Contents I. Introduction
Wind energy drivetrain systems must be more compact and more efficient and must reduce the overall volume, weights, and cost [1]– [7]. It has been well demonstrated recently that the levelized cost of energy for two wind turbines with 3 MW is higher than that for one turbine with 6 MW [8]. This suggests that there is a trend to increase the wind turbine unit power per wind farm, reducing the overall cost of the wind farm, specifically on offshore wind systems. In the last decade, there were many significant technological improvements in the electrical generators applied to wind turbines, including different technologies such as wound rotor synchronous generators and doubly fed induction generators combined with a multiple-stage gearbox. Recently, there has been a tendency to apply the direct drive technology to large offshore wind turbines, removing the gearbox from the drivetrain [9]– [15]. The direct drive technology requires a lower rotational speed, less than 20 r/min, and consequently, for large power, mechanical torque is in a range of tens of millions of newton meters. Additionally, the air-gap shear stress of large conventional liquid-cooled electrical machines rarely exceeds 100 kNm/m 3 [16]. For this reason, the standard radial dimensions of conventional wind generators for offshore applications are very large [18], [19]. At present, most of the large electrical power generators for wind energy applications are installed with a direct drive system or with a single-stage gearbox [1], and the majority of conventional large-power synchronous generators installed with a direct drive system use rare earth permanent-magnet technology to increase the overall air-gap shear stress of the electrical generator [1], [15], consequently reducing their total volume. Another alternative to increase the air-gap shear stress is the use of high-temperature superconductor synchronous generators (HTSSGs) [19]. HTSSGs can work with second-generation high-temperature superconductors (2 GHTSs) in the inductor part, allowing a high magnetic induction in the air gap and consequently a high shear stress two to four times higher than the conventional shear stress of a wind turbine electrical generator [19]– [25]. For these reasons, 2 GHTS could be a great opportunity to be applied to large wind turbine electrical generators, increasing the megawatt-per-wind-turbine unit without increasing the radial and axial main dimensions and overall weights compared with conventional electrical wind generators by using their higher high-temperature superconductor (HTS) current density [20], [26]. The motivation of this paper stems on the research of the innovative configurations of wind turbine electrical generators using HTSs at extremely air-gap magnetic flux density conditions (∼2.5 T) and temperature (∼30 K) in order to highly reduce the radial and axial dimensions. The first step to demonstrate the electromagnetic feasibility of a 15-MW HTSSG using 2 GHTS technology after an analytical calculation approach goes through a 2-D FE electromagnetic analysis (TDFEEA) using electromagnetic commercial software tools such as FLUX 10.4.1 from CEDRAT and thermal lumped-element circuits commercial tools specifically for electrical machines such as Motor-CAD v7.3 from Motor Design Ltd. The TDFEEA design of a 15-MW HTSSG for offshore wind energy includes different electrical studies such as a static current test, a no-load test, and a rated-load test. Before this analysis, 2 GHTS parameterization using the power law formulation [27]– [29] and Kim Anderson formulation [30], [31] for different operational temperatures and external magnetic fields was performed using the SCS12050 from SuperPower Inc. due to its suitable dimensions and thermal and magnetic properties [32]. By using the formulation described in (1) and (3), thermal and magnetic parameters were identified. Therefore, reducing the 2 GHTS tape critical current density in order to take into account the 2 GHTS coils' critical current and quench margin reduction, the model is applied to each 2 GHTS coil of the HTS direct drive synchronous generator (HTSDDSG). Then, injecting an excitation voltage per phase, the appropriate electromotive force in the armature is reached.
