The main obstacle to a wide use of dc-Decay tests lies in the necessity to align the rotor with the $d$- or $q$ -axis, which is very difficult to perform for high-power units. The goal of this paper is to propose and validate standstill dc-Decay measuring methods for a rotor in arbitrary position, which consist of generalizations of the existing methods with the rotor aligned with one axis. The first new method (dc-Decay I) delivers the equivalent circuit with a few measurements. The second new method (dc-Decay II) permits to determine the full equivalent circuit of both axes by performing only one measurement, which is unique and a breakthrough in parameter identification. The field current measurement enables obtaining the characteristic reactance $x_{c}$. A new method (dc-Decay III) is used to get the rotor angle. To extract the time constants and the reactances, an enhancement of the existing parameter identification algorithms using curve fitting is presented (used for the dc-Decay I method). Simulation and experimental results validate as good as possible the new methods and show their accuracy as well as their drawback.

This review article covers the multidisciplinary challenges shaping hydropower development within a broader energy transition. Classical hydropower generation facilities are moving toward flexible operations to adapt to and facilitate the integration of variable renewable energy sources (VRES). This review article explores the paradigm shift in terms of machine design considerations, power plant operation, and grid regulator implications. Machine designers, consider magnetic saturation, energy efficiency estimation, and mechanical wear and tear. This could involve, high flux densities in certain core regions, lower weighted average efficiencies, and higher machine failures owing to insulation degradation. Moreover, power plant operators must consider the effects of the flexible power production caused by VRES. In some cases, hydropower ramping ranges from 1 to 5 megawatts (MW) per second for large hydropower plants, which pose technical challenges for operators. In contrast, grid regulators must strengthen reactive power mandates and system-bearing services to ensure stability. In certain regions, there could be longer periods of 30 to 40 percent reduction in physical inertia owing to the lower share of traditional power plants in a VRES-dominated power system.

The end-winding region is usually overlooked in the roebel design of large AC machines. However, saturated stator slots cause overhang parts to impact the circulating currents significantly. In fact, a precise knowledge of the winding overhang strand inductances is crucial when optimising the transpositions of large roebel bars, especially in the case of under-roebeling (having less than 360-transposition in the active part) where the goal is to compensate the winding overhang parasitic field with the slot-parasitic field. In this paper, a rectangular inductance calculation model (R-ICM) is proposed. It results in a circuital lumped-element model (LEM) that takes the strand dimensions, the bar bending, as well as small-scale effects into account. Moreover, the work describes how to model finite current-carrying rectangular segments (straight and arced) from first principles. Finally, the methodology was demonstrated for two prototype specimens with different bending-shapes corresponding to the fundamental elements on a stator bar in the winding overhang of large electrical machines.

The computation of three-dimensional (3-D) magnetic fields is a demanding task in the analysis of electrical machines and other electromagnetic devices. In this context, integral field calculation provides a smooth solution, high precision and resolution, ”on-demand”- calculation, and an origin-based formulation of the magnetic field and the magnetic vector potential. However, conventional elliptic methods lead to huge parallelizable computing efforts and significant errors. In this paper, a 3-D generic current-carrying arc segment with rectangular cross-section is studied. A new analytic formulation is proposed to speed up the computation of magnetic fields and reduce the error by more than three orders of magnitude. In addition, the proposed magnetic vector potential expression has similar accuracy as numerical integration. In fact, a significant reduction of the error level has been showcased clearly with respect to existing approaches. The present work is promising for improving the design methodology and optimization of large superconducting dipole magnets or arched end-winding geometries of large electrical machines.

This paper presents the exact transient solution to the unbalanced and balanced faults in the doubly-fed induction machine (DFIM). Stator currents, rotor currents, and stator fluxes have been validated using simulation and experiment. The work is meant to strengthen and fasten the predictability of large DFIMs in the design stage to comply with mechanical constraints or grid fault issues. Moreover, the analytical approach reduces the computational costs of large-scale stability studies and is especially suited to the initial phase, where a of plethora design computations must be carried out for the DFIM before it is checked for its transient interaction with the power system. The possibility to dynamically estimate the DFIM performance is simplified by original equations derived from first principles. First, case studies of two large 265 MVA DFIMs are used to verify the analytical approach, and to justify the proposed "large machine approximation" using simulation with an exact match. Finally, laboratory measurements were conducted on a 10.96 kVA and a 1.94 kVA DFIM to validate the transient current peaks predicted in the proposed analytic expressions for two-phase and three-phase faults, respectively.

The Roebel bar conventional design of large AC machines uses the classical strand-slot inductance model (CSSIM). Suitable alternatives are missing as the CSSIM is favored for its inherent simplicity based on the ideally permeable iron core hypothesis. However, saturated armature slots can lead to high variations of the slot inductance, where the CSSIM cannot represent this precisely. An accurate prediction of the strand inductances is crucial when optimizing the transpositions of large Roebel bars to be competitive on efficiency and low measurement tolerances. This fact is crucial in under-roebeling, having less than a 360-degree transposition over the active part. In the end, the goal is to compensate the winding overhang parasitic field with the slot-parasitic field. This paper proposes a differential strand-slot inductance model (DSSIM) based on the concept of differential inductance (DI). It is compatible with a circuital lumped-element model (LEM) that considers the strand topology, geometrical dimensions, saturation level, and small-scale effects. Numerical simulations showcase the performance improvement of the DSSIM against known models. Finally, a 20-strand prototype of a slot model with actual Robel bar strands corresponding to a simplified bar cross-section in a large AC machine’s slot demonstrates the presented DSSIM’s precision.