Considerations in application and selection of unit substationtransformers

This paper discusses secondary unit substations transformers (USTs) ranging from 300 up through 2500 kVA with 34.5-kV maximum primary voltages and with secondary voltages 600 V and below. Pad-mounted (compartmentalized) transformers are not considered in this paper. The first part of this paper discusses USTs belonging to two general categories, liquid type and dry type (nonliquid type). Liquid types include a discussion of the presently available insulating fluids: mineral oil, high-temperature hydrocarbon fluid, silicone fluids, and the new vegetable-based fluids. Dry types include a discussion of vacuum pressure impregnated polyester, vacuum pressure encapsulated silicone, partial cast, and full cast insulation systems. The National Electric Code (1999) requirements as they affect both liquid- and dry-type USTs for both outdoor and indoor locations are considered. The paper discusses the relative advantages of each UST within each category with respect to the following: (1) first cost; (2) operating costs (losses); (3) overload capability; (4) fan cooling; (5) space factor; (6) environmental factors; (7) sound levels; and (8) maintenance     

Effects of hydrolysis on the physical properties of short glass-fibre reinforced poly(ethylene terephthalate)

The mechanical fracture strength and toughness of short-fibre composites, injection moulded from compounds of poly(ethylene terephthalate) (PET) containing 10 and 30% (by weight) (w/o) glass, have been investigated and the dependence upon matrix hydrolytic stability determined. Mouldings have been characterised by several physical techniques to evaluate molecular weight, degradation rates, crystallinity and morphology, whilst time-dependent gravimetric data were derived to quantify sorption kinetics and allow comparisons with theoretical reaction rates to be made. During melt processing, PET is hydrolysed extremely rapidly by traces of moisture (<0.02w/o). yet the inherent strength of moulded composites declines significantly only below an apparently critical molecular weight. However, on long-term humid ageing in hot water, impact behaviour especially is rendered more complex by simultaneous crystallisation, molecular reorder and losses of interfacial bond strength.     

Half-bridge inverter using 4H-SiC gate turn-off thyristors

This paper reports on the first demonstration of a half-bridge power inverter constructed from silicon carbide gate turn-off thyristors (GTOs) operated in the conventional GTO mode. This circuit was characterized with input bus voltages of up to 600 VDC and 2 A (peak current density of 540 A/cm²) with resistive loads using a pulse-width modulated switching frequency of 2 kHz. We discuss the implications of the thyristor's electrical characteristics and the circuit topology on the overall operation of the half-bridge circuit. This work has determined the conservative critical rate of rise value of the off-state voltage to be 200 V/μs in these devices     

Contents

This article explains the basic issues that are facing the global community regarding the emission of carbon dioxide (CO/sub 2/) gas that is degrading our environment and, most significantly, the environment of generations to come. This article deals with the current state of a potential international agreement to curtail greenhouse gases and the subsequent underlying issues. A theory suggests that apparent reluctance to agree to an international treaty by the United States is directly tied to the process of electric utility reforms. The proposed Kyoto Treaty is explained as well as the consequences of its implementation. The current facts surrounding electric utility generation and CO/sub 2/ gas emissions are explained, as are some mitigation efforts.     

Theoretical analysis of control properties for the brushless doubly fed reluctance machine

The brushless doubly fed induction machine (BDFIM) has been extensively researched for approximately 30 years, but a related machine [the brushless doubly fed reluctance machine (BDFRM)], has not. This was mainly due to the fact that reluctance rotor designs were not capable of generating saliency ratios large enough to make the BDFRM competitive with other machines. However, recent developments in reluctance rotors, spurred on by research into synchronous reluctance machines, has resulted in high saliency ratio rotors that are economic to build. This, together with the promise of higher efficiency and simpler control compared to the BDFIM, means that further investigation of the BDFRM is warranted. A relatively limited amount of work to date has been published on the BDFRM. This paper attempts to fill this void by presenting a theoretical analysis of some of the important control properties of the ideal BDFRM.     

Performance and modeling of advanced Ti:LiNbO3 digitaloptical switches

High-performance Y-branch digital optical switches realized in Ti:LiNbO₃ are presented. Their switching response functions have been optimized in terms of switch voltage and crosstalk ratio. The optimization is based on analyzing different types of waveguide shaping and switching arrangements using coupled mode theory and computer simulations. Excellent switching characteristics are achieved with devices exploiting a specially shaped waveguide branch in a dilated switch arrangement. Demonstrated performances include switching voltage as low as 9 V with crosstalk suppression better than 45 dB and fiber-to-fiber losses as low as 4 dB. Polarization independence with crosstalk suppression better than 40 dB over a 1520- to 1570-nm wavelength range is achieved for any applied switch voltage greater than 18 V. These optimized digital optical switches have further demonstrated the capability to reshape electrical input signals at switching rates of several hundred megahertz