HTS Power Research Articles

Magnetization Loss Estimation of HTS Solenoid Coils Wound With CORC

CORC is one of the candidates of the HTS cable for high field magnets. We have been interested in applying CORC to HTS power transformers that require not only large currents but also very low AC losses. Striation and twisting are the solutions for reducing AC losses. We suggested an estimation method for a rough calculation of magnetization losses in a striated CORC coil. First, we measured and calculated the magnetization losses of short samples of straight CC tapes and CORC. Before considering the striation effect, we analyzed the characteristics of slit HTS tapes. In order to emulate the slitting of an HTS tape, we prepared three kinds of HTS tapes with different widths: 2, 4, and 6 mm. Slitting will certainly create the effect of reducing the magnetization loss, but the effect is not directly dependent on the number of the slits nor the applied field magnitude. We measured and compared the magnetization losses of a wide HTS tape to those of narrow HTS tapes. And the narrow tapes were arranged in parallel and measured, so the net width was same as the width of the wide tape. Finally, we applied the measured data of the magnetization loss of several samples to the calculation of the losses of solenoid coils wound with CORC in order to take into account the loss reduction effect of slit HTS tapes.

Compact 2G HTS power cable: new cold tests results

In Russian Scientific R&D Cable Institute we continue studies to develop as compact as possible 2G HTS power cables for electric power systems. Compact cables should have minimized diameter compared to other co-axial HTS cables developed before. Recently we developed the cable prototype with four HTS layers in the core and two layers in the shield. Numerical optimization of compact cables has been used together with manufacturing technology to produce small diameter cables with uniform current distribution. The first cold test of the compact cable demonstrated that we were able to control the homogeneous current distributions in a compact cable with the inner diameter of the core ~10 mm and the outer diameter of the shield ~20 mm. In this work we present more test results of this cable, including, mechanical test and high voltage tests. New test results demonstrated very high parameters of the compact cable developed. The compact cables perspectives for electric systems are discussed.

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.

Understanding Surface Flashover in Helium Gas Cooled High Temperature Superconducting Devices

Surface flashover voltage on solid insulators in gas cooled superconducting power devices is studied. The relationship between the surface flashover voltage and the dielectric strength of the gas media is established. Analysis of the data on surface flashover voltage measurements on cylindrical samples made of high-pressure fiberglass laminate (G10) in three different gas media with varying dielectric strength and pressure showed a positive correlation between the surface flashover voltage and the dielectric strength of the gas both at room temperature and 77 K. The positive relationship, however, is not linear. The results suggest that using gas media with higher dielectric strength, employing higher pressure, and lowering temperature in HTS power devices leads to higher surface flashover voltages and higher operating voltages because surface flashover is usually the limiting voltage in devices that use solid support structures and possess voltage gradients.

Анализ потерь в обмотках и стопках из ВТСП лент второго поколения

The model coil with the racetrack geometry based on second generation High Temperature Superconducting tapes has been developed for an electrical machine where a winding pack is a stack of 2G HTS tapes. It is important to evaluate transport current AC losses and possible methods to reduce them in a winding. In a device made of 2G HTS tapes, the main the AC losses are the hysteresis ones. Only numerical simulation permits to predict them in full. The FEM model for calculation of the hysteresis losses developed before for 2G HTS power cables was modified for a stack of the 2G HTS tapes in a coil. In this paper the methods to increase a computational speed are presented. Possible ways to reduce AC losses are discussed also. The details of the model and comparisons of calculations with measurements of the AC loss are presented.

Experimental investigations on power frequency electrical breakdown characteristics of liquid nitrogen for HTS power devices

Experimental investigations on power frequency electrical breakdown characteristics of liquid nitrogen for HTS power devices

Increased impedance by quench at a shield layer of HTS power cable for fault current limiting function

Increased impedance by quench at a shield layer of HTS power cable for fault current limiting function

Understanding Surface Flashover Strength in Cryogenic Helium Gas for Superconducting Devices

One of the limitations when developing GHe cooled HTS devices for power applications is determining suitable insulation designs. There would be significant reductions in cost, time, and effort if insulation designs could be accurately measured and characterized at room temperature instead of cryogenic temperatures. Our previous research on the intrinsic breakdown strength of GHe demonstrated that it is independent of temperature, and it is solely based on the mass density of the gas. However, the insulation of HTS power devices contains solid insulation materials, and it is necessary to confirm that the interaction between the solid insulation and GHe is also independent of temperature. This paper contains the development and fabrication of a surface flashover experiment setup which can be used at both room and cryogenic temperatures to characterize surface flashover in a GHe environment.

Investigation of HTS Power Transmission Lines Stability Conditions in Short-Circuit Mode

Generally in HTS cables, sample test is performed to measure the sample heating when it is more often subjected to a short-time load by a current exceeding the critical value. This test is assumed to determine the HTS line stability against short-circuit modes (SCMs). However, the characteristics of these modes depend not only on the sample properties but also on the HTS line parameters as a whole and the electric power network part under protection as well. In this paper, we aimed at the theoretical analysis of these dependences, technique of analytical calculation of cable heating in SCM, and the results of a more exact numerical calculation of all the SCM characteristics for various designs of HTS ac lines. The results show that taking into account all the electric power network parameters allows not only a more exact determination of conditions of the cable stability against SCM, but can lead to the contrary conclusions concerning the cable design ensuring this stability. In particular, at the parameters, typically for most of the ac HTS lines, extra copper added to the cable materials increases the cable heating rather than decreasing it. The removal of copper or its replacement by another material with higher electric resistance not only diminishes the cable heating in the SCM but also allows an essential decreasing of the surge current and the current reached at the end of SCM. There is also an opportunity to significantly reduce the ac losses in the rated operative mode.

An HTS power switch using YBCO thin film controlled by AC magnetic field

This report is on the design of and results from a fast-acting, high power density high temperature superconducting switch. The switch uses thin films of YBa2Cu3O7−x on a sapphire substrate switched into the off-state through application of an AC magnetic field to the superconductor, which causes any transport current to experience a dynamic resistance. Results show reliable and repeatable switching of a 10 × 30 mm section of thin film between an on-state with Ic of 100 A and an off-state of 9 mΩ, with off-state voltage exceeding 100 mV. On-state to off-state transition time is less than 1 ms and off-state to on-state transition time is around 15 ms. These results are promising for further development of a high power, fast-acting superconducting power switching device for use in applications such as flux pumps.

Hydraulic analysis of liquid nitrogen flow through concentric annulus with corrugations for High Temperature Superconducting power cable

A hydraulic analysis for turbulent flow of subcooled liquid nitrogen through an annulus within the HTS cable is numerically investigated in terms of friction factor and flow pattern. The control volume for the investigation is confined to the annular space between outer smooth surface of cable core and inner surface of corrugated cryopipe. The corrugation geometries i.e pitch and depth affect the friction factor and thus the pressure drop and pumping power of liquid nitrogen. Nine different geometries of corrugated cryostats with various combinations of pitches (6, 10 and 14 mm) and depths (5, 7 and 9 mm) are considered and compared with smooth (non-corrugated) pipe. The results are useful in selecting the cryostat configuration and the associated fluid pumping system for cooling HTS power cables.

Analysis of terminal heat loss of 10kV/6000A high temperature superconducting cable

HTS power cable terminal is an afflux unit of transition from superconducting cable operating at low temperature to high voltage bus and outlet of refrigerant in normal temperature. In order to obtain the low-temperature environment for the stable operation of 10kV/6000A superconducting cables, we designed a terminal system. The terminal is refrigerated by circulating liquid nitrogen, and the heat loss of the terminal is calculated to obtain. The current leads in the terminal are optimized and the optimal structure lead dimensions are obtained and finally it is verified by experiments. The results show that the leakage heat value of the terminal is about 590W, and the current lead is the most important leakage heat source. The calculation results provide a basis for the design and further optimization of the HTS cable terminal cryogenic system.

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.

Cold test and numerical analysis of the compact 2G HTS power cable

The R&D project is underway in the Russian Cable Institute to reduce mass and dimensions of HTS cables for such applications as electrical aircrafts or ship propulsion systems. Such compact cables should have minimized diameter compared to other co-axial HTS cables developed before. Recently we developed and tested the compact cable prototype with three layers in the core and two layers in the shield. As continuation of this project we developed the new cable prototype with four HTS layers in the core and two layers in the shield. Narrower 2G HTS tape with 3 mm width has been used for this cable. Numerical optimization of the cable with analysis of manufacturing errors influence on current distribution has been performed. The method to produce small diameter cable with uniform current distribution has been suggested. The cold test of the compact cable manufactured has been performed at the Russian Cable Institute test facility. We demonstrated that we can control the homogeneous current distributions in a compact cable with the inner diameter of the core ~10 mm and the outer diameter of the shield ~20 mm. AC loss measurements in the compact cable demonstrated that they are close to those in previously tested HTS 2G cables with larger diameters.

Revisiting the homogenized domain model for fast simulation of AC transport power losses in first generation high temperature superconducting tapes and cables

Challenges, linked with FEM modelling of HTS power cables, include the long computational time required to simulate the small domains that Bi-2223 superconductors possess in a large device. To decrease that time, in this paper the models of AC transport power losses for a tape and a simplified cable are achieved by a homogenization technique via a ”filamentary-equivalent domain” approach. In it, a single homogenous domain with effective material properties is used to represent the superconducting material, where the domain would have the approximate shape of the multifilamentary region. In order to investigate its validity for a range of transport currents, including overcurrent, four tapes from the literature are modelled in three configurations. A simplified cable model is modelled via the homogenized model and an analytical equivalent circuit model. The homogenization technique can accurately predict the transport power losses of the majority of configurations while demonstrating fast computational times and is promising for simulation of real power cables.

Effective Vacuum Pumping of a Long HTS Power Transmission Line Insulated With Heat Radiation Shield

The energy loss of an HTS power transmission line is highly correlated to the properties of the vacuum thermal insulation. However, the proper evacuation of the very long cryostat requires a number of equally spaced pumping stations. The distance between pumping stations is determined by the pumping speed, the conductance of the cryostat, and the outgassing rate from inner surfaces. In this paper, the pumping of long cryostat is considered in the example of the Ishikari-2 project, Japan. This 1000-m-long HTS dc test facility rated for 50 MW was developed in order to obtain data for the next generation of power transmission lines. The vacuum pumping of the cryogenic pipe is carried out through ports located at the pipe ends. A 475-m-long straight section No. 1 is equipped with a heat radiation shield providing excellent thermal insulation of the cable pipe, whereas 137-m-long U-bend section No. 2 and 379-m-long straight section No. 3 are protected with conventional multilayer insulation. Experimental data allow us to estimate the lower limit of the pumping length. When using a cryogenic pipe with heat radiation shield as is,'' it is possible to provide the required level of vacuum for a length of about 8 km in the nighttime and about 5 km in the daytime, because outgassing rate correlates well with the temperature of the outer pipe, varied from 4 °C to 41 °C. Therefore, if we take measures against the heating of the outer pipe by direct sunlight, and redesign the radiation shield to improve pipe conductance, the possibility to evacuate 10-km-long cryostat by two vacuum pumps installed at the ends can be achieved.

Numerical Simulation and Cold Test of a Compact 2G HTS Power Cable

Superconductinge power cables offer the advantages of lower loss, lighter weight, and smaller dimensions, in comparison with conventional cables. It is important to developed most compact cable with a small diameter to increase the transmission power density of a cable. Precise analysis should be used to provide optimal cable layout with uniform current distribution among layers. We have developed an electric circuit model of a cable, and two FEM models using a finite-element code ANSYS. The numerical simulations can provide detailed information about the current distribution inside the cable and the optimal parameters from the point of view of current distribution of the cable have been determined. On the base of the models, the compact 2G HTS cable model with three layers in the core and two layers in the shield has been produced and tested. The outer diameter of the cable was as small as ~20 mm that is less than previously 1G and 2G HTS power cables tested. The experiment with the model cable demonstrated that even rather small mistake in the diameter of a layer leads to considerable change of current distribution among layers. This phenomenon was analyzed with the simulation models developed. It was shown that change of a layer diameter as small as the thickness of one tape (~0.1 mm) led to the current distribution change observed. Calculated results qualitatively coincide with the experiments. In addition, ac losses at different frequencies have been measured to compare them with ac loss in previously tested cables with larger diameter.

Study on AC loss measurements of HTS power cable for standardizing

High-temperature superconducting power cables (HTS cables) have been developed for more than 20 years. In addition of the cable developments, the test methods of the HTS cables have been discussed and proposed in many laboratories and companies. Recently the test methods of the HTS cables is required to standardize and to common in the world. CIGRE made the working group (B1-31) for the discussion of the test methods of the HTS cables as a power cable, and published the recommendation of the test method. Additionally, IEC TC20 submitted the New Work Item Proposal (NP) based on the recommendation of CIGRE this year, IEC TC20 and IEC TC90 started the standardization work on Testing of HTS AC cables. However, the individual test method that used to measure a performance of HTS cables hasn’t been established as world’s common methods. The AC loss is one of the most important properties to disseminate low loss and economical efficient HTS cables in the world. We regard to establish the method of the AC loss measurements in rational and in high accuracy. Japan is at a leading position in the AC loss study, because Japanese researchers have studied on the AC loss technically and scientifically, and also developed the effective technologies for the AC loss reduction. The JP domestic commission of TC90 made a working team to discussion the methods of the AC loss measurements for aiming an international standard finally. This paper reports about the AC loss measurement of two type of the HTS conductors, such as a HTS conductor without a HTS shield and a HTS conductor with a HTS shield. The AC loss measurement method is suggested by the electrical method..

Temperature Distribution and Critical Current of Long HTS Cables Cooled with Subcooled Liquid Nitrogen

Cooling of the long HTS power transmission lines performs by pumping of subcooled liquid nitrogen (LN2) along the cable. The temperature of LN2 along the cable increases due to the heat losses of the cryostat and heat generation in the HTS cable. The experiment using test cable line in Ishikari shows that flow rate of 35 L/min retains increasing of LN2 temperature by 1 K per 1 km of length. The technology when the back flow of LN2 cools the radiation shield surrounding the cable pipe is also applied in Ishikari-2 project. In this case the ambient heat flow into cable pipe is 50 times less than that without radiation shield. Back flow of LN2 removes almost all heat coming from the environment. When transport current is close to the critical value the Joule heat of HTS cable is significant. This heat additionally increases the temperature of LN2 flowing along the HTS cable. Near the outlet the temperature of HTS cable is maximal and the local critical current is minimal. The current matching critical current criterion of average electrical field of E0 = 10−4 V/m provides the voltage drop and significant Joule heat at the hot end of the cable. It can lead the damage of the cable. The present work contains analysis of temperature distribution along the cable and the way to achieve the fail-safe operation of long HTS cable cooled by subcooled LN2. We also performed extrapolation of obtained results for several times longer cable lines by decreasing the LN2 flow rate.