tailieunhanh - transformer engineering design and practice 1_phần 8

Thông lượng lạc tác động đến tiến hành các bộ phận (quanh co dẫn và thành phần cấu trúc) phát sinh các dòng xoáy trong đó. Những tổn thất lạc trong cuộn dây có thể được cao trong các máy biến lớn nếu kích thước dây dẫn và phương pháp chuyển vị | 6 Short Circuit Stresses and Strength The continuous increase in demand of electrical power has resulted in the addition of more generating capacity and interconnections in power systems. Both these factors have contributed to an increase in short circuit capacity of networks making the short circuit duty of transformers more severe. Failure of transformers due to short circuits is a major concern of transformer users. The success rate during actual short circuit tests is far from satisfactory. The test data from high power test laboratories around the world indicates that on an average practically one transformer out of four has failed during the short circuit test and the failure rate is above 40 for transformers above 100 MVA rating 1 . There are continuous efforts by manufacturers and users to improve the short circuit withstand performance of transformers. A number of suggestions have been made in the literature for improving technical specifications verification methods and manufacturing processes to enhance reliability of transformers under short circuits. The short circuit strength of a transformer enables it to survive through-fault currents due to external short circuits in a power system network an inadequate strength may lead to a mechanical collapse of windings deformation damage to clamping structures and may eventually lead to an electrical fault in the transformer itself. The internal faults initiated by the external short circuits are dangerous as they may involve blow-out of bushings bursting of tank fire hazard etc. The short circuit design is one of the most important and challenging aspects of the transformer design it has been the preferential subject in many CIGRE Conferences including the recent session year 2000 . Revision has been done in IEC 60076-5 standard second edition 2000-07 reducing the limit of change in impedance from 2 to 1 for category III above 100 MVA rating transformers. This change is in line with the results of many 231 .