[حمید]

عامل اصلي در شكل گيري نيروهاي مخرب اتصال كوتاه شار پراكندگي اطراف سيم پيچيهاي ترانس مي باشد. با افزايش مقاومت مغناطيسي هسته ،شار پراكنده افزوده شده و نيروهاي اتصال كوتاه كه متناسب با حاصل ضرب شار مغناطيسي در جريان اتصال كوتاه هستند افزايش مي يابد. در واقع داریم : F=K.Isc.? که در آن F نیروی مکانیکی مخرب ناشی از اتصال کوتاه، Isc جریان اتصال کوتاه و ? شار پراکندگی لینک کننده سیمهای حامل جریان اتصال کوتاه است که با کاهش مرغوبیت هسته متناسباً افزایش می یابد. Two conductors running in parallel and carrying current in the same direction are drawn together, producing a compressive force. This force arises as a result of the flux produced by the conductors themselves. However, the conductors of each winding are also acted upon by the leakage flux arising from the conductors of the other winding. The radial component of this leakage flux, which gives rise to the axial force, will act in one direction at the top of the leg and the other direction at the bottom. Since the current is in the same direction at both top and bottom this produces axial forces in opposite directions which, if the primary and secondary windings are balanced so that the leakage flux pattern is symmetrical, will cancel out as far as the resultant force on the winding as a whole is concerned. Any initial magnetic unbalance between primary and secondary windings, that is axial displacement between their magnetic centers will result in the forces in each half of the winding being unequal, with the result that there is a net axial force tending to increase the displacement even further. In very large transformers the designer aims to achieve as close a balance as possible between primary and secondary windings in order to limit these axial forces and he will certainly aim to ensure that primary and secondary windings as a whole are balanced, but complete balance of all elements of the winding cannot be achieved entirely for a number of reasons. One is the problem of tappings. Putting these in a separate layer so that there are no gaps in the main body of the HV when taps are not in circuit helps to some extent. However, there will be some unbalance unless each tap occupies the full winding length in the separate layer. One way of doing this would be to use a multistart helical tapping winding but, as mentioned above, simple helical windings placed outside the HV winding would be very difficult to brace against the outward bursting force. In addition spreading the tapping turns throughout the full length of the layer would create problems if the HV line lead were taken from the centre of the winding. Another factor which makes it difficult to obtain complete magnetic balance is the dimensional accuracy and stability of the materials used. Paper insulation and pressboard in a large winding can shrink axially by several centimetres during dry out and assembly of the windings. Although the manufacturer can assess the degree of shrinkage expected fairly accurately, and will attempt to ensure that it is evenly distributed, it is difficult to do this with sufficient precision to ensure complete balance.