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Comparative analysis of dry and oil transformers

09.09.2009

 Comparative analysis of dry and oil transformers

     Lately consumers' interest in application of dry transformers has considerably grown.

     Certainly, in some cases the choice of substation variant with dry transformer is the only one. It happens when there is no other possibility on the object for substation installation – only inside a building, for example, in industrial shop, in socle room or on the floor overhead cover of the dwelling house or on the territory of the educational institution, in park zone etc. so in such places where requirements to transformer structure are determined by heightened requirements concerning ecological and fire safety.

     On the other hand, transformer even dry one is a kind of equipment of high voltage and that is why it can be considered as an object of heightened danger. So transformer is always installed inside package transformer substation in the enclosed cabinet. In such substation there are also switchgears of low and high voltage inaccessible from outside by means of which the required electrocution safety level is provided. It is worth mentioning that any touch to cast winding of on dry transformer leads to electrocution, thus, in case of protection absence dry transformer has larger electrocution area than oil one.

     Now people say that the use of dry transformers considerably saves time for electrical installation service. Concerning general timetable for electrical installation service one of the most important processes is periodic inspection. If we study the information of the standard chart of substation inspection (inspection intervals are different depending on local conditions but at least such inspection is carried out once a quarter) we will understand that the scope of work during substation inspection practically does not depend on the structure of the used transformer. Inspection chart taken as an example at one of the plant of “Interregional Power Supply of Siberia” is applied. Probably, in case of dry transformer installation only three items of the section “Transformer” are excluded (1, 3 and 5), all others are necessary to be processed.

     It is well-known that considerable part of all damage of distributional transformers happens because of weakening of bus rod junction on vent external low-voltage contacts and independently if it is dry or oil transformer these contacts demand regular maintenance. If we take oil transformer contact overheating leads to insulator damage or sealing depressurization but in any case the repair is not a serious problem. In case of dry transformer such kind of damage (because of less effective cooling of overheating area and intensive transmission of heat in conductor to winding) leads to cast insulation damage in the area of wires from winding. As a result there is the necessity to change winding block on the damaged phase. Certainly it is possible to take measures improving the quality of current collection but at the same time even if there are clamps with which all oil transformers of OJSC “Alttrans” with the power of 160 kVA and more are supplied, these units should be regularly inspected. In case of dry transformer it is all the more.

     Despite deep vacuum usage in the process of manufacturing of solid winding for dry transformers there exists coefficient difference of heat volume expansion of conductor materials and cast insulation. This is the reason why winding blocks of dry transformers during operation experience micro damage that results in the so-called partial discharges. What is partial discharge and what is its danger? Micro rack makes boundary contact of two different dielectric mediums, so in this area heightened strength of electric field arises. Such heightened strength leads to continuous disruption within micro rack. Insulation carbonization happens gradually in this point and then transforms into turn-to-turn and interlaminar short circuit resulting in winding burning. This process can develop week after week or even month after month but in reality it is unnoticeable up to the breakdown itself. Partial discharges can be revealed only with the help of special devices. The problem of control of partial discharge absence in distributional transformers appeared together with dry transformers appearance. In oil transformers partial discharges can theoretically appear if there are micro air bubbles somewhere in paper-oil insulation but due to degassing technology in the process of oil preparation and filling oil transformers as a rule do not experience such kind of damage.

     Insulation defects in dry transformers can also arise in case of violation of admissible climatic conditions of their application. Here we take into consideration not only transformer operational conditions but transformer storage conditions before its installation. Actually, lower temperature limit of dry transformer application equals -25˚C.

     As a conclusion it is worth mentioning that in every individual case it is necessary to consider equipment real operational conditions thus providing the best combination of transformer engineering-and-economical performance and its optimal choice.

Technical adviser: V. V. Razumov



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