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Đối với thiết kế cách điện của một biến áp phù hợp cho tất cả các loại quá áp, điện áp nhấn mạnh trong các cuộn dây cần phải được xác định. Với mục đích này, phân phối điện áp trong các cuộn dây biến áp cho điện áp thử nghiệm cụ thể được tính toán | 7 Surge Phenomena in Transformers For designing the insulation of a transformer suitable for all kinds of overvoltages the voltage stresses within the windings need to be determined. For this purpose voltage distributions within the transformer windings for the specific test voltages are calculated. For AC test voltages of power frequency the voltage distribution is linear with respect to the number of turns and can be calculated exactly. For the calculation of the impulse voltage distribution in the windings they are required to be simulated in terms of an equivalent circuit consisting of lumped R L and C elements. There are a number of accurate methods described in the literature for computation of winding response to impulse voltages some of which are discussed in this chapter. Electric stresses in the insulation within and outside the windings are obtained by analytical or numerical techniques which are described in the next chapter. 7.1 Initial Voltage Distribution When a step voltage impinges on the transformer winding terminals the initial distribution in the winding depends on the capacitances between turns between windings and those between windings and ground. The winding inductances have no effect on the initial voltage distribution since the magnetic field requires a finite time to build up current in an inductance cannot be established instantaneously . Thus the inductances practically do not carry any current and the voltage distribution is predominantly decided by the capacitances in the network and the problem can be considered as entirely electrostatic without any appreciable error. In other words the presence of series capacitances between winding sections causes the transformer to respond to abrupt impulses as a network of capacitances for all frequencies above its lower natural frequencies of oscillations. When the applied voltage is maintained for a sufficient time 50 to 100 microseconds 277 Copyright 2004 by Marcel Dekker Inc. 278 Chapter 7 .
