resistance of the stator wire that we attribute to a local quench due to the eddy currents driven by the local electromotive force (EMF) beneath the rotor magnet. This paper emphasizes the importance of frequency dependence when defining optimum operating parameters of a high-Tc superconductor (HTS) flux pump. Our results also offer the promise that very high current flux pumps may be developed through maximizing the ratio of the stator wire width to the peak width of the rotor field profile.