The phenomena that take place in a transformer when it is turned on or when it is turned off are phenomena of an unsteady state, lasting only a fraction of a second. Despite the insignificant duration of these phenomena, their study is extremely necessary, since their consequences, if you do not take certain countermeasures, can damage the transformer or the devices included in its circuit. Without setting ourselves the goal of detailing the theory of the above-mentioned phenomena, we will further restrict ourselves to only the most important points of this theory.
The transformer included in the circuit when the secondary circuit is open is in everything similar to a conventional reactive coil with iron. Suppose first that the reactive coil does not have an iron core, and the active resistance of its winding is negligible and can be neglected. In a steady state, the magnetic flux of the reactive coil changes according to the basic law of electromagnetic induction: v = w * dF / dt * 10-8, where v is the instantaneous value of the applied voltage, w is the number of coil turns, dF is the change during dt of the magnetic flow. The total change in the magnetic flux for any period of time t, counted from the zero value of the flux, will be expressed the sum of changes over the same period of time and will be equal to
Фt = 0 / tdФ = 0 / tvdt / w × 10-8
The flux Фt is a flux that permeates the coil at time t. It is an integral function of the applied voltage. Therefore, if the voltage changes along a sinusoidal curve, then the magnetic flux will also change along a sinusoidal curve with a phase shift of 1/4 period. In an unsteady turn-on mode, the magnetic flux of the reactive coil changes according to the same basic law of electromagnetic induction, but the shape of the curves of its change in time depends on the moment the coil is switched on to the primary network. Suppose that the primary voltage changes along the sinusoidal curve V1 and the coil is turned on at the moment the voltage passes through the largest value (Fig. 1a).
A real transformer connected to the primary network with no load differs from the considered reactive coil in that it has a very high self-induction coefficient and has an iron core. The presence of iron significantly increases the turn-on current. Indeed, let the switch-on occur at the moment the voltage passes through zero. In this case, the magnetic flux should increase to double its steady state value. Consequently, the induction in the iron should double, which will lead to its strong saturation and high magnetic resistance. The latter circumstance results in an excessive increase in the magnetizing turn-on current.
In modern transformers, especially with artificial cooling, the magnetic circuit is taken with a high saturation, and therefore the inrush currents when turned on must be large. Oscillograms of the turn-on currents of modern transformers show that the current surges exceed the amplitude of the normal magnetizing current by a factor of 100-120. Since the normal magnetizing current is 5-10% of the normal load current, the inrush current at turn-on can exceed the normal load current by 8-12 times. Such currents are dangerous for devices included in the transformer circuit, and are undesirable for the network to which the transformer is connected. They are also undesirable for the transformer itself due to the mechanical forces that are obtained between the winding coils. Due to their short duration, these currents are not thermally dangerous. To illustrate what has been said about turning on the transformer, Fig. 4 shows oscillograms of the turn-on currents of one transformer, and the first oscillogram corresponds to the case of turning on when the voltage passes through the largest value, i.e. through its amplitude, and the second oscillogram is the case of switching on when crossing zero. In order to weaken the switching current, switches with so-called preliminary contacts are used, with the help of which, at the first moment, a large resistance is introduced into the transformer circuit, which is short-circuited with further movement of the knife of the switch. In addition to the transient current phenomenon, transient voltage phenomena occur when the transformer is turned on, which often lead to an excessive voltage increase between adjacent turns of the winding and between the transformer terminals. The reason for these phenomena lies in free oscillations arising in a circuit consisting either of the line capacitance and self-induction of the transformer itself, when the latter is switched on with the secondary winding connected to the line, or from the capacitance of the transformer itself and its self-induction, when one higher voltage winding is switched on, which has quite large capacity. By mathematical analysis of free oscillations, it is easy to show that these oscillations can be considered as the resultant of traveling waves with a steep front, moving along the circuit in opposite directions at a very high speed, and their mutual shift and the height of the front depend on when the voltage changes the transformer is turned on. A traveling wave, moving along the winding of the transformer, gives a voltage between the turn above which the wave front is at a given moment and the next turn, which significantly exceeds the voltage that exists between the turns in the steady state. If the transformer is turned on at the moment the mains voltage passes through the largest value (amplitude), then the height of the wave front, and therefore the voltage between adjacent turns, can reach the value of the mains voltage amplitude, i.e., dozens of times exceed the normal voltage between turns, equal to V / w, where w is the number of winding turns.
For low voltage transformers, which have a large margin of dielectric strength of the insulation compared to the voltage being serviced, such an overvoltage between the turns is not dangerous. It is dangerous for high voltage transformers, in which the insulation works closer to the breakdown voltage. A means of combating breakdowns from local overvoltages is to strengthen the insulation of the first turns of the winding and turn on a reactive coil in front of the winding. In addition to local overvoltage, traveling waves, under favorable conditions for switching on, can also give an overvoltage at the winding terminals, reaching double the normal voltage.