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transformer engineering design and practice 2_phần 1

Có một số phương pháp chính xác được mô tả trong các tài liệu để tính toán trả lời quanh co điện áp xung, một số trong đó được thảo luận trong chương này. Căng thẳng điện trong vật liệu cách nhiệt bên trong và bên ngoài các cuộn dây thu được bằng các kỹ thuật phân tích hoặc số đó được mô tả trong chương kế tiếp. | 9 Cooling Systems The magnetic circuit and windings are the principal sources of losses and resulting temperature rise in various parts of a transformer. Core loss copper loss in windings PR loss stray loss in windings and stray loss due to leakage high current field are mainly responsible for heat generation within the transformer. Sometimes loose electrical connections inside the transformer leading to a high contact resistance cause higher temperatures. Excessive temperatures due to heating of curb bolts which are in the path of stray field can damage gaskets refer to Chapter 5 . The heat generated due to all these losses must be dissipated without allowing the core winding and structural parts to reach a temperature which will cause deterioration of insulation. If the insulation is subjected to temperatures higher than the allowed value for a long time it looses insulating properties in other words the insulation gets aged severely affecting the transformer life. There are two principle characteristics of insulation dielectric strength and mechanical strength. The dielectric strength of insulation aged in oil remains high up to a certain temperature after which it drops rapidly. At this point the insulation material becomes brittle and looses its mechanical strength. Thus it is primarily the mechanical strength which gets affected by the higher temperatures and aging which in turn affects the dielectric strength. Hence the dielectric strength alone cannot always be depended upon for judging the effect of temperature on the insulation 1 . Accurate estimation of temperatures on all surfaces is very critical in the design of transformers to decide the operating flux density in core and current densities in windings connections. It helps in checking the adequacy of cooling arrangements provided for the core and windings. It also helps in ensuring reliable operation of the transformer since the insulation life can be estimated under overload conditions and .