Hae-Jin Sung

Practical Design of a 10 MW Superconducting Wind Power Generator Considering Weight Issue

Recently, gearless type generators, which have very low rotation speed and high torque, have been preferred over the geared type due to the problem of the gearbox reliability. In particular, a high-temperature superconducting generator is promising for wind power applications because of its advantages of weight, size, and efficiency. In this paper, a 10-MW class superconducting wind power generator is designed using Y-Ba-Cu-O and Bi-Sr-Ca-Cu-O wires, and the weight of the superconducting generator is estimated. The finite elements method is used to analyze the magnetic field distribution, and the 3-D CAD program is used to calculate the weight of the super-conducting synchronous generator. The designed 10-MW class superconducting generator is analyzed and discussed considering the proper weight. The designed 10-MW superconducting generator will be effectively utilized in the construction of the 10-MW class wind power generation system.

Operating Characteristics of an Insulationless HTS Magnet Under the Conduction Cooling Condition

This paper considers the insulationless high-temperature superconducting (HTS) magnet as a new and innovative concept for dc applications due to its benefits as compared to conventional HTS magnets with insulation layers between HTS conductors. The purpose of this paper is to study the operating characteristics of an insulationless HTS magnet. The normal operating characteristics of the insulationless HTS magnet were investigated according to the various current levels. Thermal stability of the insulationless HTS magnet was also analyzed. The critical current of the insulationless HTS magnet was measured at 40 K. The temperature distribution of the insulationless HTS magnet was measured using thermocouples. The experiment results demonstrate superior thermal stability characteristics in the insulationless HTS magnet. Thus, it is confirmed that the thermal performance of large-scale HTS magnets can be enhanced by applying insulationless magnets instead of general insulated HTS magnets.

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.

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.

Stator Winding Fault Influence on the Field Coil of a 10 MW Superconducting Synchronous Generator

It is important to analyze the transient characteristics of large synchronous generators according to the trend of the extension of wind turbines. The fabricated field coil of a synchronous generator using high-temperature superconducting (HTS) wire is somewhat influenced by the external magnetic field of the stator in a real environment. This paper describes the electromagnetic analysis of an HTS field coil of a 10-MW superconducting synchronous generator during stator winding faults. The real-time digital simulator is used to simulate a fault condition of the 10-MW SCSG. Also, the finite-element method is used in each operating condition, including normal and transient conditions, to analyze the influence of the HTS field coil. Based on the simulation results discussed here, there do not appear to be any significant problems regarding the electromagnetic operation of the HTS field coil during transient conditions of the stator. This paper will be used in the analysis of the transient characteristics of a large superconducting synchronous generator during stator winding faults.

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.

A study on the required performance of a 2G HTS wire for HTS wind power generators

YBCO or REBCO coated conductor (2G) materials are developed for their superior performance at high magnetic field and temperature. Power system applications based on high temperature superconducting (HTS) 2G wire technology are attracting attention, including large-scale wind power generators. In particular, to solve problems associated with the foundations and mechanical structure of offshore wind turbines, due to the large diameter and heavy weight of the generator, an HTS generator is suggested as one of the key technologies. Many researchers have tried to develop feasible large-scale HTS wind power generator technologies. In this paper, a study on the required performance of a 2G HTS wire for large-scale wind power generators is discussed. A 12 MW class large-scale wind turbine and an HTS generator are designed using 2G HTS wire. The total length of the 2G HTS wire for the 12 MW HTS generator is estimated, and the essential prerequisites of the 2G HTS wire based generator are described. The magnetic field distributions of a pole module are illustrated, and the mechanical stress and strain of the pole module are analysed. Finally, a reasonable price for 2G HTS wire for commercialization of the HTS generator is suggested, reflecting the results of electromagnetic and mechanical analyses of the generator.

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.