Abridgment of effect of transient voltages on power transformer design

When an ordinary transformer is subject to transient voltage excitation, local concentration of voltage takes place in which the capacitance charging current of the coils to ground is supplied through the winding. This is because the ratio of inductance and capacitance of the various parts throughout the winding is not constant. Calculations and tests of voltage distribution in the winding, caused by the impact of (a) damped high-frequency oscillations, and (b) unidirectional traveling waves, are given. In order to make the analysis clearer, the transformer winding is considered as a network of inductances and capacitances, and this term “network” is used throughout the paper. Certain simplified and typical networks are considered. Transformers having one terminal grounded, such as are used in three-phase star connection, particularly in high-voltage systems, are frequently built with the insulation graded to other windings and ground, in the order of the normal frequency voltage stress. The danger of such a practise is shown in power transformers which are subject to transient overvoltage, since voltage oscillation in the winding may raise the voltage to ground at intermediate points above the terminal voltage, unless the design of the winding eliminates oscillation. The theoretical and experimental data given show that the distribution and magnitude of voltage stresses existing during recognized standard insulation tests are essentially different from stresses created by transient voltages. This permits the construction of transformers that would satisfactorily pass standard insulation tests but at the same time would not be suitable for average service. A new type of a transformer called “non-resonating,” for use on grounded neutral systems, is described. In transformers of this type, voltages of all frequencies distribute uniformly along the windings, as the possibility of internal voltage resonance is eliminated by a proper balance of distributed capacitance and inductance of the winding. This is accomplished principally by means of conducting surfaces (shields) placed outside of the winding and connected to its line terminal. The action of the shields is similar to that of the shielding ring on an insulator string. It neutralizes the effect of the capacitance current from the inside surface of the winding to ground, by supplying to every point of the winding a “charging” current equal to the “discharging” current of that point to ground. In some cases, the application of the shield reduces the local stresses to one-eightieth. Up to the present time, the total capacity of this new type of transformer exceeds half a million kva.