Influence of pole-pair combinations on the characteristics of the brushless doubly fed induction generator

The brushless doubly fed induction generator (BDFIG) is an alternative to the doubly fed induction generator (DFIG), widely used in wind turbines which avoids the need for brush gear and slip rings. The choice of pole numbers for the two stator windings present in the BDFIG sets the operating speed, typically in the medium speed range to eliminate a gearbox stage. This paper focuses on how both the total number of poles and the assignment of poles between the windings affect machine performance. Analytical expressions have been developed for parameters including pull-out torque, magnetizing current and back-iron depth. The results show that the pole count can be increased without unduly compromising pull-out torque and that in cases where more than one combination of pole number is acceptable only the back iron depth is significantly affected. In addition an output factor has been introduced to enable a direct comparison to be made with conventional DFIGs. The torque density of a brushless DFIG is compromised to a degree relative to a comparable DFIG as a consequence of the presence of two magnetic fields and finite element analysis is needed to achieve an optimized design. Finally, predictions of the performance of multi-MW machines are made based on data from an existing 250 kW machine which show that suitable efficiencies can be obtained and excessive control winding excitation can be avoided

A study of rotor eccentricities effects on brushless doubly fed machines performance

This paper studies the unbalanced magnetic pull (UMP) in the Brushless Doubly-Fed Machine (BDFM). Analytical study is performed to derive the UMP, then, Finite element (FE) analysis, which has been verified experimentally, is used to verify the analytical method. The BDFM with different types of rotor eccentricities including static and dynamic eccentricities, are also modeled in FE method and their resultant UMPs are obtained. The results are compared with the case at which a perfectly constructed rotor is considered. The study has been carried out on a prototype D400 250 kW BDFM

A Light-Weight Rotor Design for Brushless Doubly Fed Machines

In this paper, a new optimized rotor design for Brushless Doubly Fed Machines (BDFMs) is proposed. The BDFM is considered as an attractive generator particularly for offshore wind power generation and also as a replacement for doubly fed slip ring induction generators. This is due to its higher reliability and lower maintenance reported in the literature. It is shown in this paper from the study of the magnetic field distribution in the rotor core that the conventional design of the BDFM rotor iron core can be modified, leading to a lighter machine. The proposed design method is supported by an analytical study and its practicality is validated using 2-D Finite Element (FE) analysis. A 250 kW experimental BDFM with frame size D400 is considered as the prototype machine

Numerical Analysis of Stator Magnetic Wedge Effects on Equivalent Circuit Parameters of Brushless Doubly Fed Machines

This paper studies the effects of magnetic wedges used for closing stator open slots on the Brushless Doubly Fed Machines' (BDFM) equivalent circuit parameters. The BDFM is an attractive generator solution for offshore wind power and can replace doubly-fed slip-ring induction generators. It is shown in this paper that the use of magnetic wedges, commonly used in large induction machines, reduces the stator windings magnetising currents, reflected in the values of magnetising inductances. But they also increase the leakage flux of the stator windings and hence change the series inductance in the equivalent circuit. The series inductance significantly affects the machine performance as well as the rating of its converter. 2-D Finite element analysis of a 250 kW experimental BDFM is used to investigate the effects of magnetic wedges on the machine's magnetic field distribution and how these can alter the machine's parameters values. Experimental tests have also been carried out to validate the analysis

Impact of Monovalent Metal Halides on the Structural and Photophysical Properties of Halide Perovskite

This chapter discusses the importance and impact of metal halide additives into perovskite to enhance its semiconductor quality and realize highly efficient and stable perovskite photovoltaic devices. Monovalent metal halides have been introduced as the most promising candidates due to their loading capacity and chemical compatibility with the perovskite materials, as well as ease of incorporation and their remarkable positive impact on the crystal growth, optoelectronic properties, and subsequently the performance of perovskite solar cells (PSCs). Among all the monovalent metal cations, Cs is the only one that could fit in the perovskite structure and forms photoactive perovskite. The other monovalent cations are located at the interstitials sites, grain boundaries, and crystalline surfaces. We also discuss the key roles of monovalent metal halide additives that include modulating morphology of perovskite films, modification of structural and optoelectronic properties, adjusting energy level alignment in PSCs, inhibiting non-radiative recombination in perovskites, eliminating hysteresis, and enhancing operational stability of PSCs

A new optimized rotor design for brushless doubly fed machines

The brushless doubly fed machine (BDFM) is an alternative to the doubly fed induction generator, widely used in wind turbines, without use of brush gears and slip rings. Rotor design is important for designing an optimal multi-MW BDFM. To date, nested-loop rotors have been extensively used in various BDFMs but they may not be suitable for larger machines. In this paper, different methods of BDFM rotor equivalent circuit parameters determination are presented and validated by experimental tests. Then, a design optimization of BDFM rotors is proposed based on equivalent circuit analysis with the aim of minimizing the rotor parameters. Two optimized rotors, one bar cage rotor and one nested-loop rotor were designed and built from the outcomes of the optimization method for a BDFM with frame size D180. The characteristics of the conventional and optimized rotors in terms of the rotor equivalent circuit parameters and iron saturation at rated operating conditions are compared using analytical and Finite Element Analysis (FEA) methods

Mesoporous TiO₂ electrodes with different thickness for dye sensitized solar cell application

Mesoporous TiO2 films with different thicknesses were prepared for dye sensitized solar cell application using dip coating method. The crystal structure and morphology of the films were studied by scanning electron microscope and X-ray diffraction. The optical properties of the films were investigated through UV–Vis absorption. With increasing film thickness from 3.1 to 13.9 μm, the efficiency increases from 0.81 to 3.09 %

Chemical Modifications and Passivation Approaches in Metal Halide Perovskite Solar Cells

This dissertation describes our study on different physical properties of passivated and chemically modified hybrid metal halide perovskite materials and development of highly efficient charge transport layers for perovskite solar cells. We first developed an efficient electron transport layer via modification of titanium dioxide nanostructure followed by a unique chemical treatment in order to have clean interface with fast electron injection form the absorber layer in the perovskite solar cells. We then explored monovalent cation doping of lead halide perovskites using sodium, copper and silver with similar ionic radii to lead to enhance structural and optoelectronic properties leading to higher photovoltaic performance of the resulting perovskite solar cells. We also performed thorough experimental characterizations together with modeling to further understand the chemical distribution and local structure of perovskite films upon monovalent cation doping. Then, we demonstrate a novel passivation approach in alloyed perovskite films to inhibit the ion segregation and parasitic non-radiative losses, which are key barriers against the continuous bandgap tunability and potential for high-performance of metal halide perovskites in device applications, by decorating the surfaces and grain boundaries with potassium halides. This leads to luminescence quantum yields approaching unity while maintaining high charge mobilities along with the inhibition of transient photo-induced ion migration processes even in mixed halide perovskites that otherwise show bandgap instabilities. We demonstrate a wide range of bandgaps stabilized against photo-induced ion migration, leading to solar cell power conversion efficiencies of 21.6% for a 1.56 eV absorber and 18.3% for a 1.78 eV absorber ideally suited for tandem solar cells. We then systematically compare the optoelectronic properties and moisture stability of the two developed passivation routes for alloyed perovskites with rubidium and potassium where the latter passivation route showed higher stability and loading capacity leading to achieve substantially higher photoluminescence quantum yield. Finally, we explored the possibility of singlet exciton fission between low bandgap perovskites and tetracene as the triplet sensitizer finding no significant energy transfer between the two. We then used tetracene as an efficient dopant-free hole transport layer providing clean interfaces with perovskite layer leading to high photoluminescence yield (e.g. ~18%). To enhance the poor ohmic contact between tetracene and the metal electrode, we added capping layer of a second hole transport layer which is extrinsically doped leading to 21.5% power conversion efficiency for the subsequent solar cells and stabilised power output over 550 hours continuous illumination

Experimental Analysis of Noise and Vibration for Large Brushless Doubly Fed Machines

In this work, a large-scale Brushless Doubly Fed Machine (BDFM) built in frame size D400 is tested at different operating conditions in order to examine its noise and vibration levels. A series of measurements were conducted on the machine to establish the main modes of excitation of vibration and noise developed by the machine. The harmonic spectrums of the vibration amplitudes are presented at two different rotor speeds at which the vibration level is highest, in order to determine the vibration components frequencies existed in the vibration spectrum. A harmonic analysis of the BDFM is then proposed in order to assess the sources of vibration in the machine