CONTENTS & ABSTRACTS
In English. Summaries in Estonian
Proceedings of the Estonian Academy of Sciences.
Volume 8 No. 4 December 2002
Solving line balancing problems with model checking; 211–222
Jüri Vain, Ingmar Randvee, Tiit Riismaa, and Juhan Ernits
Abstract. A two-step technique for solving large-scale assembly line balancing problems is proposed. In the first step, an approximate global solution is found to the task assignment problem using the algorithmic branch-and-bound method. In the second step, the workstations with critical workload are selected and their load recalculated using local models of finer granularity. The workstation models in the second step are defined as parallel compositions of timed automata to which the parametric model checking procedure can be efficiently applied. An illustrative example is given.
Key words: flexible manufacturing, behavioural modelling, timed automata, line balancing, model checking.
Feed-forward neural networks for smooth operation of the high voltage power transmission network; 223–247
Abstract. The article describes algorithms based on feed-forward neural networks for short-term forecast of high-voltage power consumption and fault prediction, implemented in the software “Event Navigator” of Eesti Energia AS.
Key words: high-voltage power transmission, fault prediction, short-term forecast.
Evaluation of the Gurson–Tvergaard material model by using damage mechanics and thermomechanics; 248–269
Abstract. Gurson–Tvergaard material model has been evaluated by using damage mechanics and thermomechanics. The evolution law of Tvergaard for the void volume fraction is rigorously derived. It follows that the modification for rate of the void volume fraction by Chu and Tvergaard and Tvergaard and Needleman cannot be accepted since it violates the axiom of conservation of mass. An extension of the Gurson–Tvergaard model has been derived. Using the extended model it has been shown that under compression there are stress states by which the Gurson–Tvergaard material model does not satisfy the Clausius–Duhem inequality.
Key words: material model, Gurson–Tvergaard model, porous materials, damage mechanics, thermodynamics, thermomechanics.
Simultaneity of the wind power in Estonia; 270–275
Abstract. Analysing synchronous wind speed and power databases at six characteristic sites in West and North Estonia, the simultaneous wind power on the Estonian coastal line is studied. Estonia is big enough not to be covered by the calm totally, but about 33% of the time wind turbines over the whole Estonia would be performing at an insignificant capacity. The wind energy resource in West Estonia is nearly twice bigger than that in North Estonia due to favourable wind conditions on islands. If wind on islands is not employed (due to environmental restrictions or lack of electrical lines), no big effect can be expected from wind utilization in the rest of the Estonian territory.
Key words: wind energy, geographical variation of wind.
Influence of wind speed gusts on power generation; 276–285
Teolan Tomson and Maire Hansen
Abstract. Based on the analysis of the Estonian offshore and onshore wind database, the boundaries of the probability of wind speed gusts and lulls, which lead to disturbances in the electrical grid, are assessed. These boundaries are assigned to the probability distribution function (pdf) of wind speed gusts and lulls on one hand and to the pdf of power deflections on the other. The probability of electrical disturbances remains between these boundaries. For the evaluation of the pdf of gusts and lulls, standard deviation of the wind speed can be used.
Key words: wind speed gust (lull), wind power gust (lull), disturbances in the electrical grid.
Olav Aarna 60; 286–287
Contents of volume 8; 288