High Temperature Superconducting Field Winding Research Articles

AC loss analysis on an HTS field winding of the generator with T-A formulation

In this study, we proposed a coupling model utilizing finite element software to estimate AC loss on the high-temperature superconducting (HTS) field winding of a 1 MW generator. The model comprises three distinct physical modules: rotating machinery and magnetic (RMM), electrical circuit (CE) and partial differential equation (PDE). These modules collaborate to resolve the generator's magnetic field distribution, rated output parameters and degree of current magnetization on the HTS field winding, employing Maxwell equations and T-A formulation. Given the placement of the HTS field winding on the rotor core, accurately calculating its loss in an electromagnetic field that varies both temporally and spatially presents a considerable challenge. To address this issue, we adjust the coordinate system within the PDE module to vary with the material rather than space. Our analysis reveals that the instantaneous AC loss of an HTS field winding comprises two primary components: the attenuation envelope and the oscillatory variation, resulting from the charging process and external harmonic magnetic fields. Additionally, we comprehensively discuss and analyze the influence of current excitation speed on instantaneous AC loss. Accurately estimating AC loss in HTS field windings is a critical and meaningful aspect of preliminary machine design. This study provides an essential reference for addressing this issue.

Study on Electrical Design of Large-Capacity Fully Superconducting Offshore Wind Turbine Generators

A topological structure of 10 MW fully superconducting offshore wind turbine synchronous generator (FSWTG) is proposed, which stator and rotor windings are both composed with high temperature superconductors (HTS) in this paper. And an electrical design method is presented, which integrates the generator structure design, calculation analysis, and parameter optimization. It is all completed based on the electrical design method that the basic parameter design and the optimization of the number of poles, the pole pitch and the electrical load of the 10 MW FSWTG. Finally, the different design parameters are compared and analyzed for FSWTG, partial superconducting wind turbine synchronous generators (PSWTG: traditional armature winding + high temperature superconducting field winding) and the traditional wind turbine synchronous generators (WTG). The results show that the FSWTG has higher efficiency, lower weight and smaller size than those.

Investigation Into Multi-Phase Armature Windings for High-Temperature Superconducting Wind Turbine Generators

High-temperature superconducting (HTS) generators are being considered as a competitive candidate in large direct-drive (DD) wind turbines because of their features of being lightweight and compact. Normally a large air gap is inevitable in partially HTS generators, sacrificing the torque producing capability. In this paper, multi-phase armature windings for HTS generators are investigated to reduce the air gap length in HTS generators while not compromising generators’ performance. Therefore, the torque density of HTS generators can be improved without any added costs. Five different multi-phase armature winding schemes are studied in the paper. Their performance regarding torque production and rotor losses in a 10 MW DD HTS generator are examined. The findings show that employing multi-phase armature windings can reduce the mechanical air gap without generating extra eddy current losses in the rotor, and the torque production can be improved by up to 9.1%. In addition, the alternating magnetic field reaching the HTS field winding are also reduced by using multi-phase armature windings, resulting in lower AC losses and cooling costs.

Commissioning of the World's First Full-Scale MW-Class Superconducting Generator on a Direct Drive Wind Turbine

High temperature superconducting (HTS) generators could enable a lightweight and cost-effective direct drive (DD) wind turbines with large power ratings. The EU-funded EcoSwing project successfully demonstrated the world's first full-scale MW-class HTS generator on a commercial DD wind turbine. This paper focuses on the commissioning of the EcoSwing HTS generator on the wind turbine. The commissioning campaigns, including the rotor cool-down, excitation of the HTS field winding, and the power production of the generator, are presented in the paper. In the testing period, the generator was grid-connected for more than 650 hours and accumulatively produced more than 600 MWh to the grid. The target output power of the 3 MW class was reached. Throughout the real-life testing on the wind turbine, the generator performed well from the electromagnetic, thermal, and mechanical perspectives. Moreover, the generator even sustained three sudden short circuits in the converter system. The work reported has shown that HTS generators are technologically feasible for wind turbine applications, and the technology readiness level of HTS wind turbine generators has been improved to 6 $\sim$ 7 for the first time.

Dynamic loss of HTS field windings in rotating electric machines

High-temperature superconducting (HTS) coated conductors (CCs) are frequently applied under complex electromagnetic fields to develop powerful, compact and efficient rotating electric machines. In such electric machines, field windings constructed by HTS CCs are adopted to increase the magnetic loading of the machines. The HTS field windings work with DC currents and due to the time-varying magnetic field environment, dynamic losses occur. In addition to the AC magnetic field, there is a large DC background field, which is caused by the self-field of the HTS field windings. This paper investigates the dynamic loss in HTS CCs using an H-formulation based numerical model for a wide range of combined DC and AC magnetic fields under various load conditions, and two different methods have been used for calculating dynamic loss. The results show that a DC background field plays a vital role to accurately predict the dynamic losses in HTS CCs. A DC background field of 75 mT can triple the dynamic loss as compared to only applying an AC magnetic field. In addition, the theoretical definition for the dynamic region for the case of solely an AC field has been found inapplicable in the case of a DC background field. Finally, a case study is done based on our double claw pole power generator to estimate the dynamic loss in an actual rotating machine, which was found to be 13.3 W. A low dynamic loss was achieved through the generator field winding design, which prevents high magnetic field fluctuations in the winding, since it is located at a distance from the air gap and armature coils. Furthermore, the rotational speed is very low and hence the resultant magnetic field frequency is low as well.

Performance of Multi-Layer and Stator-Shifting Fractional-Slot Concentrated Windings for Superconducting Wind Turbine Generators Under Normal and Short-Circuit Operation Conditions

High temperature superconducting (HTS) generators are being considered for large offshore direct-drive (DD) wind turbines as they are expected to be lightweight and compact. However, short circuit torques of an HTS generator with integral-slot distributed windings (ISDWs) are too high for wind turbine constructions, mainly due to the large magnetic air gap. Fractional-slot concentrated windings (FSCWs) can be considered to address this issue since their high leakage inductance can limit short circuit currents and torques. Unlike ISDWs, FSCWs produce great contents of space harmonics that induce excessive losses in rotor components. Multi-layer and stator-shifting windings have been proposed to effectively reduce such losses. Based on a conventional 12-slot 10-pole configuration, this paper evaluates the effects of multi-layer and stator-shifting FSCWs on torque production and loss reduction in a 10 MW DD HTS generator. The examined losses include eddy current losses in the rotor shields and AC losses in the HTS field winding. This paper also checks if these FSCW schemes maintain the advantage of achieving a low short circuit torque. The results show that a 6-phase stator-shifting winding is the best choice for applying FSCWs to HTS generators.

Experimental Validation of a Full-Size Pole Pair Set-Up of an MW-Class Direct Drive Superconducting Wind Turbine Generator

The article presents a full-size pole pair set-up of an MW-class direct drive (DD) high temperature superconducting (HTS) wind turbine generator. The set-up serves as a precursor to the world's first full-scale DD HTS generator developed within the EU-funded EcoSwing project. The set-up built for de-risking employs an HTS field winding in the rotor and a conventional copper armature winding in the stator, representing a minimum independent unit of the corresponding full generator. Such a prototyping approach well reflects the performance of the full generator at a low cost. The article first explains the main design considerations of the full generator, followed by designing and construction details of the set-up. Afterward, a number of tests for determining the key performance of the set-up are reported. The testing results showed that the HTS field winding ran stably at its rated current of 480 A and was well maintained below 30 K. Moreover, the tangential force in the set-up reached 29.8 kN, projecting the 2 MW target output power of the full generator. The set-up developed effectively validated the design ideas and engineering feasibilities of the full generator, leading to successful kick-off of the EcoSwing project.

Designing and Basic Experimental Validation of the World's First MW-Class Direct-Drive Superconducting Wind Turbine Generator

High temperature superconducting (HTS) technologies are expected to be a key enabler for lightweight and cost-effective direct-drive (DD) trains for large wind turbines. This paper reports the designing and basic experimental validation of the world's first full-scale DD HTS generator demonstrated on a commercial wind turbine. The HTS generator has its rotor with an HTS field winding working below 30 K, which is achieved by using off-the-shelf Gifford-McMahon cryocoolers. The stator of the generator is essentially conventional, except that the armature winding has four segments to limit fault torques in case of sudden short circuits due to converter failures. Compared to an existing DD permanent magnet generator on the turbine, the air gap shearing stress of the HTS generator is doubled, and the weight is reduced by 24%. The overall design requirements from the turbine integration perspective, as well as the topological considerations, are first described in this paper. The electromagnetic and cryogenic designs are then presented, followed by performance testing of HTS coils. The basic experimental validation shows that the cryogenic design is satisfactory and the measured no-load voltage matches the finite element calculation very well.

Short-Circuit Characteristics of a High Temperature Superconducting Wind Turbine Generator Employing a Segmented Armature Winding

High temperature superconducting (HTS) wind turbine generators (WTGs) are expected to offer a compact and lightweight direct drive train for large offshore wind turbines. However, short circuits occurring at armature winding terminals can pose great challenges, such as high peak torques due to a large magnetic air gap in HTS generators, field current rise over the critical current that may cause a quench, and AC losses in HTS field windings that may cause local overheat. Segmentation of armature windings could be a solution to mitigate such challenges. In this paper, an optimized design of a 10-MW direct drive HTS generator using 2G HTS wires in the field winding is studied based on a well-verified generator topology. The generator design has two scenarios, i.e., without segmentation and with four segments in the armature winding. These two winding scenarios are compared through a phase-to-phase short circuit occurring at the terminals of the armature winding. The effects of segmentation are revealed on 1) electromagnetic torque, 2) current rise in the HTS field winding, and 3) AC losses in the HTS field winding after the short circuit occurs. The results show the great benefits of using segmented armature windings in an HTS WTG: the peak torque is reduced by a factor of 4, and the AC losses in the HTS field winding are nearly halved although the maximum field current is almost unchanged.

Design and Study of a 2G HTS Synchronous Motor With Brushless HTS Flux Pump Exciters

A high temperature superconducting (HTS) synchronous motor with brushless HTS flux pump exciters was designed and is under construction. This HTS synchronous motor is composed of copper armature windings, HTS field windings, HTS flux pump exciters, vacuum chamber, rotary gas seal, etc. The double racetrack-type HTS field coils are wound with second-generation (2G) YBCO tapes and cooled directly by the cold heads. The designed operation temperature is about 30 K. An HTS flux pump was designed to be used as DC power supply to inject large currents into the HTS field coils, which eliminates the slip ring in the motor and avoids thermal load from current leads. The novel design of multiple sources enables us to control the excitation current of the HTS field winding for each pole separately and to prevent all the series field coils from stopping at the same time due to the quench of one coil. To describe the performance characteristics of the HTS motor, an electromagnetic design was carried out with 2-D finite-element method models. Finally, the total losses of HTS synchronous motor are calculated. The next step is to manufacture and install the HTS exciters, HTS field windings, etc. into the HTS synchronous motor.

Topology Analysis, Design, and Comparison of High Temperature Superconducting Double Stator Machine With Stationary Seal

This paper proposes a high temperature superconducting (HTS) double stator machine (HTS-DSM), based on the magnetic field modulation principle. In the proposed HTS-DSM, the HTS field winding and the copper armature winding are separately located on the two stators, so that it overcomes the drawbacks caused by the coupling device for cryogenic transfer, the torque tube, the carbon brushes, and slip rings in the conventional HTS synchronous machines with the rotating field winding, bringing the merits of high reliability and low maintenance cost. Moreover, a method of amounting a ferromagnetic ring outside the HTS field winding is also proposed to reduce the negative effects of the armature reaction field on the HTS field winding, since the alternating current (AC) losses induced by the armature reaction field easily cause the quench of the HTS field winding. Totally four different topologies are proposed, and the operation principle of the HTS-DSMs is analyzed based on the magnetic field modulation principle. Then the design and optimization of the HTS-DSM is discussed in detail to present the main design parameters of the four topologies. Finally, the performance of the four different topologies are compared by the finite element analysis, the results reveal that the HTS-DSM with modular dewar and iron-teeth inner stator exhibits the merits of higher power density.

Design and Analysis of 10 MW Class HTS Exciting Double Stator Direct-Drive Wind Generator With Stationary Seal

The superconducting (SC) generators are expected to have a promising application for offshore wind generation due to the merits of high power density and high efficiency over the conventional generators. In this paper, a 10-MW conceptual high temperature superconducting (HTS) exciting double stator direct-drive wind generator (HTS-DSDDG) with the stationary seal is proposed and investigated. The proposed HTS-DSDDG has two spatially independent stators: to place HTS field windings and copper armature windings, respectively. Thus, it realizes the stationary seal of the cooling system and removes brushes and slip rings and the other additional excitation device; hence offering the advantages of the high reliability and the low operation & maintenance cost. The main design considerations and the initial design specifications of the 10 MW HTS-DSDDG are given in detail. Furthermore, the electromagnetic characteristics of the HTS- DSDDG under no-load, rated load, and three-phase short-circuit fault conditions are analyzed by the finite element method (FEM). Subsequently, its size, weight, and cost are compared with the existing conceptual 10 MW wind generators. The results reveal that the HTS-DSDDG has excellent electromagnetic performances, such as good sinusoidal voltage, low cogging torque, and torque ripple; besides, its output power per volume is twice than that of the permanent magnet direct-drive generator (PMDDG), being comparable to that of the existing conceptual MgB2-HTS direct-drive generators.

Design of a Combined Screening and Damping Layer for a 10-MW-Class Wind Turbine HTS Synchronous Generator

This paper is focused on the design of a single protection layer to cover both the screening and damping characteristics of a large-scale high-temperature superconducting (HTS) wind turbine synchronous generator under sudden short-circuit conditions. Due to small synchronous reactance of HTS generators, short-circuit condition causes substantial fault current in the stator windings, which produces high unbalance variable magnetic field as well as substantial mechanical fluctuations issues. The former one can lead to unstable condition for HTS field windings, due to induced ac loss and field overcurrent, and the latter one can produce large fluctuations on the generator rotational speed and rotor torque angle. The proposed protection layer is designed in a way that not only screening capability is provided for the HTS windings by preventing unbalance magnetic field penetration, but desirable damping ratio is also achieved so that the fluctuations are suppressed in a proper transient time. A 10-MW-class HTS synchronous generator is modeled and simulated using finite element method under different short-circuit conditions. The temperature variation study and field overcurrent analysis are provided to show the compatibility of the proposed model. In addition, magnetic force distribution and static eccentricity fault detection analyses are provided, which guarantee the proper dynamic performance of the studied generator using the proposed protection layer. Simulation results confirm that with the proposed protection layer, both the screening and damping characteristics of the 10-MW-class HTS generator are obtained and proper transient dynamic responses are deduced for wind power applications.

Electromagnetic Design of a Large-Scale Double-Stator Direct Driving HTS Wind Generator

High-temperature superconducting (HTS) direct driving wind generators have been extensively researched due to their inherent high torque density and small mass. In this paper, a double-stator HTS wind generator with rotating HTS field windings is proposed to further improve the torque density. The electromagnetic design of this generator is made and electromagnetic performances of different topologies with air-gap windings or iron teeth are compared and analyzed. In addition, two regular HTS wind generators and one double-stator permanent magnet wind generator with the same scale are put forward to make comparison with proposed double-stator HTS machines, and the feasibility and practical value of these five topologies are evaluated in terms of the weight, volume and cost of active materials. The results reveal double-stator HTS wind generator has great advantages in volume and mass reduction.

A Superconducting Linear Variable Reluctance Machine for Urban Transportation Systems

This paper presents a new superconducting linear variable reluctance machine for urban transportation applications. The proposed linear machine has a doubly salient structure. Its armature adopts segmented configuration and houses both the copper armature winding and the high-temperature superconducting field winding, while the salient-pole rail consists of iron core only with no excitation sources. Performances of the proposed machine are analyzed and evaluated by the finite element method. By using segmented armature design, the force ripple can be suppressed to 5.5% that is very desirable for urban transportation systems.

Short-Circuit Fault Simulations in an HTS Wind Generator With Different Mechanical Conditions

The high-temperature superconducting (HTS) generators are proposed as a promising method to reduce the weight and size of the large-scale off-shore wind generators. However, the lower reactance of an HTS generator makes short-circuit faults heavier. The short-circuit fault problems of an HTS generator are complicated. A simulation is an effective way to study the generator's performances under fault conditions. However, different mechanical conditions affect the simulation results. In this paper, the performance of a 2.5 MW HTS wind generator under short-circuit fault conditions is studied by coupling the finite element method model and the equivalent circuit model. The mechanical conditions are set by three different methods that are the constant rotation speed method, the constant torque method, and the constant power method. The results in these three methods show similar peak values in transient processes, but different developing processes and different final states. The peak values of the electromagnetic torque, armature current, and field winding current appearing in the transient process should be considered when designing the HTS wind generators. This study provides a criterion to set mechanical conditions in the short-circuit fault model of an HTS wind generator, and also provides data for future work about the quench protection of the HTS field windings.

A Superconducting Vernier Motor for Electric Ship Propulsion

A megawatts (MW)-class direct-drive vernier motor for electric ship propulsion is proposed in this paper. The proposed vernier motor adopts the homopolar structure and utilizes the high-temperature superconducting wire for electrical excitation. The stator houses the superconducting field winding and the copper armature winding. The rotor consists of an iron core without excitation sources. Thus, no cryo-moving components are required. A dual-body Dewar for cooling the pancake high-temperature superconducting field winding is proposed. The motor performance is analyzed and evaluated using three-dimensional finite-element analysis. Both the brushless ac and brushless dc operation modes for the proposed motor are assessed and compared.

A New Linear Vernier Permanent-Magnet Machine Using High-Temperature Superconducting DC Field Excitation

In this paper, a new linear Vernier permanent-magnet machine is proposed. By using high-temperature superconducting (HTS) winding to provide an additional field excitation, a hybrid excitation machine is achieved. The HTS field winding is accommodated in the stator and PMs are mounted on the mover surface. By regulating the dc current flowing in the HTS field winding, the air-gap flux can be flexibly controlled. First, the proposed machine and the cooling system for the HTS field winding are presented. Second, the operating principle of the proposed machine is discussed. Third, by using the 3-D finite element method, the performance of the proposed machine is evaluated. Finally, a conclusion is drawn.

Short Circuits of a 10-MW High-Temperature Superconducting Wind Turbine Generator

Direct Drive high-temperature superconducting (HTS) wind turbine generators have been proposed to tackle challenges for ever increasing wind turbine ratings. Due to smaller reactances in HTS generators, higher fault currents and larger transient torques could occur if sudden short circuits take place at generator terminals. In this paper, a finite element model that couples magnetic fields and the generator's equivalent circuits is developed to simulate short-circuit faults. Afterward, the model is used to study the transient performance of a 10-MW HTS wind turbine generator under four different short circuits, i.e., three-phase, phase-phase clear of earth, phase-phase-earth, and phase-earth. The stator current, fault torque, and field current under each short circuit scenario are examined. Also included are the forces experienced by the HTS field winding under short circuits. The results show that the short circuits pose great challenges to the generator, and careful consideration should be given to protect the generator. The findings presented in this paper would be beneficial to the design, operation and protection of an HTS wind turbine generator.

A New High-Temperature Superconducting Vernier Permanent-Magnet Machine for Wind Turbines

A vernier permanent magnet (VPM) machine using high-temperature superconducting (HTS) winding for dual excitation is proposed for wind power generation. The proposed VPM machine adopts the parallel hybrid excitation configuration, the stator consists of an identical tooth pair connected together by the stator yoke and the rotor contains two sets of consequent-pole permanent magnets joined together by the rotor yoke. The armature winding is located at the stator main slots while the field winding using HTS coils is wound on the stator yoke. With the benefit of the HTS field winding, the air-gap flux can be flexibly adjusted by controlling the magnitude of the dc current. Therefore, the machine performance is improved to cope with different operating conditions.