Conductor Winding Research Articles

Mathematical model for calculating steady-state conditions and static characteristics of salient-pole synchronous machines

Salient-pole synchronous machines occupy a paramount position in both the generation and consumption of electrical energy. Recent achievements in the design, manufacturing, and operation of salient-pole synchronous machines are the result of successful advancements in the fields of electromechanical engineering technology, semiconductor technology, and mathematical modeling. To calculate the nominal steady-state operation of salient-pole synchronous machines and their static characteristics (no-load, angular, V-shaped, regulatory, and external), a mathematical model has been developed. Its computer implementation enhances the technical and economic performance of synchronous pole machines. During the creation of the mathematical model, the following assumptions were made: the magnetic field of the machine is conditionally divided into a working field and scattered fields; the magnetic field of the machine is flat and parallel; the magnetic field in the active layer has only a radial component, while in the stator and rotor yokes, only a tangential one; the winding conductors are located in an infinitely thin layer and distributed along the slot according to a harmonic law; losses in the steel are absent. To create the mathematical model of salient-pole synchronous machine, a system of equations for the electrical and magnetic state is formulated. To obtain equations with coefficients independent of the rotor position, the coordinate transformation method (transformation to the dq0 rectangular coordinate system stationary with respect to the rotor) was employed.

Multimodal analysis of saddle micro-terrain prone to wind disasters on overhead transmission lines

Pervasive micro-terrain is a significant contributor to wind disasters on transmission lines. This study explores the effect of saddle micro-terrain on the wind field of transmission lines and proposes relevant models and analysis methods. Firstly, the characteristic elements and parameters of saddle micro-terrain are extracted using DEM and established representative cross-sections for classification. Subsequently, a multimodal computational model is developed, considering the geographical and meteorological features and the sag model of transmission lines under micro-terrain. This study calculates wind field distribution and conductor wind loads for three types of saddle micro-terrain conditions, revealing an exponential growth trend of wind loads with increasing wind speeds. The results indicate that in transmission lines at saddle areas, the sag region does not intrude into the boundary layer, with a wind speed growth rate of only 0.18, resulting in relatively stable wind loads. In contrast, for transmission lines at saddle areas in secondary mountain ranges and dual-mountain saddle regions, wind speed growth rates reach 0.97 and 1.53, respectively, indicating higher disaster risks. This research provides a basis for distinguishing and disaster prevention in mountainous transmission lines' micro-terrain variations, offering significant contributions to enhancing wind-resistant design standards in mountainous regions.

Creep failure analysis of crimp joints in aluminium and copper windings of distribution transformers

The present work investigates creep performance and contact resistance of Distribution Transformers (DTs) winding crimp joints under the influence of temperature and stress, aiming to reduce the failure rate of DTs. The integrity of crimp joints is crucial, as the field study revealed that a majority of crimp joints become unstable and show elevated contact resistance due to service exposure. These changes affect the steady-state creep rate of the crimp joint and the surrounding winding conductors, ultimately diminishing the operational lifespan of DTs. For experimentation, crimp joints with varying crimp points were fabricated for a 25 kVA DT using aluminium and copper conductors with diameters of 0.83 mm and 0.6 mm, respectively. For all types of joints i.e., Al-Al, Al-Cu and Cu-Cu, the 2-point crimp joint was identified as optimum in terms of tensile strength and contact resistance. The creep tests for the crimp joints were conducted under varying temperatures of 100 °C and 140 °C with stress levels of 40 MPa and 55 MPa as per ASTM E139-11. The steady-state creep rate and contact resistance of Al-Al crimp joints increased with an increase in temperature and stress. The creep life of DTs due to Al-Al and Al-Cu crimp joints is significantly less than the designed life of a typical DT. On the other hand, creep behaviour of copper winding crimp joints supports a significantly higher DT creep life even at a higher temperature of 140 °C and elevated stress of 55 MPa.

Research on Dynamic Characteristics Detection Method of Tall Power Transmission Tower

In addition to the gravity of the structure itself and the supported object, the open-air high-rise supported steel structure tower mainly bears dynamic loads. Take the transmission line tower as an example, such as wind load, conductor galloping, tower vibration and sudden unbalanced tension caused by line breaking. It is necessary to study the dynamic characteristic parameters to know the structural resistance to dynamic load. Therefore, from the perspective of dynamic safety evaluation, this paper designed the acquisition method of dynamic characteristic parameters and established a reliable finite element modal analysis model of transmission tower. Finally, the dynamic characteristic parameters such as natural frequency, vibration mode and amplitude of transmission line steel tower were obtained. It was found that the inclined material of the transmission tower might be unstable when the high-order natural frequency resonance occurred, which had important practical significance for structure design optimization and safety evaluation.

Circulating and Eddy Current Losses in Coreless Axial Flux PM Machine Stators With PCB Windings

Printed circuit board (PCB) stators in coreless axial flux permanent magnet (AFPM) machines have been proposed, designed, and studied for use in multiple industries due to their design flexibility and reduction of manufacturing costs, volume, and weight compared to conventional stators. This paper investigates mechanisms and methods of approximating open circuit losses in PCB stators within example wave and spiral winding topologies for a dual rotor, single stator configuration using 3D FEA, analytical hybrid techniques and experiments. The effect of rotor magnet shape, end winding, and active conductor geometry on eddy currents is studied, and some mitigation techniques are proposed. Through stator equivalent circuit analysis, circulating current losses caused by mechanical abnormalities and magnetic circuit asymmetry are assessed. Possible strategies and schemes to minimize circulating current losses are also described. The trade-off between stator loss components and some practical design considerations are outlined in detail. The open circuit power losses of a prototype coreless AFPM motor were experimentally tested using multiple example PCB stators and emulated rotor asymmetries, with the findings being comparable to the FEA and hybrid analytical methods results.

Development and assessment of simplified analytical method for current distribution among REBa2Cu3O y parallel conductors in armature windings for fully superconducting rotating machines

Fully superconducting rotating machines employing REBa2Cu3O y (REBCO, RE = rare earth elements or Y) superconducting armature and field coils, are particularly interesting for aircraft applications, owing to their high output power density (kW kg−1). To achieve high current capability in superconducting coils, we have proposed a cabling design for transposed multi-strand parallel conductors. In the parallel conductor design, the REBCO strands are insulated from each other, except for both terminal ends, and transposed during the winding process to achieve uniform current distribution by cancellation of interlinkage magnetic flux between the strands. In this study, a simplified analytical method considering inductances was developed based on Laplace’s equation in cylindrical coordinates to roughly calculate the current distributions of multi-strands under armature coil conditions. The validity of the analytical method was investigated through current distribution measurements of the sample coils wound with two-strand parallel conductors. Consequently, the analytical method was validated with approximately 10% deviation under the experimental coil conditions. To establish a more accurate analysis method, certain improvements are needed.

Experimental Study on Wind Noise Distribution Law of Transmission Lines

Aiming at the problem of wind noise generated by transmission lines under windy weather conditions, this paper selects three groups of transmission lines in Shandong Province, China. Through the fixed-point measurement of the wind noise pressure of the conductor at the ground projection point of the side, phase conductor sags as the starting point, and the outward extension of 50 m as the endpoint, the frequency spectrum curve characteristics of the wind noise of the transmission line, the predominant frequency, the cut-off frequency, and the overall A sound level are analyzed. The results show that the transmission line conductor wind noise is mainly concentrated in the range of 500 - 2000 Hz, and its predominant frequency is around 650 Hz, which belongs to low-frequency noise. The wire wind noise attenuates with the increase of the distance from the projection point, and the attenuation is completed at 50m, and the wind noise fixed-point measurement decreases by 8 dB.

Analytical modelling of high‐frequency losses in toroidal inductors

AbstractToroidal inductors are widely used in power electronic systems. With the increasing switching frequency of power devices, the AC losses of high‐frequency inductor windings have become an issue that cannot be ignored in design. A method with low computational time cost and high accuracy is urgently needed by designers. This paper proposes a modified analytical method for winding loss calculation of toroidal inductors based on the one‐dimensional Dowell method and the two‐dimensional Ferreira method, and proposes a compensated analytical model considering the proximity effect of inter‐turn conductors of toroidal windings. The analytical method calculates the non‐linear porosity of toroidal windings, and can accurately solve the AC losses of toroidal inductors with arbitrary porosity winding encapsulation. The analytical results are in good agreement with the finite element method (FEM) and experimental data. The experimental measurement data verifies the validity of the proposed model within 1‐MHz frequency, the calculation error of the modified analytical method is less than 25%, and the calculation error of the compensated analytical model is reduced to 10%.

MAGNETICS ELEMENTS FOR POWER ELECTRONIC CONVERTERS

Magnetics devices are an important part of every power electronics converter. Always, the realisation of the magnetic elements must be connected with the converter design. The power electronics devices projects include models and design the converter, and models and design the magnetics elements too. The describes design of magnetics parts for power electronics converters is the topic of this article. In this paper, basic theory is reviewed, including magnetic circuits equation and inductor modelling. Power losses in winding conductors ae also important. This can lead to copper losses significantly in excess of the value predicted by the dc winding resistance. The "skin effect" and the "proximity effect" are most pronounced in high-current conductors of multilayer windings, particularly in high-frequency converters. This article explains and provides practical methods to compute these losses. The aim of the paper comprise calculation of magnetic devices, their losses, and design of magnetic devices for power electronics converters.

DETERMINATION OF THE TEMPERATURE FIELD DISTRIBUTION IN THE WINDING RODS OF THE STATOR OF TURBOGENERATORS OF TYPE TVV FOR PURPOSES OF CONTROL AND DIAGNOSTICS

Turbogenerators were and remain the most unreliable element at nuclear power plants in Ukraine. The most dangerous disturbances in the operation of turbogenerators of the TVV type is complete or partial blockage of the hollow conductors of the stator winding. The systems of operational control and technical diagnostics today, in a number of cases, cannot cope with the identification of these phenomena. The complex application of adequate mathematical models, indications of the standard thermal control system and new diagnostic features give the opportunity with sufficient accuracy for practical purposes to detect blockages in the hollow conductors of the stator windings of these turbogenerators. The aim of the work is to develop a program for calculating the three-dimensional temperature field in the rods of the stator winding of a turbogenerator of the TVV-1000-2U3 type, taking into account the temperature field of the stator core, to carry out a test calculation and to compare the experimental and to calculated data. The problem is solved with the help of the finite difference method using the integro-interpolation method.

Electrical Power Transformer Damage on the Power Grid in Benin: Causes, Consequences and Solution Approaches

The transformer exploits the phenomenon of electromagnetic induction of wound conductors. One of its important properties is that it can act as an impedance matcher. Faults on the electrical network often affect the grounding resistances of power transformers and eventually damage them. Benin's electricity network, like those of other countries in the world, is frequently tested by electrical transformer damage. Through this work, from the static data obtained from the SBEE (Beninese Electric Energy Company), the state of the transformers installed on the electricity network, from 2016 to 2022, is realized. The analysis of their different operating conditions and their installation and protection methods has made it possible to identify the main causes of their damage. Their consequences on the transport and distribution of electrical energy in Benin are highlighted. It appears from this work that most of the faults are due to overvoltage due to discharges, overloads and short circuits. The poles most affected are those on poles (H61) followed by those on frames. Finally, a series of recommendations are proposed to minimize the number of defective transformers.

Comparative Analysis of AC Copper Loss With Round Copper Wire and Flat Copper Wire of High-Speed Stator-PM Flux-Switching Machine

In the high-speed flux-switching permanent magnet (FSPM) machine, the ac copper loss issue in the armature winding conductor is very prominent, due to the high-frequency leakage magnetic field in stator slot, which seriously affects the machine performance. To model and calculate ac copper loss more accurately, a coupled electric circuit-magnetic field model is constructed based on 2D-finite element method, and consequently the influence of modeling methods and winding types on ac copper loss calculation of a 12-slot/10-pole (12/10) 54.7 kW/10 kr/min FSPM machine is investigated. First, ten simplified ac copper loss calculation models are established to deal with the problem of large consumption of calculation time and computer resources caused by the excessive number of wires, small size of winding conductors, high precision modeling, and mesh refinement. Second, the influences of voltage-/current-source and steady/rotating rotor are considered, and the ten models are evaluated in terms of computing time and accuracy. Third, based on the preferred model from the ten models, the effects of frequency, round/flat wires, and wire sizes on ac copper loss are further studied. Fourth, to investigate the influence of flux leakage at the end of winding on ac copper loss, a 3-D ac copper loss calculation model with different axial lengths of winding end space is established. Fifth, the change process of ac copper loss and transient temperature of winding under short time overload condition are obtained combined with the bidirectional coupling model between electromagnetic and thermal field method through multiple iterations of electromagnetic loss and temperature. Finally, the stator modules of round and flat copper wires are manufactured, respectively, and an experimental modular device is built to measure the ac copper loss, which confirms the improved accuracy of the predictions by the proposed model.

Effect of winding methods: transport AC losses in CORC coils

AC losses undoubtedly increase the burden on the cooling system and affect the overall performance of high-temperature superconducting (HTS) cables. Therefore, AC losses in HTS cables are important design factors for large-scale HTS equipments. Extensive research has been conducted on straight, densely wound conductors. However, AC losses of bending/circular HTS cable conductors have not been sufficiently studied. In this article, the transport AC losses in a circular HTS conductor coil bended by a conductor on round core (CORC) type cable were investigated using experiments and numerical simulations. The effect of different winding methods was also studied. First, the modeling method of the three-dimensional simulation and the experimental procedures were presented. The simulation results of the straight densely wound conductor (SDWC) and circular densely wound coil (CDWC) were compared with the corresponding experimental results. Finally, the winding method for the CDWC was optimized using both the simulation and experimental approaches. Both the experiments and simulations showed that reducing the tape-to-tape frontal area was able to decrease the transport AC loss, and the results show that the optimum winding angle of the HTS CORC coil was 24.15°. Overall, the modeling and experiments provide useful optimization strategies for designing winding structures of high-performance CORC coils/magnets.

Increasing reliability of traction electric motors of diesel locomotives taking into account thermophysical parameters of insulation and armature winding conductors

Introduction. The issue of ensuring the operable state of the DC traction motor is relevant due to its large-scale use on diesel locomotives, including modern powerful motors, operating on lines with increased train masses. At present, a rather difficult situation has developed in the locomotive industry with the failure of diesel locomotive traction electric motors due to a decrease in the insulation resistance of the armature windings of the electric motors and their subsequent breakdowns: up to 28 % of the total number of failures of electric motors are due to breakdown of the hull insulation and interturn short circuit of the armature and 13 % of cases are due to reduction in the resistance of the insulating material.Materials and methods. The paper considers the main directions of scientific research on the causes of insulation integrity failure, which lead to unscheduled repairs of traction motors. The theoretical substantiation of the root causes of insulation destruction is based on the importance of taking into account the coefficients of thermal linear expansion of copper and its insulating materials. In order to study thermodynamic processes in the winding of a traction motor, a computational finite element model of a winding coil laid in the groove of the armature core has been developed. The winding model is represented separately by a conductor and insulation, between which contact conditions are specified. The conductor of the calculation model heats up to 120 °C from the current flow. Mathematical apparatus embedded in the MSC calculation program, Patran – Nastran, made it possible to evaluate the deformation of the conductor relative to the insulation as a result of a linear increment due to thermal expansion.Results. With the help of mathematical modelling and based on the results of finite element analysis, the confirmation of the theoretical justification is clearly shown. The difference in elongation during heating of the motor armature conductor and insulation, obtained by mathematical modelling, is 0.6 mm and is significant for the winding (consisting of a conductor and insulation), which is usually considered as a single whole body.Discussion and conclusion. The obtained result shows the need for more detailed studies to select the technology for the insulation of the DC traction motor. The use of insulating materials for the armature winding with coefficients of thermal linear expansion equal to the coefficient of thermal expansion of the copper conductors of the winding will improve the reliability of traction electric motors of diesel locomotives in operation.

Current Center Line Calculation Method and Results for ITER Poloidal Field Coils

Three out of six Poloidal Field Coils (PFC) are already delivered to the ITER Organization. The PF Coils are built winding, impregnating, and vertically stacking double pancakes (DPs) of NbTi Cable-in-Conduit conductors into a Winding Pack (WP). Later, the Winding Pack (WP) is impregnated for ground insulation and clamping devices are installed for structural support and interface with the rest of the ITER machine. One of the main parameters characterizing the PFCs is the Current Centre Line (CCL), defined as the barycentre of its WP conductors. Ideally, the CCL would be in the WP's symmetry plane but due to solutions in the construction design and manufacturing deviations, it may vary. Double Pancakes (DPs) may be wound with different dimensions, or a deviation during their stacking would cause a misalignment of all the conductors contained in that ill-positioned DP, affecting the CCL. The manufacturing process starts with the conductor winding and insulation, forming a DP. The DP is then impregnated with resin and a scan of its surface is used to reconstruct the DPs virtually. The DPs are then stacked, forming a WP, and measurements of points on the surface are used to recreate the process virtually. Finally, the WP is insulated and impregnated with resin. At this stage, both surface scans and point measurements are used to align the WP in the coordinate system of the ITER machine. This paper explains the process to calculate the CCL of the three first PF Coils using manufacturing data, defining the uncertainty associated with the calculation and comparing against the target tolerances defined for the proper ITER machine operation.

Strategy for Developing the EU-DEMO Magnet System in the Concept Design Phase

Fusion power plants offer the prospect of a new sustainable source of energy for future generations. The design and R&D of future reactors is expected to largely benefit from the experience gained in the design, construction and operation of ITER. However, deploying reliable fusion power plants, requires to overcome the design challenges and to address the remaining readiness gaps. Europe is starting the Conceptual Design Phase for building a superconducting DEMOnstration Fusion Power Plant (DEMO) and starting operations around the middle of the century. The aim is demonstrating the production of 500 MW of net electricity, with a closed tritium fuel cycle and adequate plant availability. For the design of the Superconducting Magnet System several variants of coils have been investigated in the pre-conceptual design phase, which has concluded in 2020. Some of them are very close to ITER design, therefore have a relatively high technology readiness level. The “alternative” proposed solutions, that are very promising in terms of costs and performances, need a validation to industrial scale in order to be eligible for the down-selection expected in 2024. In particular, the validation regards the design of the layer-wound graded TF winding pack based on React-and-Wind Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn conductors, and the hybrid, layer-wound graded CS coil. The challenging aspect is that the CS magnet is made of REBCO conductors in high-field, React-and-Wind Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn conductors in medium field and NbTi conductors in low field. Also innovative techniques need to be studied for insulating critical parts of the coils, as penetrations, discontinuities and joints. The main issues of the technological development are illustrated. A R&D work plan is presented for the Conceptual Design phase to validate the technology within 2024 and demonstrate the operational capabilities by testing superconducting insert coils, that is about 50-m long wound conductors, by 2027.

DC Characterization of a Low-Field Nb3Sn Prototype Conductor for a DEMO TF Coil

A Low Field W&R (Wind and React) conductor, developed in the frame of the conceptual design studies for the Toroidal Field (TF) coils of DEMO, has been designed to be constituted of a small number of superconducting Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn strands and a high number of stabilization copper wires. It has been extensively characterized at the SULTAN facility at Swiss Plasma Center (SPC) in February 2021. The conductor has been operated up to its target current, 70.8 kA, and characterized up to a background magnetic field of 10.78 T. The test program included AC loss measurements, current sharing temperature ( <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T_cs</i> ) and critical current ( <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">I_c</i> ) measurements before and after cyclic loading. Critical temperature ( <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T_c</i> ) measurements were performed on the virgin conductor at the beginning and at the end of the test campaign. In this work we are focusing on the DC characterization, while the AC measurements analysis will be the object of a following work. Concerning DC tests, premature quench phenomena prevented from operating the conductor at the target current – field conditions. Nevertheless, at lower currents <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T_cs</i> stability was observed, with no performance degradation due to cycling. The present paper investigates these results and tries to give an explanation through measurements and tomographic analysis. Furthermore, the achieved strain distribution in the conductor, obtained by means of inductive measurements analysis, will be shown. The implications of the design approach on the performance, based on the outcome of such tests, are discussed.

Bending Strain Sensitivity of Critical Current in REBCO Coated Conductor Winding for Coil Application at 77 K

High-temperature superconducting (RE)Ba₂Cu₃ O_7-x (REBCO)-coated conductor (CC) tapes are known for their capacity to tolerate high tensile stress under uniaxial tension, which is typically in the range of 600-700 MPa at 77 K. However, when used in a coil, there is a limit to the allowed strain before degradation of the critical current (<i>I</i>_c) occurs. This is attributed to the combined strain due to bending and axial tension which was caused during fabrication, cool down, and operation of coils and magnets and corresponds to the irreversible strain limit of <i>I</i>_c degradation. Applied strain along the CC tape&#x0027;s edgewise direction (hard bending strain) also affects the irreversible strain limits of <i>I</i>_c considerably. With such conditions, degradation of the REBCO coil performance can be expected. Therefore, the current study aims to systematically investigate the mechanism for such <i>I</i>_c degradation in REBCO coils. Linear superposition of strains induced in different modes of easy and hard bending were studied analytically and experimentally. The <i>I</i>_c degradation behaviors were examined in IBAD&#x002F;REBCO CC tapes in windings with different pitches on mandrels with different diameters. The investigation was done at 77 K and in self-field conditions using an apparatus designed for coil testing. The bend strain parameters in coils were analyzed, including the radius of curvature and pitch between CC tape turns. Through the linear superposition of strains induced in the REBCO coils, an appropriate approach is suggested to effectively handle the <i>I</i>_c degradation under winding including edgewise bending.

Redesign of the Coils for the 60T Controlled-Waveform Magnet at NHMFL

Driven by the1.4 GW generator, the 60T Controlled Waveform (60 TCW) magnet was the most powerful controlled waveform system in the world and had always been one of most important magnets to the National High Magnetic Field (NHMFL) and high-field research community because of its following unique features: (1) quasi-static field up to 60 T with 100 ms flat-top and total pulse-length of about 2000 ms, (2) variable magnetic field waveforms such as staircase and triangle with flat-top (3) relative large bore (32 mm) and (4) very fast cooling time (20 minutes) between pulses [Boebinger, 2001], [Crooker <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">et al.</i> 2001]. The magnet is composed of nine concentric coils, with each coil consisting of several conductor winding layers reinforced by a high-strength metallic shell. The magnet underwent a catastrophic failure in 2000 and all the coils had to be rebuilt. In late 2014 the second magnet version failed near the mid-plane of coil 7. The simulations afterward that incident indicated that the overall strength of the coil would be increased by replacing a section of the reinforcing shell with Zylon fiber-epoxy composite. This reduces the stress and thus significantly lowers the level of plastic deformation in the windings. The role of the metal and Zylon fiber reinforcing layers in bearing the axial and radial Lorentz forces has been studied to optimize the magnet design. The results of the optimization will be discussed as well as challenges that have been presented in rebuilding the individual coils of the magnet.

AC Loss Calculation and Analysis of Hollow Conductor for Doubly Salient Brushless DC Generator

The hollow conductor has attracted attention in variable reluctance machines (VRMs) working in harsh environment due to its good heat dissipation performance. The abundant harmonic content of the magnetic field in the slot of VRM and the complex structure of the hollow conductor make ac loss characteristics of the hollow conductor complex. Accurate and fast ac loss calculation is necessary to optimize the efficiency of VRM and provide the basis for thermal design. The doubly salient brushless dc generator (DSBLDCG), as the VRM, is analyzed in this article. First, the ac loss of the hollow conductor under a uniform alternating magnetic field is modeled numerically. The loss characteristics of hollow conductor in DSBLDCG are obtained by combining the analytical model and the simplified finite element (FE) model. Second, the refined FE model of DSBLDCG with the hollow conductor winding is established to verify the proposed method, and the influence of different structural parameters and operating conditions on the ac loss of the hollow conductor is also analyzed. Finally, the ac loss test experiment is designed for experimental verification.