A study on the surface flashover characteristics for a HTS cable termination

Terminal technology is important component to a HTS cable as well as a conventional cable. HTS cable terminations are required when the insulated shield HTS cables connect with other conductors such as a bus or a overhead lines. HTS cable terminations must span a temperature range from 77 K to 300 K. The termination is insulated with insulating oil or air, cryogenic gaseous nitrogen and liquid nitrogen. Particularly, difficult conditions for high voltage insulation had to be overcome with HTS cable. And, several environments can substantially raise the flashover possibility at the HTS cable termination. Therefore, in order to insulating design of HTS cable termination, this paper will report on experimental investigations of the surface flashover characteristics under various surface length and GFRP thickness in the atmospheric air, transformer oil, LN₂ at atmospheric pressure and complex condition.     

AC surface flashover strength and barrier effect of LN2 for HTS transformer with simulated electrode

In the response to an increasing demand for electrical energy, much effort aimed to develop and commercialise HTS power equipments is going on around the world. For the development, it is necessary to establish the dielectric technology in LN₂. Hence many types of dielectric tests should be carried out to understand the dielectric phenomena at cryogenic temperature and to gather various dielectric data. Among the many types dielectric tests, the characteristic of surface flashover and the barrier effect were conducted with the simulated electrode after analysing the insulating configuration of the pancake-coil-type HTS transformer. The influence of a barrier on the dielectric strength was measured according to the size of the barrier, the position of the barrier and the effect of the back-electrode. It was shown that the effectiveness, namely the ratio of the breakdown voltage in presence of barrier to the voltage without barrier, is highest when the barrier is placed at the needle electrode side. The effect increased up to 1.8 times when collar length is 10 mm. The flashover characteristic with back-electrode was remarkably lower than the characteristic without one in the case the electrodes located at the same surface of dielectric plate. On the contrary, in the case the barrier was placed between the electrodes, the characteristic was even improved slightly.     

Aging characteristics of cryogenic insulator for development of HTS transformer

In the response to the demand for electrical energy, much effort aimed to develop and commercialize high temperature superconducting (HTS) power equipments has been made around the world. Especially, HTS transformer is one of the most promising devices. For the development of HTS transformer, the cryogenic insulation technology should be established. In this paper V−t characteristics of polyimide (Kapton) tape and GFRP used as turn-to-turn and structural insulations, respectively were studied. Moreover, breakdown hole site of GFRP after breakdown was also discussed. The experimental results show that the time to breakdown is conditioned on applied electric stress and the lifetime indices n of Kapton tape decrease slightly as the number of tape increases while the lifetime indices n of GFRP decrease strongly with increasing thickness. Furthermore, the breakdown holes of GFRP were not at the contact point, at which the electric field is maximum value, between sphere electrode and GFRP sample and its location depends on applied voltage as well as sphere diameter.     

Influence of bubble size and flow velocity on AC electrical breakdown characteristics of LN2

To evaluate insulation design of high voltage for high T_c superconducting (HTS) power apparatus, the effect of bubble and electrode arrangements on the AC breakdown characteristics of LN₂ were investigated. Supposing that an outbreak of quench, the three electrode models were employed for breakdown voltage measurement in LN₂ with bubbles. Experimental results for various quench conditions revealed that the breakdown voltage of LN₂ with increasing the bubble size, flow velocity and electrode distance. Then, the relationship between the bubble conditions and the AC breakdown characteristics of LN₂ were clarified. Also, bubble movement phenomena were observed with an electrode model which consist of plane to cylindrical needle electrode immersed LN₂ for the simulation of the insulation environment in HTS pancake type coils and others of the quenching state.     

Breakdown characteristics of Z continuous winding in HTS transformer

For the development of electrical insulation design of a HTS transformer with Z continuous winding, we have discussed insulation composition and investigated breakdown characteristics such as breakdown of LN₂, polymer and surface flashover on FRP and breakdown-surface combination in LN₂. Also we have designed and manufactured a bobbin that has spiral slot for the Z continuous winding. A Z continuous winding mini-model from Kapton film insulated Cu tape for simulated 22.9 kV class HTS transformer has been constructed using 0.1% breakdown strength obtained by Weibull distribution. The winding model was measured with respect to its insulation characteristics such as ac (50 kV, 1 min) and impulse (154 kV, 1.2 × 50 μs full wave, 3 times) withstand test and its excellent performance was confirmed.     

A magnetic levitation rotating plate model based on high-Tc superconducting technology

With the wide requirements of the training aids and display models of science, technology and even industrial products for the public like schools, museums and pleasure grounds, a simple-structure and long-term stable-levitation technology is needed for these exhibitions. Opportunely, high temperature superconducting (HTS) technology using bulk superconductors indeed has prominent advantages on magnetic levitation and suspension for its self-stable characteristic in an applied magnetic field without any external power or control. This paper explores the feasibility of designing a rotatable magnetic levitation (maglev) plate model with HTS bulks placed beneath a permanent magnet (PM) plate. The model is featured with HTS bulks together with their essential cryogenic equipment above and PMs below, therefore it eliminates the unclear visual effects by spray due to the low temperature coolant such as liquid nitrogen (LN₂) and additional levitation weight of the cryogenic equipment. Besides that, a matched LN₂ automation filling system is adopted to help achieving a long-term working state of the rotatable maglev plate. The key low-temperature working condition for HTS bulks is maintained by repeatedly opening a solenoid valve and automatically filling LN₂ under the monitoring of a temperature sensor inside the cryostat. With the support of the cryogenic devices, the HTS maglev system can meet all requirements of the levitating display model for exhibitions, and may enlighten the research work on HTS maglev applications.     

Electrical characteristics of a dc superconducting cable

A dc superconducting cable is ideal for transmitting large blocks of electrical power over a long distance. However, it must be designed to operate reliably within the constraints of the electrical system. Therefore, system analysis must be performed for each application. The conductor losses caused by the harmonics on the dc must be within the design goals; a system fault should not drive the cable normal with eventual damage to the cable and interruption of power flow; and, the dielectric system of the cable must be designed to be compatible with the expected transient voltages by proper insulation coordination. Transient overvoltages are of concern to electrical power systems; these are especially critical to cryogenic cables because of the susceptibility of the cryogenic enclosures to these transients. This paper discusses the electrical system constraints which are particularly applicable to a dc superconducting cable and shows how such a cable can be designed to be compatible with the electrical system.This paper also summarizes the work on low temperature dielectrics performed at Los Alamos. It shows the variation of break down voltage of dielectric materials, in sheet form and cable configuration, with temperature and pressure under dc and impulse voltage. The surface flashover characteristics with large creepage distance as well as electrical conductivity of dielectric materials at cryogenic temperatures are discussed. These studies are essential for the design of high voltage apparatus operating in cryogenic environments.     

A capacitance-graded cryogenic high voltage bushing for vertical or horizontal mounting

The cryogenic high voltage bushing for the terminations of the 138 kV, 4100 A superconducting cable prototype at the Brookhaven National Laboratory (BNL) is described. The bushing is energized with the line-to-ground voltage between the coaxial centre and outer surrounding conductors, in the axial direction there is a temperature difference from ambient to about 6 K.The capacitively graded bushing can be mounted in any position. It is vacuum tight and withstands pressures of more than 20 bar. The heat leak of the insulation body in axial direction is only 8.5 W.The electrical specifications of the bushing are: 825 kV, 1.2 × 50 μs impulse voltage; 240 kV rms ac 2h test voltage: and > 160 kVs partial discharge inception voltage.This bushing concept may easily be modified to meet most conceivable thermal, mechanical and electrical requirements of cryogenic high voltage engineering.     

Determination method of equivalent insulation test voltage at room temperature for high temperature superconducting power apparatus with coil structure

This paper deals with the determination method of the equivalent insulation test voltage at room temperature that secures the reliability of the cryogenic electrical insulation of the high temperature superconducting (HTS) power apparatus. The high voltage test is related to the apparatus with coil structure at the stages of its development, manufacturing and shipment. In the test method, the equivalent insulation test voltage at room temperature is derived from the standard test voltage at cryogenic temperature, based on the idea that the HTS power apparatus should be operated without partial discharges (PDs). The conversion factor between the two voltages is given by the product of the two medium factors, i.e. the one relating to the potential distribution along the coil winding and the other relating to the PD inception condition at the weakest part in the electrical insulation system. In order to determine the latter factor concerned with the non-linear phenomena against electrical stresses, the PD inception voltages at cryogenic and room temperatures are theoretically and experimentally estimated for modeled turn-to-turn insulation system. The results show that both estimated and measured values are in fairly good agreement and the proposed method is useful for the equivalent high voltage test at room temperature for the HTS power apparatus.     

Development of 500 m HTS power cable in super-ACE project

A high temperature superconducting power cable (HTS power cable) is highly promises as a low cost and large capacity power line. An HTS cable is also effective in increasing power capacity of underground cable in a city part. A demonstration of a 500 m HTS cable that contributes to research for commercial applications was planned in a part of “Super-ACE project” of METI (Ministry of Economy, Trade and Industry). Furukawa Electric has been taking charge of designing, manufacturing and installation of the 500 m cable. The cable is a 77 kV 1 kA single-core cable with liquid nitrogen (LN₂) impregnated paper insulation. The manufacturing and the installation of the cable have been completed in November 2003, and now preparations of peripheral equipments are proceeding for a test starting in March 2004. This paper describes the design, manufacturing and installation of the 500 m HTS cable.     

Numerical study of cryogenic micro-slush particle production using a two-fluid nozzle

The fundamental characteristics of the atomization behavior of micro-slush nitrogen (SN₂) jet flow through a two-fluid nozzle was numerically investigated and visualized by a new type of integrated simulation technique. Computational fluid dynamics (CFD) analysis is focused on the production mechanism of micro-slush nitrogen particles in a two-fluid nozzle and on the consecutive atomizing spray flow characteristics of the micro-slush jet. Based on the numerically predicted nozzle atomization performance, a new type of superadiabatic two-fluid ejector nozzle is developed. This nozzle is capable of generating and atomizing micro-slush nitrogen by means of liquid-gas impingement of a pressurized subcooled liquid nitrogen (LN₂) flow and a low-temperature, high-speed gaseous helium (GHe) flow. The application of micro-slush as a refrigerant for long-distance high-temperature superconducting cables (HTS) is anticipated, and its production technology is expected to result in an extensive improvement in the effective cooling performance of superconducting systems. Computation indicates that the cryogenic micro-slush atomization rate and the multiphase spraying flow characteristics are affected by rapid LN₂-GHe mixing and turbulence perturbation upstream of the two-fluid nozzle, hydrodynamic instabilities at the gas-liquid interface, and shear stress between the liquid core and periphery of the LN₂ jet. Calculation of the effect of micro-slush atomization on the jet thermal field revealed that high-speed mixing of LN₂-GHe swirling flow extensively enhances the heat transfer between the LN₂-phase and the GHe-phase. Furthermore, the performance of the micro-slush production nozzle was experimentally investigated by particle image velocimetry (PIV), which confirmed that the measurement results were in reasonable agreement with the numerical results.     

Electrical characteristics of high-Tc superconducting mini-model cable under mechanical stresses in liquid nitrogen

To develop 22.9 kV class high-T_c superconducting (HTS) cable in Korea, we have been studying electrical insulation properties of dielectric paper, such as breakdown voltage, partial discharge, which is one of the HTS cable structure elements. However, the research on the mechanical stress of dielectric paper compared to breakdown properties of dielectric paper is insufficient. A cracking and variation of the electrical insulation due to mechanical stresses during cooling and bending of HTS cables in cryogenic temperature is a serious problem. Thus, we investigated tensile stress and breakdown stress of dielectric paper under mechanical stress. Moreover, we manufactured mini-model cables investigated breakdown stress under bending stress to design a cable drum for conveyance. In the AC, impulse and partial discharge properties, all test results showed a similar tendency, and the suitable bending radius ratio R/r was decided to be more than 25.     

Cryogenic design of liquid-nitrogen circulation system for long-length HTS cables with altitude variation

Liquid-nitrogen (LN₂) circulation systems with altitude variation are investigated and designed for application to practical long-length HTS cables. This study is motivated by the KEPCO’s immediate plan to install new HTS cables in existing utility tunnels including inclined and vertical sections. The distribution of pressure and temperature along the LN₂ circulation loop is examined for various geographic conditions, taking into account the gravitational effect of altitude variation. The cryogenic cooling requirements are defined in terms of the pressure and temperature of LN₂, and a design process is graphically demonstrated on phase diagram. It is concluded that the LN₂ flow rate along the cable with altitude variation should be carefully determined with the constraints on pressure and temperature, and the proposed graphical method is useful in the design.     

On conduction-cooling of a high-temperature superconducting cable

Current generation high-temperature superconducting (HTS) power transmission cables use liquid nitrogen as a coolant that circulates along the cable. In this work, the use of axial conduction-cooling in attaining HTS temperatures in transmission lines is proposed. Liquid coolant use is envisioned only at periodic length intervals along the transmission lines, in combination with insulation and copper. The proposed concept is feasible due to the high thermal conductivity of pure copper at cryogenic temperatures. A basic design for the insulated cable is proposed and a detailed numerical simulation of heat transfer in such a cable is carried out for various case studies considering the superconducting materials MgB₂ and BSCCO-2223.     

Investigation of cryogenic cooling effect in reaming Ti-6AL-4V alloy

This research article is based mainly on the investigation of the effect of cryogenic machining, while reaming Titanium grade 5 alloy (Ti-6Al-4V) material. Cutting speed (V_c) and feed rate (f) are two input parameters at three different levels for a constant depth of the hole. The output parameters considered by using a cryogenic LN₂ cooling compared to a conventional flood cooling are torque (M_t), thrust force (F_t), cutting temperature (T), quality of the hole (circularity and cylindricity), surface roughness (R_a) and chip morphology. The results show cryogenic liquid nitrogen (LN₂) cooling resulting in 15–31% reduction in the cutting temperature, 23–57% reduction in the thrust force and 14–65% reduction in torque. Higher surface roughness, circularity (Cir) and cylindricity (Cyl) were observed in the cryogenic LN₂ cooling condition. Furthermore, better chip breakability was observed in the cryogenic LN₂ cooling condition. No drastic change in the microstructure was observed in both flood and cryogenic LN₂ cooling. Increase in microhardness by 10–16% and 8–19% in cryogenic LN₂ cooling over flood cooling was observed.     

Low-temperature tensile strength of the ITER-TF model coil insulation system after reactor irradiation

The windings of the superconducting magnet coils for the ITER-FEAT fusion device are affected by high mechanical stresses at cryogenic temperatures and by a radiation environment, which impose certain constraints especially on the insulating materials. A glass fiber reinforced plastic (GFRP) laminate, which consists of Kapton/R-glass-fiber reinforcement tapes, vacuum-impregnated in a DGEBA epoxy system, was used for the European toroidal field model coil turn insulation of ITER. In order to assess its mechanical properties under the actual operating conditions of ITER-FEAT, cryogenic (77 K) static tensile tests and tension-tension fatigue measurements were done before and after irradiation to a fast neutron fluence of 1×10²² m⁻² (E>0.1 MeV), i.e. the ITER-FEAT design fluence level. We find that the mechanical strength and the fracture behavior of this GFRP are strongly influenced by the winding direction of the tape and by the radiation induced delamination process. In addition, the composite swells by 3%, forming bubbles inside the laminate, and loses weight (1.4%) at the design fluence.     

Surface flashover characteristics in liquid nitrogen for application of superconducting pancake coils

For the development of superconducting power apparatus, it is necessary to establish the dielectric technology in coolants like LN₂. Among the dielectric technology, surface flashover characteristics are studied with several simplified spacers at the structural aspects. Double pancake coil can apply to transformer and fault current limiter, etc. To design dielectric system of high temperature superconducting transformer consisting of double pancake coils, this study discusses an effective insulator composition. Circular shape insulator divided into two parts should be inserted between coils and the insulator should cover electric stresses concentrated at the circumference of the coils which are in the same section of double pancake coils facing each other.     

Performance evaluation of cryogenic cooling in reaming titanium alloy

Removal of materials in metal cutting operations through drilling and reaming of hard materials is a difficult process. Wear of the tool and high cutting zone temperature have big effect on it. In this study, experiments have been carried out in a reaming operation on titanium alloy material under flood and cryogenic LN₂ cooling separately. Cutting speed, feed rate, and hole depth (constant) are the three input variable parameters. Torque (M_t), thrust force (F_t), cutting temperature (T), quality of the hole, surface roughness (R_a), and chip morphology are the output parameters. In both cooling conditions, each of the nine experiments based on orthogonal array (OA) L₉ were conducted under both cooling conditions. Based on the results obtained, cutting temperature was reduced by 12–21%, thrust force reduction is 17–32%, and torque reduction is 7–30% in cryogenic LN₂ cooling. Surface roughness is increased by 4–15% and hole quality (circularity and cylindricity) parameters are affected in cryogenic LN₂ cooling with respect to flood cooling. Better chip breaking was found in both flood and cryogenic LN₂ cooling. No drastic changes were observed in microstructure under both cooling conditions.     

Effect of the connection gap on the heat-load characteristics of a liquid nitrogen bayonet coupling

A transfer system for liquid nitrogen (LN₂) installed at National Synchrotron Radiation Research Center (NSRRC) to provide LN₂ required for the superconducting equipment and experimental stations has a LN₂ transfer line of length 160 m and pipeline of inner diameter 25 mm, a phase separator (250 L) and an automatic filling station. The end uses include two cryogenic systems, one Superconducting Radio Frequency (SRF) cavity, five superconducting magnets, monochromators for the beam line and filling of mobile Dewars. The transfer line is segmented and connected with bayonet couplings. The aim of this work was to investigate, by numerical simulation, the effects on the heat load of the gap thickness of the bayonet assembly and the thickness of vacuum insulation. A numerical correlation was created that has become a basis to minimize the head load for future design of bayonet couplings.     

Cryogenic system with the sub-cooled liquid nitrogen for cooling HTS power cable

A 10 m long, three-phase AC high-temperature superconducting (HTS) power cable had been fabricated and tested in China August 2003. The sub-cooled liquid nitrogen (LN₂) was used to cool the HTS cable. The sub-cooled LN₂ circulation was built by means of a centrifugal pump through a heat exchanger in the sub-cooler, the three-phase HTS cable cryostats and a LN₂ gas-liquid separator. The LN₂ was cooled down to 65 K by means of decompressing, and the maximum cooling capacity was about 3.3 kW and the amount of consumed LN₂ was about 72 L/h at 1500 A. Cryogenic system design, test and some experimental results would be presented in this paper.