Abstract
The use of power Darlington transistors for an inverter of a 40-kVA uninterruptible power supply (UPS) system is discussed, and the storage time, dead time, and switching aid network contributions to the total harmonic content of the output voltage are examined. A review of common switching aid circuits, for nominal and short-circuit conditions, is presented, and their limitations are discussed. A solution, conceived specifically for power Darlington transistors, is presented. The solution that fits the most relevant characteristics of this technology is analyzed regarding commutation under nominal conditions, commutation under short-circuit conditions, and contribution to harmonic distortion in the output voltage. Conclusions are drawn about the optimal switching frequency for this type of application. The discussion and conclusions are supported by experimental and simulation results.

Abstract
A problem consisting of the optimal design of a segmented wall on a surface having a nonlinear slope contour is considered. It is formulated as an optimal control problem and a solution procedure based on the Discrete Maximum Principle is developed. An application to a real case, which previously had been solved by a Dynamic Programming based method, is included together with some critical comments on the above-mentioned technique.

Abstract
Proportional hazards modelling is a powerful technique which can be used to investigate the effects of various explanatory variables on the life length of equipment. In this paper the analysis of aircraft cargo door complaints is considered, as this can cause serious delay problems. Particular attention is given to the assumptions supporting the application of the model. Consideration is given to problems in the choice of the appropriate basic metric time, definition of covariates and adapting the methodologies to this particular problem. Emphasis is placed upon the validation of the results by attempting several different structures of the dependent variables. Copyright © 1989 John Wiley & Sons, Ltd.

Abstract
Fuzzy modelling of power systems is proposed, to take in account the qualitative aspects and vagueness or uncertainty that have not, in fact, a random nature and therefore cannot be modelled by a probabilistic approach. An operative procedure to derive fuzzy load diagrams associated with different types of energy consumption is presented. The feasibility of the development of fuzzy linear programming models for power system planning is demonstrated. A DC and an AC fuzzy load flow formulations are developed and the quality and usefulness of the results obtained with them is made evident with the help of an example.

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