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The paper presents mathematical model - TOPSIS method, which was utilized on insulating state of distribution transformer to analyze and sensibility of individual measurements methods mutual comparison. We can uniquely determine the importance of these measurements methods with this mathematical apparatus in these measurements methods in insulating state of transformers.

This article is focused on the theory of unacceptable effect on transformer winding, mainly the overvoltages and the mechanical forces. In the next part of the article is described the measurement of selfdischarging voltage frequency dependence on transformer. The second measurement shows one of the possible type of SFRA method. The measurement results lead us only to the conclusion that some changes of the winding condition have occurred.

Power transformers are key equipment for transfer and distribution of the electric power. Considering the significance of the power transformers in the electric system, their price and possible damages occurred by accidents, it is necessary to pay attention to their higher prevention. To prevent failure states of transformers, we perform different types ofmeasurements. They shall illustrate a momentary state of the measured equipment and if necessary to draw attention in advance to changes of parameters, which have specific relationship to no-failure operation of the equipment. The conditions under which breakdown of composite liquid/ solid insulation can occur, e.g. in transformer, play an important role in designing such insulation. The liquid, mainly mineral oil, generally constitutes the weakest part of insulation and a great amount of work has been devoted to the study of streamers, which appear in the gaseous phase, and most often are triggering the failure of insulation.

The fundamental characteristics and properties of SF6 and its mixtures with N2 and CO2 are given. Current wave-forms of initial stages of negative corona discharge formation in a short negative point-to-plane gap have been measured with nanosecond time resolution in the mixtures N2 + SF6, at the pressures 50 kPa and for various contents of SF6 and overvoltages. Current pulses in this mixture with low concentration of SF6 are described using the streamer-based theory.

In the article the diagnostic and measuring systems dedicated for complex output tests of power transformers aswell as their diagnostic is dcscribcd. The aim of research in this area was to elaborate the problem of so-called open loop measuring system controlled by PC. The attribute "open" means the possibility to adapt the system for different electric equipment, different measurands and an zdaptation of the way of monitoring, evaluation and distribution of output information according to specific requirements the controlled transformer.

The windings of power transformers must be designed so, that the effect of short-circuits currents, which can emerge in an operation, will not start up on them the destruction or the deformation of the mechanical or thermal character. Except for permanently deformation results of the effects of short-circuit current come to also by correct ditnensioning of electric equipment to progressive ageing. Which can make worse his mechanical properties. The article deals Ivith the diagnostic method. Which enables to anticipate such state.

This article deals with a description of methods of an experimental analysis concerning the actual condition of windings and magnetic circuit of the autotransformer, which is required in power transmission companies all over the world.