In traction applications based on long primary and short secondary type, contactless electrical energy transmission can offer distinct advantages over the conventional energy transmission based on ...catenary system to provide the required on-board power. In this paper, a linear brushless doubly fed machine with dual-primary windings and a reluctance secondary mover is proposed as a means of providing decoupled traction and on-board power. The machine primary contains two three-phase windings with different number of poles while an additional third winding is added around rotor saliencies forming a third output electric port to provide the required on-board power. A prototype machine is designed and simulated using 2D finite element analysis to verify the proposed concept.
Magnetic gearbox (MGB) is a contactless magnet-based transmission system that transmits mechanical power between two shafts. MGBs offer significant advantages over mechanical gears such as ...contactless power transfer, high gear ratios, inherent overload protection, high torque density, potential for high efficiency, and maintenance free. Cogging torque minimization is a challenging design aspect and stands as one of the parasitic demerits of MGBs. The proper selection of the combination of the number of pole-pairs of both high and low speed rotors can effectively minimize cogging torque magnitude. However, only some fixed gear ratios can be achieved. For any general desired gear ratio, the torque ripple magnitude may be unacceptable. This paper introduces a new technique for torque ripple reduction in MGB using the axial pole pairing technique, which is usually applied for torque ripple mitigation in conventional PM machines. A conventional 14/4 MGB, which normally results in a significant cogging torque magnitude, is designed and simulated using 3D finite-element analysis (FEA) for different pairing angles for both high and low speed rotors. Based on 3D FEA results, the required optimum pairing angle that optimizes torque transmission while minimizes the corresponding cogging torque is determined.
Volatility of firm performance is one measure of firm-specific risk that is priced in the marketplace. Because CEOs are typically undiversified investors in their companies stocks, the negative ...valuation effects of this risk are asserted to provide them the incentive to reduce earnings volatility. Furthermore, such an incentive is expected to increase in risk aversion. Two alternative measures of risk aversion are generated: one derives from wealth, and the other derives from CEOs choice of compensation structure as to the risky and safe components. The results consistently show a negative relationship between risk aversion and the volatility of each performance measure: earnings and operating cash flows. The results survived several tests of robustness. However, industry analysis shows that these results do not hold for certain industries such as public utilities. PUBLICATION ABSTRACT
Flywheels as energy storage systems are good candidates for numerous power system applications such as voltage support, serving fluctuating loads, frequency regulation and renewable energy ...utilization. Most of these applications utilize flywheel energy storage systems (FESSs) at medium voltage ranges. This paper discusses the optimization of the medium voltage high power converters interfacing the FESS with the grid. A dual three phase induction machine (IM) is proposed to drive the flywheel and is controlled via two series-connected three phase voltage source front end converters (VSCs). The proposed configuration is believed to reduce the converters ratings by a half compared to the conventional configuration where three-phase IM with single three-phase VSC is employed. The machine modelling and control are presented, in addition to converters DC links balancing technique. The performance of the proposed system is verified via simulation using MATLAB/Simulink.
DFIGs are reported to suffer serious reliability problems due to the presence of slip-rings, brushes and gearbox. These disadvantages encouraged several research groups to investigate the viability ...of employing single-frame or double frame Brushless Cascade DFIGs (BCDFIGs) in grid connected multi-megawatt wind turbines especially off-shore and in solar chimney power plants. In this regard, this paper tackles three main issues. Firstly, the expected reduction in gear ratio and maintenance frequency and cost by replacing DFIGs with BCDFIGs of the same power. Secondly, a detailed comparison is held between DFIG-WTs and BCDFIG-WT regarding their behaviour under unbalanced grid voltage. Finally, the paper studies to what extent a multi megawatt BCDFIG-WT is grid code compliant by its nature in terms of fault ride-through capability. This is illustrated by comparing and quantifying the response of a DFIG-WT and a BCDFIG-WT to a severe three phase voltage dip. Results indicate a promising behaviour of BCDFIGs during disturbances. (7 pages)
This paper proposes a recovery strategy for wind energy-fed voltage source converter high voltage DC transmission systems, capable of maintaining power balance between the AC and DC sides during ...different AC faults. This prevents the DC link voltage rise as a result of trapped energy. This reduces the voltage and current stresses on the switching devices. Also, the strategy ensures the converters remain connected to the AC network to provide the necessary voltage support, within the converter reactive power capability. The recovery strategy is implemented by introducing a flywheel energy storage system based on induction machine in parallel with the grid side inverter. Due to the parallel combination of the grid side inverter and flywheel induction machine (FW-IM), the trapped energy in the DC link during AC faults can be eliminated; as a result of finding another path for the power of the wind turbines during AC faults. To illustrate the feasibility of the proposed HVDC system, this paper assesses its dynamic performance during steady-state and network alterations, including its response to AC side faults. (6 pages)
Variable frequency transformer (VFT) has been recently used as an alternative to HVDC to control power flow between asynchronous networks. VFT consumes less reactive power than a back-to-back HVDC ...system, provides faster initial transient recovery, and has better natural damping capability. VFT is simply a doubly-fed induction machine (DFIM) where the machine torque controls the power flow from stator to rotor and vice versa. The main disadvantage of this VFT is the slip rings and brushes required for the rotor circuit, especially in bulk power transmission. The brushless doubly fed machine (BDFM) with nested cage rotor machine is proved to be a comparable alternative to conventional DFIM in many applications with the advantage that all windings being in the stator frame with fixed output terminals. In this paper, the BDFM is used as a variable frequency transformer (BVFT). A prototype machine is designed and simulated to verify the system validity. (7 pages)
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