Splice Resistance Research Articles

Evaluation of contact and noncontact lap splices in concrete block masonry construction

Contact and noncontact lap splices, where the lapped bars were located in adjacent cells, were tested in both double pullout and wall splice concrete block specimens. Eight replicate specimens of each type were tested to evaluate whether differences in the reported quantitative results were statistically significant. Visual observations of the incurred damage were also reviewed to identify the resulting failure modes. A statistically significant difference was found between the results of the double pullout and wall splice specimens with the same reinforcing arrangements, and for the different reinforcing arrangements in the same specimen type. Specimens with contact lap splices failed due to bar pullout. Evidence of bond loss at the grout–block interface was observed for specimens with noncontact lap splices and appeared to have influenced the resulting failure mode and lap splice resistance.

Test and Analysis of Spliced DI-BSCCO HTS Tapes

Abstract For some applications, short unit lengths of HTS wires should be spliced if longer lengths are necessary and short unit lengths of HTS wires should be utilize by applying the splice technology to reduce the total wire cost in the application. The splice technology has been developed for DI-BSCCO Type HT-CA tapes by Sumitomo Electric and spliced tapes were tested in Russian Cable Institute. The test program included: measurements of splice's resistance, critical current anisotropy, thermo cycling tolerance, mechanical properties, overload tests and magnetization measurements. In the paper the results of tests are presented and discussed. The test results demonstrated that splices can be used for cable production if twisting and bending limitations are taken into account.

Electrical resistance of Nb3Sn/Cu splices produced by electromagnetic pulse technology and soft soldering

The electrical interconnection of Nb3Sn/Cu strands is a key issue for the construction of Nb3Sn based damping ring wigglers and insertion devices for third generation light sources. We compare the electrical resistance of Nb3Sn/Cu splices manufactured by solid state welding using electromagnetic pulse technology (EMPT) with that of splices produced by soft soldering with two different solders. The resistance of splices produced by soft soldering depends strongly on the resistivity of the solder alloy at the operating temperature. Using solid state welding, splice resistances below 10 nΩ can be achieved with 1 cm strand overlap length only, which is about four times lower than the resistance of Sn96Ag4 soldered splices with the same overlap length. A comparison of experimental results with finite element simulations shows that the electrical resistance of EMPT welded splices is determined by the resistance of the stabilizing copper between the superconducting filaments, and confirms that welding of the strand matrix is indeed achieved. EMPT allows the ductile, unreacted strands to be interconnected, which reduces the risk of damaging the brittle reacted Nb3Sn strands.

A Bi-2223 Layer-Winding Coil Using 540 m Tape Including a Joint Inside the Winding

High Temperature Superconductors (HTS) are now widely used in many applications such as power transmission cables, motors, etc. Recent (Bi,Pb) <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> Sr <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> Ca <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> Cu <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> (Bi-2223) conductors have been much improved in transport and mechanical properties, and their application as insert coils for high field generation is expected. In order to demonstrate the feasibility and the potential of Bi-2223/Ag tapes for magnet applications, we fabricated a layer-wound coil using Bi-2223/Ag/Cu-alloy tapes. We used two tapes, 330 m- and 210 m-pieces, in order to examine the possibility of extension of conductor length through a lap-joint inside the coil. These two tapes were soldered without removing Cu-alloy laminations. The winding has the dimensions: inner dia. 57.1 mm, outer dia. 88.9 mm, coil height: 153.9 mm, 44 layers (2,355 turns). We tested this coil at 4.2 K in 0 ~ 12 T. The test coil carried 193.3 A (0.1 μV/cm criterion) and generated 3.3 T in the axial direction of the coil in external 12 T (15.3 T in total). The critical current is close to the short sample performance in the perpendicular fields. Splice resistance at the joint stayed about 1 μΩ at 4.2 K, 10 T.

The Resistance and Strength of Soft Solder Splices Between Conductors in MICE Coils

Two of the three types of MICE magnets will have splices within their coils. The MICE coupling coils may have as many as fifteen one-meter long splices within them. Each of the MICE focusing coils may have a couple of 0.25-meter long conductor splices. Equations for the calculation of resistance of soldered lap splices of various types are presented. This paper presents resistance measurements of soldered lap splices of various lengths. Measured splice resistance is shown for one-meter long splices as a function of the fabrication method. Another important consideration is the strength of the splices. The measured breaking stress of splices of various lengths is presented in this paper. Tin-lead solders and tin-silver solders were used for the splices that were tested. From the data given in this report, the authors recommend that the use of lead free solders be avoided for low temperature coils.

Splice Resistance Measurements on 2G YBCO Coated Conductors

The Oak Ridge National Laboratory has been investigating the electrical splice resistance of second-generation (2G) YBCO coated conductor. The purpose of the experimental investigation is to study the splice resistance of 2G YBCO coated conductor as a function of: a) operating temperature, b) magnetic field strength (B-field), and c) magnetic field orientation (thetas). Understanding the splice resistance with its corresponding variation as a function of surface preparation and operating conditions is essential to the practical implementation of electric utility devices; e.g., motors, generators, transformers, cables, and fault-current limiters, etc. Preliminary test results indicate that the 2G YBCO splice resistance shows a weak temperature dependence and a significantly stronger dependence upon magnetic field strength and magnetic field orientation. Surface preparation conditions are also briefly discussed.

Evaluation of Electrical Properties of Lap Joints for BSCCO and YBCO Tapes

The joint process between tapes of coated conductors is a critical issue for the most of the applications of high temperature superconductors (HTS). Using different fabrication techniques joints of YBCO coated superconductors were prepared and characterized through electrical measurements. For soldering material low melting point eutectic alloys, such as In-Sn (m.p. 116degC) and Sn-Pb (m.p. 189degC) were selected to prepare lap joints with effective length between 1 to 20 cm. The splice resistance and the critical current of the joints were evaluated by I-V curve measurements with the maximum current strength above the critical current, in order to evaluate the degree of degradation for each joint method. Pressed lap joints prepared with tapes without external reinforcement presented low resistance lap joint nevertheless some critical current degradation occurs when strong pressing is applied. When mechanical pressure is applied during the soldering process we can reduce the thickness of the solder alloy and a residual resistance arises from contributions of high resistivity matrix and external reinforcement. The lap joints for reinforced tape were prepared using two methods: the first, using ldquoas-suppliedrdquo tape and the other after reinforcement-removal; in the latter case, the tapes were resoldered using Sn-Pb alloy. The results using several joint geometries, distinct surface preparation processes and different soldering materials are presented and analysed. The solder alloy with lower melting point and the longer joint length presented the smallest joint resistance.

Electrical and Mechanical Properties of DI-BSCCO Type HT Reinforced With Metallic Sheathes

Electrical and mechanical characteristics of Type HT tape, which is standard 4.2 mm wide DI-BSCCO tape reinforced with metallic tapes, have been evaluated. Longitudinal distributions of critical current and n-index in kilometer long Type HT tape has proved uniform from end to end just like the original insert tape, which is Type H tape. Type HT-CA reinforced with 50 mum thick heat-resistant copper alloy is highly balanced tape with high mechanical properties and low splice resistance. Type HT-SS reinforced with 20 mum thick stainless steel has the best mechanical properties, which has been demonstrated under the actual environment in high field magnet, namely the hoop stress load test energizing a one-turn coil in external high magnetic field and liquid helium.

An HTS Insert for Phase 2 of a 3-Phase 1-GHz LTS/HTS NMR Magnet

As a component of Phase-2 LTS/HTS NMR magnet of a 3-phase program to complete a 1-GHz LTS/HTS NMR magnet by 2009, an HTS insert magnet has been assembled from 48 double-pancake (DP) coils, each wound of high-strength Bi-2223/Ag/stainless steel composite tape. The paper describes the design and actual parameters; and preliminary performance results of the insert. The results include: 1) voltage vs. current data of double pancakes; and 2) splice resistances between adjacent double-pancake coils.

Fatigue of Joints with Staggered Holes

It is customary in contemporary design for fatigue resistance of bolted connections to use slip-critical joints. However, many existing bridges are more likely to have bearing-type joints that use either rivets or high-strength bolts. Moreover, in many of these cases, hole patterns are staggered. Fatigue fracture of tension members with bearing-type joints that use staggered holes is observed to take place on a plane perpendicular to the axis of the member. The s2/4g rule, commonly used for static strength design of bolted tension members, is not applicable for this case, and current design rules do not make it clear just what net section is to be used to calculate the stress range. The fatigue resistance of bearing-type shear splices was investigated to assess the effect of bolt-hole stagger and gauge distance on fatigue resistance. Thirty-one symmetrical bearing-type shear splices were tested at different stress ranges. Staggers varying from zero to 75mm were investigated on four sets of three specimens...

A high-temperature superconducting double-pancake insert for an NMR magnet

For a low- and high-temperature superconducting (LTS/HTS) NMR magnet, an HTS insert, comprised of 50 double pancake (DP) coils, each wound with Bi2223/Ag tape, has been built at FBML. The paper describes design, assembly, and performance data (critical current, splice resistance, field homogeneity) of the HTS insert.

Electrical splice resistance measurements and analysis on high purity aluminum

Low temperature electrical splice resistance measurements were performed on samples of high purity aluminum (RRR>5500). The samples were spliced together using two different joint configurations and adhering techniques: 1) soldered lap joints of Cu coated aluminum and 2) TIG welded butt joints. Splice resistance measurements of both the TIG welded butt joints and the Cu coated lap joints were made at 300, 77, and 4.2 K using a dc transport current technique in the absence of an applied magnetic field. Results showed that the soldered samples had splice resistivities ranging from 10 −6 to 10 −7 Ω-cm 2 at 4.2 K, and were of the order of 10 −5 Ω-cm 2 at room temperature. Results on all of the TIG welded samples showed splice resistivities <10 −8 Ω-cm 2 at 4.2 K. An analysis was performed to determine the current flow within a lap joint of non-superconducting materials with rectangular cross sections. A generalized expression describing the current distribution within the lap joint was derived.

Test results for a Nb/sub 3/Sn dipole magnet

A cosine theta type dipole magnet using Nb/sub 3/Sn conductor have been designed, built and tested. D19H is a two-layer dipole magnet with a Nb/sub 3/Sn inner layer and a recycled NbTi outer layer. Coil-pairs are connected with two of the four Nb/sub 3/Sn splices in a high field region, and compressed by a ring and collet system. The ramp-rate sensitivity and the splice resistances were pleasingly low; and the 4.4 K training was rapid. At 1.8 K, however, the unusually high frequency of outer-coil fast-motion events increased with current, effectively creating a training-ceiling at 90% of the expected outer-layer limit (10.2 T). A low end-load applied to a relatively fluffy outer layer is believed to have caused this training limit. The end-load was increased; but a retest was aborted after the magnet failed a precautionary hipot test.

Test results for a subscale (100 kA) SMES splice

The design for the 20 MW-hr SMES-ETM for the Bechtel concept calls for two splices per turn of conductor, and over 100 turns. The design value of resistance for the splices is on the order of 10/sup -11/ ohms (0.4) W/splice at 200 kA), which is an order of magnitude less than the state of the art for high current devices. The splice design utilizes a superconducting braid wrapped around lapped subcables for an extremely low resistance joint. A history of the manufacturing development for the splice is presented. The performance of a sub-scale version of the splice joint has been measured at Texas Accelerator Center. Values of splice resistance at 1.8 K and background fields up to 5 T are reported. Performance of a 100 kA conductor is also reported. >

Degradation process and fluctuation mechanism of contact resistance for twisted conductor splice

AbstractDynamic and static properties of the contact resistance of twisted conductor splices have been investigated. The splice resistance in a static condition increases (degraded) with time. This is described in terms of a probability density function in which the logarithm of the splice resistance is regarded as a Markovian process. Furthermore, properties of the splice resistance when different mechanical qualities are involved in it are described by a composite probability density function. The theoretical prediction agrees well with observation. The dynamic properties are attributed to temporal change in the displacement of the conductors and in the contact force of the core conductors which is caused by an external force applied on them. Their dependence is given explicitly and a formula for evaluating the aging effect has been derived. Finally, the validity of this model is confirmed by the experimental results.

Use of Cable Surface Temperature to Detect High-Resistance Splices

Mine power trailing cables are subject to considerable abuse and are quite often damaged severely enough to require a repair splice. Although recently developed splice materials provide a splice that is initially sound both mechanically and electrically, continual flexing of the cable can rapidly cause the splice connectors to loosen, resulting in a significant increase in electrical resistance. Although the added splice resistance does not affect the delivery of power through the cable, it can cause extreme local heating. If heavy current is drawn for several minutes, the splice can catch fire or rupture. Such a failure disrupts production and can be dangerous. Since the added resistance of the splice does not significantly alter the overall cable resistance, it cannot be detected by any measurement of the bulk properties of the cable. A method is suggested for determining splice resistance from a small number of temperature measurements made on the cable jacket while the cable is in use. The advantage of the method is that it requires no knowledge of cable current flow or other electrical parameters but only the cable size and its temperature profile. A theoretical analysis of cable splice heating is presented, relating the results to laboratory experimental data. Data are also presented from several cables in the field, including one which experienced a splice fire shortly after the data were taken. Results are encouraging, suggesting that the method can be developed into a practical periodic check of cable splice characteristics.

Quench behavior of a superconducting accelerator magnet

Data are presented on the minimum energy required to cause quenches to propagate in an accelerator dipole magnet. The amount of stored energy dissipated into the magnet was measured as a function of dipole excitation current. This in turn determines the maximum coil temperature reached in a given magnet. Quench velocities in the longitudinal direction of the conductor were as high as 11m/sec. The azimuthal velocities or turn to turn velocities were found to be a function of the number of fiberglass layers of insulation that the quench had to cross and were on the order of a few tens of centimeters/sec. The field shape of a given magnet was found to be unchanged for more than 100 quenches. The coil to coil connection and inter-coil splice resistances were found to be less than a nano-ohm and therefore of little consequence in the cryogenic load considerations. No definitive answers were found on how to decrease the rate of training (130 Gauss/Quench average) required from 4.0T to 5.1T.