Turbine, generator, system modeling and impact ofvariable-frequency ripple currents on torsional stressing of generatorsin Poland and Sweden: Lithuania/Poland and Sweden/Poland HVDC links

This paper analyzes rectifier ripple current superimposed on the direct current of a 600 MW asynchronous bipolar link between Lithuania and Poland and a 600 MW asynchronous monopolar link between Poland and Sweden in evaluating possible torsional vibrations in steam turbine-generator-exciter shafts. The studies are performed for machines in Northeastern Poland and in Southern Sweden close to the inverter station which are most at risk. Analyses are performed for the generators in Poland for the Lithuania/Poland link and for the generators in Sweden for the Poland/Sweden link. The paper examines system scaling factors for the Lithuania/Poland Link. It then examines AC system scaling factors and generator scaling factors for modulation product harmonic currents impressed on generators connected to the Swedish Grid Network by the inverter. Amplitude of shaft torsional torque due to resonant excitation imposed by the link on the generators in Sweden is evaluated for different scenarios of system and generator load. First, theory which relates magnitude of harmonic AC current injected into the AC system by the inverter is summarized. Dispersion of the injected harmonic AC current to proportion the current to individual generators is then reviewed, considering impedance of the generators, loads, and other system elements. Significance of simple and detailed shaft train models of machine shafts as well as approximate (Id, Iq) and detailed (2d, 3q) generator damper models on resonant torques is reviewed. Then, effects of both power factor and generator load on generator scaling factors is illustrated     

Russia/Germany HVDC Link: Analysis of System Scaling Factors for Evaluating Torsional Stressing of Generators on the Polish Grid System

This paper examines phenomena relating to the effect HVdc link ripple current superimposed on the direct current of an asynchronous link has on torsional stressing of steam turbine-generator-exciter shafts of generators in Poland in close proximity to a proposed 1GW HVdc inverter station. It makes an indepth analysis of dispersion of noncharacteristic harmonic alternating currents, which are injected into the Polish Grid Network by the inverter due to HVdc link ripple current in an asynchronous link. It also investigates the magnitude of ripple current at the inverter due to unbalanced bridge operation (as well as for balanced bridge operation) not analyzed in depth in the literature heretofore.     

Space-time overlays for convolutionally coded systems

We consider the design of space-time overlays to upgrade single-antenna wireless communication systems to accommodate multiple transmit antennas efficiently. We define the overlay constraint such that the signal transmitted from the first antenna in the upgraded system is the same as that in the single-antenna system. The signals transmitted from the remaining antennas are designed according to space-time coding principles to achieve full spatial diversity in quasi-static flat fading channels. For both binary phase-shift keying (BPSK) and quaternary phase-shift keying modulation systems, we develop an algebraic design framework that exploits the structure of existing single-dimensional convolutional codes in designing overlays that achieve full spatial diversity with minimum additional decoding complexity at the receiver. We also investigate a concatenated coding approach for a BPSK overlay design in which the inner code is an orthogonal block code. This approach is shown to yield near optimal asymptotic performance for quasi-static fading channels. We conclude by offering a brief discussion outlining the extension of the proposed techniques to time-varying block fading channels.     

Status, plans of action, further developments, and recommendations for power pools in Africa

This paper focuses on power pools in Africa: status, plans of action, further developments, and recommendations. First, the Southern African Power Pool (SAPP) formation, current status, and future challenges are addressed. Then the Central Africa Power Pool (CAPP), the economic community of Central African States (ECCAS), and the New Partnership for Africa's Development (NEPAD) is examined. CAPP is a new sub-regional institution, created in Brazzaville in April 2003, under the auspices of ECCAS that is presently the focal point for discussions on regional power markets, particularly as the Ingar hydropower complex when developed could generate close to 40 GW. Security issues of power system interconnections in Africa are then reviewed. Also discussed is the current status and developments of the West Africa Power Pool (WAPP) and the East Africa Power Pool (EAPP). Also considered is Hydropower and African Grid Development in relation to a rights-based perspective. Finally, the future of SAPP, CAPP, WAPP, and EAPP is examined.     

Impact of shaft torsionals in steam turbine control

Shaft torsional vibrations are discussed along with their effect on steam turbine-generator control following severe supply-network disturbances and the destabilization of shaft torsional modes through the action of high-speed electrohydraulic controllers. It is shown that shaft flexibility can exert a significant influence on steam turbine-generator response following a severe supply-system disturbance, particularly when the turbine has nonlinear valve stroking and fast valving. The effect can be minimized by the careful location of a speed sensor along the turbine shaft and by filtering speed or acceleration signals to reduce the speed input of troublesome low-frequency torsional vibrations to an insignificant level. The effect of shaft torsionals on control system response to an islanding situation and on the above mentioned destabilization is evaluated. Digital implementation of steam-turbine control systems in relation to quantization, sampling, response time in protection systems, and software integrity is examined     

Low-threshold proton-implanted 1.3-/spl mu/m vertical-cavity top-surface-emitting lasers with dielectric and wafer-bonded GaAs-AlAs Bragg mirrors

We demonstrate a new structure for long-wavelength (1.3-/spl mu/m) vertical-cavity top-surface-emitting lasers using proton implantation for current confinement. Wafer bonded GaAs-AlAs Bragg mirrors and dielectric mirrors are used for bottom and top mirrors, respectively. The gain medium of the lasers consists of nine strain-compensated AlGaInAs quantum wells. A record low room temperature pulsed threshold current density of 1.13 kA/cm/sup 2/ has been achieved for 15-/spl mu/m diameter devices with a threshold current of 2 mA. The side-mode-suppression-ratio is greater than 35 dB.     

Fatty acid composition of spanish peanut oils as influenced by planting location,soil moisture conditions,variety, and season

Nine varieties or strains of Spanish type peanuts were grown in the National Variety Test in Georgia and Oklahoma with and without irrigation for two growing seasons. The oil of sound kernels was analyzed for fatty acid composition. Although no consistent pattern was found, in general, variation in fatty acid composition due to variety was much less in Oklahoma than in Georgia for both the irrigated and nonirrigated peanuts. Also, location effect and soil moisture conditions gave higher percentages of palmitic and oleic with a lower percentage of linoleic in the Georgia samples and in the nonirrigated samples from both locations. Several significant first and second order interactions with the three major fatty acids involving variety, location, and soil moisture conditions were observed. Presented at the AOCS Meeting, New Orleans, May 1973. Journal paper 2707 of the Oklahoma Agricultural Experiment Station. ARS, USDA.     

Voltage dips due to direct connection of induction generators inlow head hydro electric schemes

This paper analyses phenomena which affect voltage dip due to direct connection of induction generators running close to synchronous speed to electrical distribution systems in low head hydroelectric power schemes. The paper examines transient phenomena on connection of induction generators at close to synchronous speed in determining whether costly system reinforcement may be avoided thus making many uneconomic developments viable while satisfying constraints of permissible voltage dip if the transient phenomena is taken duly into account. More use of renewable energy in electricity supply thus reducing greenhouse gas emissions from fossil plant will therefore result. The analysis is based on a detailed induction generator model with single and double rotor bar simulation using Park's equations with simulation of rotor dynamics on connection of the generator to the electrical supply