HTS Synchronous Generator Research Articles

AC Loss Estimation on HTS Armature Windings for a 30-kW Marine Generator Prototype

In this study, we propose a numerical method to estimate the AC loss of a 30-kW HTS synchronous generator during rated operation. As HTS armature windings are installed around the stator core in a rotating machine, it is a troublesome task to calculate the AC loss accurately in such a time-varying electromagnetic field. Additionally, the nonlinear E-J characteristics and high aspect ratio of superconducting tapes make the total work more difficult and time-consuming. To address these problems, we apply appropriate boundary conditions to connect two submodels, one for the rotating machine and the other for superconducting materials, to calculate the magnetic field and actual current distributions on the HTS armature windings. In addition, the AC loss per turn of an HTS armature winding is simulated for comparison, and it should be noted that the size of the boundary has an influence on the AC loss estimation. The closer the boundary is to the superconducting material, the more reliable the result. AC loss estimation on HTS armature windings in a machine is an inevitable and meaningful issue during a preliminary design. This study could offer an indispensable reference for this issue.

Comparison of Electromagnetic Performance of 10-MW HTS Double-Stator Flux Modulation Wind Generators With Different Topologies

For conventional HTS synchronous generator, the superconducting (SC) field windings or armature windings are rotating, which could complicate their industrial feasibility for the offshore wind generation. In order to overcome this drawback, a kind of HTS double-stator flux modulation generator (HTS-DSFMG) without rotating windings is proposed. This paper compares the electromagnetic performance of 10-MW HTS double-stator flux modulation generators with two different topologies, i.e., iron-cored stators with both field and armature coils, air-cored stator with field windings and iron-cored stator with armature windings. In addition, the system comparison of HTS-DSFMG with conventional HTS synchronous machine is conducted under the no-load, rated load and short-circuit fault conditions. It is found that the proposed machine can dramatically reduce the peak current and peak torque during a symmetrical three-phase short-circuit fault, and its active material mass and cost are comparable with the synchronous one under rated load condition. Finally, the ac losses analysis of the SC coils for HTS-DSFMGs is conducted and three effective methods are proposed to reduce the ac losses.

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-Tc 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.

Comparison of Theoretical and Numerical Electromagnetic Modeling for HTS Synchronous Generator

Comparison of Theoretical and Numerical Electromagnetic Modeling for HTS Synchronous Generator

Loss measurement and analysis for the prototype generator with HTS stator and permanent magnet rotor

A prototype HTS synchronous generator with a permanent magnet rotor and HTS armature windings was developed. The rated armature frequency is 10Hz. The cryogenic Dewar is tightly surrounded outside the iron core. Both HTS coils and the iron core were cooled by using conduction cooling method. During the process of no-load running, the no-load loss power data were obtained through the torque measurement. The temperature evolution characteristics of the stator was measured by PT-100 temperature sensors. These results show that the no-load loss power at around 77K are much larger than that at room temperature. The possible reason for the no-load loss increment is discussed. The ac loss power of one individual HTS coil used in this generator was also tested. Compared with the iron loss power, the ac loss power is rather small and could be neglected.

Further Testing of an “Iron-Cored” HTS Synchronous Generator Cooled by Liquid Air

The systematic performance test program of the 100 kW iron-core HTS synchronous generator has been underway since 2007 at the University of Southampton. The early measurements were conducted at liquid nitrogen temperatures (77 K) and sub-cooled liquid nitrogen close to its triple point (63 K). Further measurements at these operating temperatures for both open circuit and loaded configurations have enabled us to confirm the machine's synchronous reactance. Circulating liquid air through the closed loop cooling system enabled us to lower the rotor temperature down to 61.7 K and boost the critical current I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> of the HTS winding up to 144 A. A number of transient tests were carried out to assess the impact on the machine, in particular the current in the HTS field winding when the stator circuit was suddenly shorted. The results show that the peak current can be almost 3 times the initial operating field current in the first cycle of the response. No `thermal quench' of HTS winding was witnessed, despite the occurrence of large over currents. The recovery of the HTS winding back to the initial operating current occurred in several seconds and depended on the initial operating current and the reaction of the power supply.

Testing of a Lightweight Coreless HTS Synchronous Generator Cooled by Subcooled Liquid Nitrogen

A lightweight `coreless' (100 kW), 2-pole HTS rotor, designed and built at the University of Southampton has been completed and is now in the testing phase of the project. By removing the iron core from the new cold rotor, the weight has been halved in comparison to the previous rotor. The `cold' components of the rotor assembly make up a sixth of the total weight, which has provided significant time and cost savings in the cool down process. A lightweight doughnut shaped cryostat houses the HTS winding. The winding itself is constructed from sixteen double pancake coils, wound from BSCCO HTS tape. Practical testing of the generator began with a series of stationary tests. The critical current (I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> ) of the HTS winding is 77 A at liquid nitrogen temperature (77.4 K) and increases to 156 A, when cooled with liquid air (61.6 K). The air-gap flux density is ~ 0.3 T at 156 A. The machines total harmonic distortion (THD) was predicted to be around 5.4% with the measured THD=5% and this despite using an unoptimized stator. Finally, the synchronous reactance, <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">X</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">s</sub> was evaluated to be 0.24 p.u. This is a 1/5 of the value found for conventional synchronous machines and is largely due to its air-core structure.

Performance Test of a 100 kW HTS Generator Operating at 67 K–77 K

A systematic test program is in progress to fully characterize a 100 kW HTS synchronous generator which was successfully constructed in 2004. The machine was one of the first HTS synchronous generator/motors to operate at liquid nitrogen temperatures while achieving a power rating relevant to practical application. It has a conventional 3-phase stator and a cold rotor with a magnetic core and a superconducting winding consisting of 10 HTS Bi2223 pancake coils separated by magnetic flux diverters. The test program includes a series of tests at various speeds, field currents and temperatures (65 K-77 K) with the machine in open circuit to determine the critical currents of the HTS rotor, the waveform and harmonic characteristics of generated voltage at different levels of iron saturation. Stationary measurements of the rotor critical current are carried out using dc current in the stator windings to quantify the influence of stator field on the performance of the superconducting winding. The voltages and temperatures of the rotor are measured using a radio frequency telemetry system.

The Design of a Lightweight HTS Synchronous Generator Cooled by Subcooled Liquid Nitrogen

A final design of a dasiacorelesspsila 100 kW HTS synchronous generator, to be built at the University of Southampton is under way. The new generator will use the same conventional 2-pole, 3 phase stator used by for the dasiairon coredpsila generator. The new HTS rotor has no central core and the rotor winding is built with 15 double pancake coils. Each coil has 38 turns of BiPb2223 superconducting tape, with nominal current of 180 A at 77 K. In this design, only two flux diverters are used at the top and bottom of the winding to help shape the magnetic field. The coils are located on the inner wall of the cryostat. The construction of the cryostat poses our greatest mechanical challenge. Although we have avoided problems with differential thermal contractions by making the cryostat entirely from stainless steel, the cryostat requires considerable reinforcements. Additional supports and stiffening ribs must be welded to the thin-walled structure. Distortion/rippling during welding must be eliminated to ensure the final welds to the ring flanges can be made. The pole-pieces, which help to improve the waveform characteristics of the machine, are located in the warm space between the cold flange of the cryostat and the inner wall of the vacuum vessel. Unfortunately, the overall magnetic performance has been affected by the parallel stacking of the coils. Some suggestions for altering the current stator have been presented which show that improvements to the voltage waveforms can be achieved.

Development of a coreless HTS synchronous generator operating at sub-cooled liquid nitrogen temperatures

This paper presents the current design concepts for a 100kW high temperature superconducting synchronous generator currently being designed at the University of Southampton, UK. The new generator will use the same conventional 2-pole 3-phase stator that was used in the HTS synchronous generator previously constructed at Southampton. The windings consist of 18 pancake coils made from BiPb2223 superconducting tape with a nominal current of 180A at 77K, provided by Sumitomo Electric Industries, Ltd. Tests were performed to determine the mechanical strain limits of the tape. The rotor has no central core, but magnetic pole pieces are used to improve the waveform of the generator. The coils are separated by magnetic diverter rings to reduce the normal field in the tape. The coils and diverter rings are supported by fibreglass formers that extend to the centre of the rotor so that they can be located by the through bolts that hold the stack together. A stainless steel tube encapsulates the rotor to provide a chamber for liquid nitrogen to flood the rotor.

Coreless HTS Synchronous Generator Operating at Liquid Nitrogen Temperatures

In this paper we report on our new design of a liquid nitrogen/air cooled 100 KW synchronous generator with core-less rotor. This follows our successful completion of 100 KW generators with a 9 wt% Ni steel core operating at 77 K. In the new design, we demonstrate that a coreless rotor using commercial BSCCO tape is a realistic choice while maintaining the cooling at 57-77 K rather than 25-30 K and still achieving reasonable air-gap flux density. This is made possible by a combination of improved HTS wire technology and a careful optimization of HTS winding and flux diverters. The maximum flux density normal to the broad face of the tape is kept below 0.13 T, even when the air-gap density is raised to 0.5 T. The superconducting winding consists of 22 pancake coils with a total length of around 1250 meters of HTS tape. The required temperatures down to 57 K are achieved by a purpose-designed refrigeration system where liquid cryogen is circulated via a network of well insulated pipes and a stationery-rotating liquid coupling junction.