HTS Power Applications Research Articles

Experimental and Numerical Study of Quench Propagation Characteristics of Different Epoxy Impregnated REBCO Coated Conductors

REBCO coated conductors are very promising for various HTS power applications. However, a detailed knowledge about their quench characteristics is crucial for developing effective quench detection methods. This article experimentally and numerically analyzed the quench propagation characteristics of REBCO coated conductors impregnated with different epoxy. The quench parameters, including minimum quench energy (MQE), voltage profiles, and normal zone propagation velocities, obtained from the experimental results are validated by numerical model. The experimental phenomenon can be further described by the numerical model. The analysis shows that higher thermal conductivity of epoxy increase the MQE of the epoxy impregnated conductor, indicating that maintaining a relatively high thermal conductivity of epoxy is helpful to decrease the quench damage of the epoxy impregnated conductor.

Effect of Transverse Compressive Loading on the Quench Characteristics of Epoxy-Impregnated REBCO-Coated Conductor

REBCO-coated conductor is widely used in various HTS power applications. The HTS coils for power applications are generally impregnated with epoxy resin to enhance their mechanical stability, and the coil conductors would be subjected to transverse compressive loading in these applications. Although the effect of various operating conditions on the quench characteristics of REBCO-coated conductor have been investigated, the effects of compressive stress on quench characteristics of epoxy-impregnated REBCO-coated conductor has rarely been reported. This study examined the effects of transverse compressive loading on the minimum quench energy (MQE) and normal zone propagation velocities (NZPV) of epoxy-impregnated REBCO-coated conductor. The critical current of the conductor shows little degradation when transverse compressive stress is applied up to 80 MPa. Under the same compressive stress, the MQE decreases but the NZPV increases with the transport current. For fixed transport current, compressive stress increases the thermal contact conductance between conductor and epoxy which allows the heater energy of the conductor dissipating faster towards the surrounding epoxy, resulting in the increase of MQE and decrease of NZPV when increasing transverse compressive stress. The results indicate that higher compressive stress is helpful to decrease the quench damage of the conductor.

Commissioning of the Cryogenic Phase Equilibria Test Stand CryoPHAEQTS

Medium-sized cryogenic applications such as local hydrogen liquefaction or HTS power applications require temperatures below 77 K at some kW cooling power. These conditions are efficiently reached by cryogenic mixed refrigerant cycles (CMRC). The CMRC development relies on physical property data of cryogenic fluid mixtures that is not available for most binary and multi-component systems. The cryogenic phase equilibria test stand CryoPHAEQTS provides precise physical property data of cryogenic fluid mixtures at temperatures from 15 K to 300 K and at pressures up to 150 bar. It also allows flammable mixtures (e.g. containing hydrogen) or oxidizing mixtures (e.g. containing oxygen). Vapor-liquid equilibria (VLE), vapor-liquid-liquid equilibria (VLLE) and the liquidus line of solid-liquid equilibria (SLE) are obtained by direct sampling from the equilibrium cell and analysis by gas chromatography. The equilibrium cell offers optical access by 4 windows to identify the coexisting phases. The cell temperature is set by a pulse-tube cryocooler and a counter-heater. As a unique feature, the specific heat capacity of the vapor phase in equilibrium is measured by the combination of a novel thermal and a Coriolis type flow meter. Having measured both caloric and thermal properties of the mixtures, we can develop new equations of state that cover all thermodynamic state variables. In this talk, we report on the progress in the construction and commissioning of CryoPHAEQTS. Validation of the measurement accuracy is presented by comparison of pure substances measurements against literature data.

Effect of Non-Uniformity of Electric Field on Breakdown Strength of Cryogenic Gaseous Insulation Media for Superconducting Power Applications

A new experimental set up for measuring the breakdown strength of gas media in weakly non-uniform electric field at cryogenic temperatures and high pressures is described. Measure of breakdown strength of helium gas at 77 K and 293 K in a non-uniform electric field with a field efficiency factor 62.5% are presented. The results suggest that the breakdown strength in weakly non-uniform electric field relate to that of uniform electric field by the field efficiency factor, η. This relationship holds good for both 293 K and 77 K data. This observation expands our previously reported systematics of dielectric strength in uniform electric field to the weakly non-uniform electric field conditions, which is important for designing HTS power applications. The established relationship eliminates the need for costly experiments for measuring the dielectric strength at a specific operating temperature and pressure and in the required non-uniformity of the electric field.

Development of 10 kW turbo-Brayton refrigerator for HTS power applications

HTS power applications have been developed actively and the refrigerator requires larger cooling power. To satisfy this request we have developed 10 kW class turbo-Brayton refrigerator which has neon gas as working fluid. The refrigerator consists of a pair of turbine-compressors which have turbine, compressor and PM motor on the same shaft. The turbine-compressor has magnetic bearings with no rubbing parts, which results in a long maintenance interval. Neon gas is compressed from 0.5 MPa at inlet to 1.0 MPa at outlet by two compressors and expands through two turbines in parallel. Design cooling capacity is 10 kW at a cooling temperature of 65 K. We made a commercial model refrigerator and tested cool down characteristics, load cycles and cooling capacity using a liquid nitrogen circulation system. Specification and configuration of the refrigerator and test results are shown in this paper.

The effects of partial insulation winding on the charge–discharge rate and magnetic field loss phenomena of GdBCO coated conductor coils

GdBCO single-pancake coils wound with Kapton insulation every 3, 6, 9 turns, and without insulation, were characterized by charge–discharge and over-current tests. The magnetic field saturation at higher currents than the critical current was affected by the portion of the insulated turns that restricted the surplus current flow away from the azimuthal current path. The charge–discharge delay decreased with the increase of the portion of the insulated turns, which suggested that partial insulation winding may be effective for use in HTS power applications. Magnetic field loss of the coils with reduced insulation at excessive high currents was mainly affected by the decrease in ampere-turns due to the electrically shorted-circuits between the non-insulated turns, which resulted in the over-current flow bypass through the turn-to-turn contacts.

Partial insulation of GdBCO single pancake coils for protection-free HTS powerapplications

The partial insulation winding was examined to ameliorate the slow charge–dischargeshown by coils wound without insulation. Single pancake coils of GdBCO coated conductorwere wound without insulation, with kapton tape every five turns, and with the full use ofkapton tape. They were characterized by charge–discharge, sudden discharge, andover-current testing. The improved charging and discharging and high thermal andelectrical stabilities of the partially insulated coil demonstrate its potential for use in HTSpower applications.

Current-Lead Design for Variable Electric Current in HTS Power Applications

A systematic design method is developed for current leads in practical HTS power systems, where the electric current varies considerably with time. If the current leads are designed on a basis of projected peak current, the conductors may be thermally safe, but must yield an additional cooling load which could be reduced if the lower current level had been considered. On the other hand, the conductor can be optimally sized to minimize the cooling load if a projected current model is given as function of time. But this design causes an overheating of conductor when the current is excessive. This paper presents a quantitative design process of constrained optimization by considering thermal safety and cooling efficiency at the same time. The conductor dimensions are determined to minimize the cooling load with a constraint that the peak temperature should not exceed a given allowed maximum. Based on Wiedemann-Franz approximation, a simple and useful formula is used to explicitly calculate the peak temperature. A step-by-step process of the constrained optimization is presented and demonstrated with examples.

Cooling performance of hybrid refrigerant of solid nitrogen and small amount of neon for the purpose of HTS power applications

We have investigated cooling performance of a hybrid refrigerant of solid nitrogen and small amount of neon by using a short sample of Bi-2223/Ag tape as a cooling target. Solid nitrogen is expected as a heat capacitor for HTS applications operated below triple point temperature of nitrogen, and we have proposed in our previous study a method for overcoming the problem of its thermal contact with a cooling target with the aid of small amount of liquid neon. This paper discusses the phase state as well as the quantity of neon required for such an improvement. Nitrogen gas was introduced into a sample chamber through a mass flow controller, and then was liquefied and solidified by a Gifford–McMahon cryocooler. After that, in order to improve thermal contact of the solid nitrogen with the sample, neon gas was also introduced into the chamber at 25 K where neon can be liquefied. Cooling performance of such a hybrid refrigerant with different inputs of neon was evaluated by measuring temperature rise of the sample with the transportation of overcurrent. As a result, neon could suppress the temperature rise even in gas state, and the existence of liquid neon could additionally suppress the temperature rise. Furthermore, the required proportion of the liquid neon to the solid nitrogen was very small. From these results, we concluded that introducing neon until neon can be liquefied was the best way to make the most of the potential of the hybrid refrigerant.

The Insulation Design of 154 kV HTS Transformer and on Load Tap Changers

The HTS transformer is one of the promising HTS power applications that are expected to be commercialized in the near future. Among the auxiliary functions of a transformer, the voltage regulation function is known to be the most important one. For the voltage regulation of HTS transformers, they are normally equipped with the on-load tap changers (OLTC). It is also required that, in order to develop a HTS transformer as a complete system, not only the basic technologies for all the components including OLTC and bushing, but also the technologies for the insulation of these components at a cryogenic temperature should be well established as applicable. Taking such facts into account, the breakdown characteristics of the insulation components have been evaluated for such application areas as turn-to-turn insulation, layer-to-layer insulation, c, drive shaft-to-drive shaft insulation, tap connecting terminal insulation and others. In overall, this paper discusses the electric insulation design and experimental results with regard to HTS transformer, OLTC and bushing.

Optimization of conduction-cooled current leads with unsteady operating current

A generalized optimization method is presented for conduction-cooled or cryocooled current leads whose operating current may vary over a period of time. This study is part of our ongoing efforts to reduce the cooling load in HTS power applications, where the actual current level varies considerably over a day and over a year. The presented method is also applicable to superconducting magnets that are not always operational at full current. When the operating current is given as a function of time, the total or accumulated cooling load at the cold end is calculated by integrating the instantaneous load over the period. The optimal length-to-area ratio of conductor is determined to achieve a minimum in the total load. After an accurate procedure taking into full account the temperature-dependent properties of conductor is developed with numerical calculations, a simple and reasonably accurate method based on Wiedemann-Franz approximation is suggested for practical use.

Heat transfer characteristics of HTS tape impregnated with solid nitrogen–solid neon hybrid refrigerant

We investigated the heat transfer characteristics of HTS tape, which is impregnated with solid nitrogen–solid neon hybrid refrigerant. We have proposed such refrigerant as high performance heat capacitor for the HTS power applications in a temperature range from 10 K to 30 K. The experiments were carried out with the use of the Gifford–McMahon (GM) cryocooler. Bi-2223/Ag tape conductors were subjected to this study, and impregnated with the refrigerant. After stabilizing the temperature of the tape with the bias current, the voltage of such tape was measured as a function of time. Furthermore, temporal traces of the temperature were also measured at different two points, i.e., surface and 2 mm apart from the tape, respectively. We repeated such measurements for different ratios of neon/nitrogen and operating temperature. It was shown that so-called dry-out phenomenon was overcome by the use of our proposed hybrid refrigerant. Detailed discussions with respect to cooling behavior of the boundary layer between the aforementioned refrigerant and the HTS tape are to be presented.

Magnetization Loss and Shield Effect in Multi-Stacked Tapes With Various Stacking Configurations

AC loss is one of the major topics in large AC power applications using high temperature superconductor such as power transformers, transmission cables and fault current limiters because it is closely related to operation efficiency. Multi-stacked tape conductors should be used to transport the large current in those power applications. A research of various arrangements of HTS tapes for multi-stacked tapes has been performed to increase the capacity of transport current in HTS power applications. In this paper, we studied magnetization loss and shield effect from several different arrangements of BSCCO tapes such as Face-to-Face type, regular matrix type (m <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$times$</tex> 2) and irregular matrix type. The results show that the magnetization loss of the Face-to-Face arrangement was lower than those of the other matrix types for the same stacking numbers. We think that the result was due to the shield effect by demagnetization of adjacent HTS tapes which were located as face to face.

Electric Field in Bi-2223 Tape Carrying DC Current and Exposed to AC Parallel Magnetic Field

A study of the degradation of the critical current of Bi-2223 tapes under AC magnetic fields is of great importance for HTS power applications. In particular, in a common AC coil design, the field component parallel to plane of the tape is much stronger than the perpendicular component. In this work we present the results of electric field measurements of Bi-2223 tape exposed to AC magnetic field parallel to the broad side of the tape with various frequencies and amplitudes. The observed E-I curves can be described as an additional voltage to the E-I curve superimposed on the zero-field curve. This additional voltage increases with amplitude and frequency of the AC magnetic field and grows almost linearly with DC current, peaks close to the zero-field critical current and decreases thereafter. We analyzed the correlation of the electric field with indicated parameters on the base of the theoretical predictions of H. Brandt and G Mikitik and found a striking agreement of the calculated and observed values. Measurements of the time dependent electric field exhibit double-frequency features and strong dependence of the electric field on the amplitude and frequency of the AC magnetic field