Ground Insulation Research Articles

Experimental characterization of the ITER TF structure cooling in HELIOS test facility

During ITER plasma operation, large thermal loads are generated in the stainless steel Toroidal Field (TF) coil casing. To minimize the impact on the temperature of the TF Cable in Conduit Conductor (CICC), these heat loads are intercepted by case cooling channels which are implemented at the interface to the winding pack. One of the design options for the case cooling channels consists of a stainless steel pipe inserted in a rectangular groove which is machined in the casing and filled by a charged resin of high thermal conductivity. A higher number of cooling pipes is arranged at the plasma facing wall of the case, thus providing a better shielding to the TF conductor at high field. To assess the efficiency of the cooling pipes and their thermal coupling with the charged resin, experimental characterizations have been performed. First of all, the thermal resistance vs temperature of some of the individual components of a TF coil has been measured on representative samples in a cryogenic bench. Further characterizations have been performed on an integrated mock-up of the TF cooling scheme at cryogenic temperature in HELIOS test facility at CEA Grenoble. The mock-up consists of a piece of TF casing that can be heated uniformly on its surface, one cooling channel implemented in the groove which is filled with the charged resin, the filler, the ground insulation, the radial plate and one insulated CICC. The cooling pipe and the CICC are cooled by supercritical helium at 4.4 K and 5 bar; the instrumentation consists of temperature, pressure and mass flow sensors. Both stationary and transient operating modes have been investigated to assess the thermal efficiency of the case cooling design. The experimental tests are presented and the first results are discussed and analyzed in this document.

Effect of direct fluorination on surface charge of polyimide films using repetitive pulsed power

As one of the basic insulating materials, polyimide films have been widely used in turn to turn insulation and turn to ground insulation in wind turbine generators. However, the pulse voltage from the converter often causes overvoltage and localized high electrical field in the insulation system. Surface charge could affect the dielectric property and cause the breakdown of insulation. Direct fluorination is a method to modify the surface layer of the polymers without changing the bulk properties. As a result of the fluorination, hydrogen atoms are replaced by the fluorine atoms and the component of the polymer surface is changed, which can influence the surface charge accumulation and affect the electrical properties. The experiment in this paper was focused on the effect of direct fluorination on surface charge of polyimide films under pulse voltage. The polyimide films with the fluorination time of 15, 30, 45 and 60 min were prepared, together with the unfluorinated one as a comparison. Surface charging was performed by corona discharging using repetitive pulse power at the temperature of 25 °C with the relative humidity of ~45%. The rising time of the pulse was 25 μs and the frequency was set at 200, 400, 600 and 1000 Hz, respectively. The surface charge decay was measured using a specific probe coupled with an electrostatic voltmeter. The results showed that the surface charge behavior was affected by the fluorination, corona discharging time, pulse voltage amplitude and frequency.

Study of the Use of Current Limiting Resistors for Protecting High Energy Superconducting Magnets

Due to their pulsed and fast discharge operation, the superconducting systems of ITER will experience high voltage variations and, as a result, the instrumentation for quench detection will also experience those high voltage levels. Therefore the risk of an electrical breakdown between instrumentation wires and ground cannot be discarded. Current Limiting Resistors (CLRs) inside the cryostat, connected in series with magnet instrumentation wires, and embedded in the ground insulation, were included in the instrumentation base line to prevent consequential damage following an electrical breakdown. A CLR can mitigate the effects of the surge currents through instrumentation wires in case of unexpected incidents. This paper describes the study of the potential installation of the CLRs and the rationale of why finally it was concluded that their use was not suitable for a superconducting magnet with the high voltage and energy that ITER needs.

Cryogenic Acceptance Tests of SST-1 Superconducting Coils

Toroidal field (TF) and poloidal field (PF) coils of steady-state superconducting tokamak (SST-1) have been fully refurbished to ensure cryostable and low-dc-resistance interpancake and intercoil joints, helium leak tightness of the winding pack, and $> 10 \hbox{M}\Omega$ winding pack to ground insulation resistance under cold conditions. As per SST-1 mission mandate, all TF coils and PF3 Top had been cold tested at full operational parameters. A dedicated large coil test facility was integrated for these tests. Specially adapted solutions such as magnet preparation cum transport stand, demountable busbar supports, demountable sensor mountings, reusable joints for busbar connections inside cryostat, etc., were developed. These measures led to timely and successful completion of cold tests and integration of the magnet system on to the SST-1 machine shell. The functionality of various diagnostics of the magnet system was also established during these tests. Details of coil refurbishment, test facility, test campaigns, and some important test results are reported in this paper.

On-Line Detection of the Coal Mine Power Cable Parameters Based on the Method of Low Frequency Signal

The coal mine power supply mainly rely on cable, its insulation level directly affects the coal mine safety production. Has long been lack of perfect insulation parameters online detesting equipment, the traditional detection methods need to stop the normal grid power supply, affect the normal production of coal mine, it is great significance that research a kind of on-line insulation parameters detection equipment. Using three-phase reactor in the neutral point insulation system to make artificial neutral point, from this point inject low-frequency voltage signal, the signal through the cable ontology, cable to ground insulation resistance and capacitance, and to a loop resistor. Measure the voltage and current signals and modulus conversion, using DSP to realize rapid changes of Fourier algorithm, can quickly and accurately calculate the cable to ground insulation parameters. The algorithm is effective to solve the traditional measuring circuit transformer reactance and unbalance current's influence to the accuracy of measurement, measurement process will not affect the normal operation of the electric power system, the insulation parameters can be accurately measure online.

Progress of the ITER Correction Coils in China

The ITER Correction Coils (CC) include three sets of six coils each, distributed symmetrically around the tokamak to correct error fields. Each pair of coils, located on opposite sides of the tokamak, is series connected with polarity to produce asymmetric fields. The manufacturing of these superconducting coils is undergoing qualification of the main fabrication processes: winding into multiple pancakes, welding helium inlet/outlet on the conductor jacket, turn and ground insulation, vacuum pressure impregnation, inserting into an austenitic stainless steel case, enclosure welding, and assembling the terminal service box. It has been proceeding by an intense phase of R&D, trials tests, and final adjustment of the tooling. This paper mainly describes the progress in ASIPP for the CC manufacturing process before and on qualification phase and the status of corresponding equipment which are ordered or designed for each process. Some test results for the key component and procedure are also presented.

LHC IR Upgrade Nb–Ti, 120-mm Aperture Model Quadrupole Test Results at 1.8 K

Over the last five years, the model MQXC quadruple, a 120-mm aperture, 120 T/m, 1.8 m long, Nb-Ti version of the LHC insertion upgrade (due in 2021), has been developed at CERN. The magnet incorporates several novel concepts to extract high levels of heat flux and provide high quality field harmonics throughout the full operating current range. Existing LHC-dipole cable with new, open cable and ground insulation was used. Two, nominally identical 1.8-m-long magnets were built and tested at 1.8 K at the CERN SM18 test facility. This paper compares in detail the two magnet tests and presents: quench performance, internal stresses, heat extraction simulating radiation loading in the superconducting coils, and quench protection measurements. The first set of tests highlighted the conflict between high magnet cooling capability and quench protection. The second magnet had additional instrumentation to investigate further this phenomenon. Finally, we present test results from a new type of superconducting magnet protection system.

Advanced Examination Techniques Applied to the Assessment of Vacuum Pressure Impregnation (VPI) of ITER Correction Coils

The ITER Magnet System includes a set of 18 superconducting correction coils (CC) which are used to compensate the error field modes arising from geometrical deviations caused by manufacturing and assembly tolerances. The turn and ground insulation are electrically insulated with a multi-layer fiberglass polyimide interleaved composite, impregnated with epoxy resin using vacuum pressure impregnation (VPI). Adequate high voltage insulation (5 kV), mechanical strength and rigidity of the winding pack should be achieved after impregnation and curing of the insulation system. VPI is an effective process to avoid defects such dry spots and incomplete wet out. This insulation technology has also been developed since several years for application to large superconducting coils and more recently to ITER CC. It allows the coils to be impregnated without impacting on their functional characteristics. One of the critical challenges associated with the construction of the CC is the qualification of the VPI insulation. Sections issued from representative VPI test samples with real scale side correction Coil (SCC) cross-section have been delivered and characterized at CERN. High resolution micro-optical inspections have been carried out on large areas through digital microscopy. The aim was to identify lack of impregnation, areas of pure resin and void entrapments. The areas near the filling fibre glass rope received special attention. High precision dimensional and geometrical assessments have been performed with the help of image analysis. Compression and pull-out tests have been also carried out. Finally, high-resolution 3D-computed tomography has been applied for a full volumetric inspection of the sections, enabling the reconstruction in three dimensions of the VPI samples and allowing to fully detect, confirm, and image the volume defects already identified by micro-optical observations.

Analysis of the ITER TF Winding Pack During Cold Tests at Reduced Current

To check the superconducting properties of the ITER TF coils winding pack prior to its enclosure in the case and final impregnation, a hypothetical cold test experiment encompassing cool-down and a reduced energization is being considered. As part of the supporting analysis to this type of test, in this paper, we present a thermo-electromechanical analysis of the ITER winding pack as reacted, insulated, transferred in the final radial plate and fully impregnated but not enclosed in the TF case, which provides structural stiffness during operation. The winding pack is placed in horizontal position and is free to slide on its supporting surface. By means of global/local models we evaluate the stresses arising after cool-down and reduced energization (up to 20% of the full current), in particular, in the turn and ground insulation. The FE code ANSYS has been used for all the analyses.

Reaction Heat Treatment and Epoxy Impregnation of a Large <formula formulatype="inline"> <tex Notation="TeX">$\hbox{Nb}_{3}\hbox{Sn}$</tex></formula> CICC Coil for a Series-Connected Hybrid Magnet

Recent activities that were conducted on the Helmholtz Zentrum Berlin Series-Connected Hybrid outsert coil included the reaction heat treatment and vacuum-pressure impregnation. A thermocouple was wound into the windings for diagnostic purposes during processing of the coil. To maintain a reasonable temperature differential across the coil, the ramp rate was limited to approximately 2.7°C/hr. Epoxy impregnation was conducted with an epoxy developed in-house. The epoxy is transferred in vacuum but cured with one atmosphere gage pressure applied. Subsequently, the winding hardware was removed including the stainless steel mandrel through a machining operation. After assembly of voltage breaks and voltage taps, Paschen tests to 4 kV were conducted successfully that qualified the ground insulation. In addition, surge tests to qualified the internal insulation with 3 kV applied across the leads were conducted. Impedance measurements were conducted to confirm the internal insulation integrity.

Structural Analyses of the Cold Mass for High Field Dusty Plasma Experiment

A magnet to study the properties of dusty plasmas in the presence of a magnetic field has been proposed. This magnet has four operating modes to vary the experimental test region and target magnetic field values. The design of the cold mass for the high field magnetized dusty plasma experiment is presented and analyzed. The cold mass includes four coils, ground insulation covering the winding packs, and coil cases used to safely support the coils. Dimensions of the four coils are specified by the magnetic design, and the equipment used to view the dusty plasma apply spatial constraints to the cold mass design. The design also considers the manufacturing process for the cold mass and the stresses resulting from electromagnetic loads coupled with thermal loads. Only one operating mode is considered in this analysis as the others produce significantly lower electromagnetic forces. Resulting stresses compare favorably to failure criteria at critical locations.

Busbar System for ITER Magnets

The superconducting busbar system for ITER magnets includes the NbTi cable-in-conduit busbars themselves, the support structure consisting of rigid ducts and clamps to take the electromagnetic loads experienced by the busbars, and the joints for electrical and hydraulic connection. The main challenges arise from considerable displacements of the coil terminals relative to the cryostat, and high electromagnetic, thermal and seismic loads on the busbars, and supporting system. In addition, the busbar ground insulation has to withstand a voltage of up to 30 kV in fault conditions. This article describes how the individual engineering challenges have been met. This includes design concepts and the analysis method for the iterative design of a support structure complying with different, often contradictory, requirements.

Dynamic behavior of surface charge on direct- fluorinated polyimide films

Polyimide film, as a special type of engineering plastic film, is a kind of basic insulating material and is widely applied in the aerial, nuclear, microelectronic industry, turn to turn insulation and turn to ground insulation of inverter-fed motors. However, premature insulation failures occur frequently in motors with the disadvantage of pure PI inherent in all organic materials. The existence of surface charge has a great effect on breakdown characteristic and is the main reason leading to dielectric breakdown. Fluorination as change the chemical component in surface layer of polymers should give rise to the corresponding change in electrical properties of the surface layer thus influence the charge injection from electrodes when they are used as an insulator. This paper presents a study aimed at clarifying the effect of fluorination time on surface charge accumulation and decay behaviors of fluorinated polyimide film. Samples were surface fluorinated in a laboratory vessel at about 328 K (55 °C) using a F2/N2 mixture with 20% F2 by volume and 0.05 MPa (500 mbar) for respectively 15, 30, 45 and 60 minutes. Corona charging tests were performed at room temperature with a relative humidity of ~ 40%. The charge distribution was measured by means of an electrostatic voltmeter. Obtained results show the dependence of the charge density as well as the charge decay rate upon the fluorination time of samples, varying as a function of the charge polarity and charging time. It is suggested that the fluorination can significantly improve the decay rate of the surface charge in polyimide films.

Testing Results for Nb-Ti, 120-mm-Aperture, Low-B Quadrupole Models for the LHC High-Luminosity Insertion

The design and construction of a 120-mm wide-aperture, Nb-Ti superconducting quadrupole magnet for the Large Hadron Collider (LHC) insertion region is part of a study towards a luminosity upgrade of the LHC at CERN, envisaged for 2020-22. The main challenges for this accelerator quality magnet are to operate reliably with the high heat and radiation loads that are predicted in the insertion magnet regions. Calculations give approximately 500 Watts over the 30-m-long string of insertion magnets, while today LHC is operating for a nominal heat load of 12 Watts. To extract this heat, the model magnets incorporate new features: Open cable insulation, open ground insulation, open magnet structure, and a quench heater that has open channels to help extract the steady state heat load. This paper presents results from tests at room temperature and 1.8 K for the initial model magnet. We report magnet training, transfer function and field quality measurements, quench heater performance, and heat extraction studies using imbedded heaters to simulate the deposited beam heating profile.

R&D of Insulating and Vacuum Pressure Impregnation Equipment for PF1 Coil Double Pancakes

The PF1 coil is one of six poloidal field coils of the ITER magnet system. The PF1 coil winding is constructed from eight double pancakes (DPs), which are plane cylindrical solenoids of about \varnothing 9 t0.1nm in size and about 20 ton in weight. The monolithic electrical insulation of the PF1 coil is designed under ITER requirements and consists of the conductor multilayer turn insulation, the interlayer insulation of the pancakes, and the ground insulation of the winding. Vacuum pressure impregnation (VPI) with the epoxy resin provides the insulation strength after high-temperature curing. This paper presents the research and development results concerning insulating and VPI equipment for PF1 coil insulating. It involves the 12-roll insulation device for lapping of polyimide and glass tape layers, the vacuum chamber for double pancake impregnation and curing, and automated devices and heaters for mixing and supply of the resin under necessary vacuum and temperature conditions. All equipment has been successfully tested using DP mock-ups. The biggest one was manufactured as a full-scale DP sector. Following modeling, the standard insulation samples were taken from the mock-up to prove the mechanical properties of the insulation to meet the ITER requirements. The test results are also given in this paper.

Structural Analyses of the JLab Hall D Replacement Solenoid

The conceptual design of the replacement solenoid for the 12 GeV upgrade at Jefferson Lab is analyzed structurally. First, stress distributions within a detailed cross-section of the winding pack under Lorentz and thermal loads are discussed. These stress distributions are used to evaluate the integrity of the copper conductor and fiberglass insulation. Slip planes are included between the ground insulation of each coil and the mandrel. Second, the design of the support rods that carry the axial EM forces attracting the cold mass to the iron, carry the weight of the cold mass and provide lateral elastic stiffness offsetting attraction of the cold mass to the iron yoke in the event the cold mass is not concentric with the iron yoke is also presented. Third, the integrity of the cryostat is evaluated while bearing loads from the cold mass supports, the attraction of the cold mass to the iron yoke, atmospheric pressure, and the step-stops preventing cryostat axial displacements. Fourth, the stresses in the conductor during winding are evaluated.

Recognition of Branch Insulation Parameters in Coal Mine Distribution Network using Least Squares Method

The precondition for leakage protection is to identify insulations parameters exactly and quickly in coal mine distribution networks. A novel identification method of insulation parameters is presented. Time domain differential mathematic model of man-made leakage test is built, and then the least squares method is used to solve model coefficient, ground insulation resistances and ground capacitance are obtained correctly. This insulation identification method can not only calculate the total ground insulation resistance and the total ground capacitance but also provide the insulation parameters of each branch in detail. The results of simulation for a 660V coal mine distribution network indicate that this identification method is accurate and effective and can identify each insulation parameters rapidly in 15ms after adding a resistor to one phase of line.

Development of Insulation Technology With Vacuum-Pressure-Impregnation (VPI) for ITER Correction Coil

ITER Magnet System includes 18 Correction Coils (CC), made with NbTi cable-in-conduit 10 kA conductor, wound into multiple pancakes. The turn and ground insulation were electrically insulated with a glass tape/polyimide interleaved multilayer composite, which should be impregnated with epoxy resin. Some technical issues remaining have to be addressed in the manufacturing process of these coils. One of the issues is the impregnation and curing of the insulation system, which should achieve good mechanical properties and sufficient high voltage insulation (5 kV). The authors performed manufacturing trials of the insulation system using the vacuum pressure impregnation (VPI) method, which is an effective process to remove defects such as dry spots and over rich resins in the insulation system. In order to simulate and optimize the feasible VPI process, two VPI test samples with the length of 1000 mm have been fabricated. In this paper, we introduce the detailed VPI process including the insulation materials and insulation structures for CC and present the results of the related mechanical and electrical tests performed. The mechanical properties, including tensile strength, inter-laminar shear as well as compressive shear (45 ) were evaluated at room temperature and liquid nitrogen temperature (77 K). In addition, the dielectric strength of the turn insulation and the ground insulation on the test samples were also examined. The results of the tests were discussed versus the design requirements.

Transient Electrical Voltages Within ITER Poloidal Field Coils

High-voltage insulation co-ordination of the coil system is essential for reliable operation of ITER. Transient electrical excitations occur on the terminals of superconducting coils, for example in case of a fast discharge of the coils and if a failure occurs within the components of the electrical circuit. Transient voltage on the coil terminals may lead to non-linear voltage distribution and oscillations within large superconducting coils because of their large dimensions and high number of turns. Thus, the internal voltages may even be higher than the voltages on the coil terminals. This effect depends on the rise time of the excitations and was measured within the ITER Toroidal Field Model Coil which is about three times smaller than the ITER toroidal field (TF) coil. The calculations of the transient electrical behavior of the coils provide a basis for high-voltage insulation co-ordination and definition of test voltages and waveforms. The high voltage tests during the manufacturing process will control the quality of the electrical insulation and ensure reliable operation of the coils. The transient electrical behavior of ITER poloidal field (PF) coils was calculated using the PF 3 and PF 6 coils as examples. Maximum voltages for ground, layer and turn insulation were calculated for four scenarios. Results of these calculations are presented in detail in this paper.

Experimental Investigation Into the Characteristics of Micro-SMES' Magnet

Superconducting magnet is a key component of Superconducting Magnetic Energy Storage (SMES), so it is necessary to investigate into the characteristics of superconducting magnet before the SMES run in power system. In the paper, a superconducting hybrid magnet based on YBCO wires and BSCCO wires, which were from Superpower and American Superconductor Company respectively, was designed and produced, and then through the cooling test, critical current test, energy storage test, ground insulation test and exchanging power experiments with power system, the electrical specifications and low temperatures insulating performance of the superconducting magnet were investigated. The results proved that the performance of the hybrid magnet satisfied the design requirements.