Nenad Mijatovic

Superconducting wind turbine generators

We have examined the potential of 10?MW superconducting direct drive generators to enter the European offshore wind power market and estimated that the production of about 1200 superconducting turbines until 2030 would correspond to 10% of the EU offshore market. The expected properties of future offshore turbines of 8 and 10?MW have been determined from an up-scaling of an existing 5?MW turbine and the necessary properties of the superconducting drive train are discussed. We have found that the absence of the gear box is the main benefit and the reduced weight and size is secondary. However, the main challenge of the superconducting direct drive technology is to prove that the reliability is superior to the alternative drive trains based on gearboxes or permanent magnets. A strategy of successive testing of superconducting direct drive trains in real wind turbines of 10?kW, 100?kW, 1?MW and 10?MW is suggested to secure the accumulation of reliability experience. Finally, the quantities of high temperature superconducting tape needed for a 10?kW and an extreme high field 10?MW generator are found to be 7.5?km and 1500?km, respectively. A more realistic estimate is 200?300?km of tape per 10?MW generator and it is concluded that the present production capacity of coated conductors must be increased by a factor of 36 by 2020, resulting in a ten times lower price of the tape in order to reach a realistic price level for the superconducting drive train.

Calculation of alternating current losses in stacks and coils made of second generation high temperature superconducting tapes for large scale applications

A homogenization method to model a stack of second generation High Temperature Superconducting tapes under AC applied transport current or magnetic field has been obtained. The idea is to find an anisotropic bulk equivalent for the stack such that the geometrical layout of the internal alternating structures of insulating, metallic, superconducting, and substrate layers is “washed” out while keeping the overall electromagnetic behavior of the original stack. We disregard assumptions upon the shape of the critical region and use a power law E–J relationship allowing for overcritical current densities to be considered. The method presented here allows for a computational speedup factor of up to 2 orders of magnitude when compared to full 2-D simulations taking into account the actual dimensions of the stacks without compromising accuracy.

Design Study of 10 kW Superconducting Generator for Wind Turbine Applications

We have performed a design study of a 10 kW superconducting slow rotating generator suitable for demonstration in a small scale wind turbine, where the drive train only consists of the turbine blades connected directly to the generator. The flux density in the superconducting rotor is chosen as B = 1 Tesla to be similar to the performance of permanent magnets and to represent a layout, which can be scaled up in future off-shore wind turbines. The proposed generator is a 8 pole synchronous machine based on race-track coils of high temperature superconducting tapes and an air cored copper stator enclosed in an iron shield.

Development of superconducting wind turbine generators

In this paper, the commercial activities in the field of superconducting machines, particularly superconducting wind turbine generators, are reviewed and presented. Superconducting generators have the potential to provide a compact and light weight drive train at high torques and slow rotational speeds, because high magnetic fields can be produced by coils with very little loss. Three different superconducting wind turbine generator topologies have been proposed by three different companies. One is based on low temperature superconductors; one is based on high temperature superconductors; and one is a fully superconducting generator based on MgB2. It is concluded that there is large commercial interest in superconducting machines, with an increasing patenting activity. Such generators are, however, not without their challenges. The superconductors have to be cooled down to somewhere between 4 K and 50 K, depending on what type of superconductor is employed, which poses a significant challenge both from a construction and operation point of view. The high temperature superconductors can facilitate a higher operation temperature and simplified cooling, but the current price and production volumes prohibit a large scale impact on the wind sector. The low temperature superconductors are readily available, but will need more sophisticated cooling. Eventually the Cost of Energy from superconducting wind turbines, with particular emphasis on reliability, will determine if they become feasible or not and for such investigations large-scale demonstrations will be needed.

Towards Faster FEM Simulation of Thin Film Superconductors: A Multiscale Approach

This work presents a method to simulate the electromagnetic properties of superconductors with high aspect ratio such as the commercially available second generation superconducting YBCO tapes. The method is based on a multiscale representation for both thickness and width of the superconducting domains. A couple of test cases were successfully simulated and further investigations were made by means of structured (mapped) meshes. Here, large aspect ratio elements were used to simulate thin material layers with a reduced number of elements. Hence, more complex geometries can be studied at considerable lower computational time. Several test cases were simulated including transport current, externally applied magnetic field and a combination of both. The results are in good agreement with recently published numerical simulations. The computational time to solve the present multiscale approach in 2D is estimated as two orders of magnitude faster than other 2D methods.

Design Study of Fully Superconducting Wind Turbine Generators

In this paper, two fully superconducting generators employing MgB2 armature winding, with YBCO and MgB2 field winding respectively, are presented and analyzed. The ac loss in armature winding is estimated, and a simple comparative study is carried out. The results show that both electromagnetic designs for fully superconducting generators are promising with respect to the power density. However, the cost of removing ac loss in armature winding is as high as

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

900 000. It is also noted that with the current price of YBCO tape, the generator employing MgB 2 field winding would have lower cost.

Superconducting generators for wind turbines: Design considerations

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 generators 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.

Finite-Element Model-Based Design Synthesis of Axial Flux PMBLDC Motors

The harmonic content of high temperature superconductors (HTS) field winding in air-core high temperature superconducting synchronous machine (HTS SM) has been addressed in order to investigate tendency of HTS SM towards mechanical oscillation and additional loss caused by higher flux harmonic. Both analytical expressions for flux distribution and current sheet distribution have been derived and analyzed. The two main contributors to the AC loss of HTS rotor winding are also identified and their influence addressed on general level.

Improved

This paper discusses the design synthesis of a permanent-magnet brushless dc (PMBLDC) machine using a finite-element (FE) model. This work differentiates itself from the past studies by following a synthesis approach, in which many designs that satisfy the performance criteria are considered instead of a unique solution. The designer can later select a design, based on comparing parameters of the designs, which are critical to the application that the motor will be used for. The presented approach makes it easier to define constraints for a design synthesis problem. A detailed description of the setting up of an FE-based design synthesis problem, starting from the definition of design variables, the FE model of the machine, how the design synthesis is carried out, and to how a design is finalized from a set of designs that satisfy the performance criteria, is included in this paper. The proposed synthesis program is demonstrated by designing a segmented axial torus PMBLDC motor for an electric two-wheeler.