It is known that industrial transformers are used to supply systems at low voltage (around 1000 V) in metallurgy and chemistry. Transformers of this kind are characterised in that they have a very wide adjustment of the output voltage which in certain cases can be as much as 100%.

Power transformers are instead used in electric power generation stations, in voltage transformer substations and in connections between lines at different voltages. These transformers have a limited adjustment (approximately 10- 20%) on the low-voltage side or on the high-voltage side.

The power levels to be transferred from the lines to the systems or between different lines across the transformers have reached very high values, on the order of hundreds of MVA.

Voltage adjustment, which is generally of the on-load type and in a few cases of the off-load type, is performed mechanically, by means of a motor or manually.

Provision of mechanical adjustment requires: -switches which are very expensive, due both to the high currents and to the large number of positions in order to minimise the voltage step for the secondary voltage; -very bulky and complicated stepped adjustment windings for the transformer.

The large number of positions required entails the need to resort to adjustment windings, which have a very small number of turns per voltage step, making it very difficult to manufacture this kind of windings.

A consequence of this is the fact that: 1) voltage adjustment is always performed in steps and therefore discontinuously and, in some solutions, nonlinearly; 2) the transformer has a high cost, since it is bulky and complicated; 3) pollution is produced due to the mineral oil of the transformer, which decomposes during switching; 4) the system is highly unsafe to operate, consequently reducing its efficiency.

In summary, the complexity and the extent of the adjustment windings considerably increase the cost of the transformer and reduce its life and therefore the efficiency of the system. This is due to the room, which they occupy, to their dielectric sensitivity during operation, with a high probability of disruptive discharges, and to the mechanical sensitivity to pulsed currents, such as starting and load currents and short-circuit currents.

A fault inside the transformer (active part or switch part) entails placing said transformer offline for the entire period of repair, which can be considerably long.

In an operating cycle in which a transformer is used it is possible to distinguish two critical steps for said transformer; a) powering-up, due to the very high starting currents; b) the opening of the load circuit, with disconnection from the voltage of the power supply line due to the over-voltages generated by the disconnector, which can trigger resonance phenomena inside the transformer, with possible disruptive discharges, especially in industrial transformers.

The aim of the present invention is to provide a transformer with electronic adjustment of secondary voltage, which allows reducing the costs of the transformer.

Within the scope of this aim, an object of the present invention is to provide a transformer with electronic adjustment of secondary voltage, which allows

reducing the risk of faults inside the transformer itself.

Another object of the present invention is to provide a transformer with electronic adjustment of secondary voltage, in which the electronic adjustment can be adapted both to industrial transformers and to power transformers.

Another object of the present invention is to provide a transformer with electronic adjustment of secondary voltage, which is relatively simple to provide.

Thus, the present invention provides a transformer with electronic adjustment of secondary voltage, comprising a primary winding and a secondary winding, said primary winding being connected to the supply voltage. The transformer, according to the present invention, is characterised in that it comprises a voltage regulator circuit, said voltage regulator circuit being associated with at least one alternate adjustment winding and being suitable to generate an adjustment voltage, which is meant to be added to, and/or subtracted from, a sinusoidal voltage generated by said secondary winding.

Further characteristics and advantages of the transformer, according to the present invention, will become apparent from the description of preferred but not exclusive embodiments of the transformer according to the present invention, illustrated only by way of non-limitative example in the accompanying drawings, wherein: Figure 1 is a block diagram of a first embodiment of the transformer according to the present invention; Figures la-lc are charts, which plot two cases of adjustment voltage subtraction and one case of adjustment voltage addition; Figure 2 is a block diagram of a second embodiment of the transformer according to the invention; Figure 3 is a block diagram of a third embodiment of the transformer according to the invention;

Figure 4 is a block diagram of a fourth embodiment of the transformer according to the invention; Figure 5 is a block diagram of a fifth embodiment of the transformer according to the invention; and Figure 6 is a block diagram of a sixth embodiment of the transformer according to the invention.

With reference to the figures above mentioned, the transformer with electronic adjustment of secondary voltage, according to the present invention, comprises, in a first embodiment shown in Figure 1, a primary winding 1, which is connected to the mains voltage VI. The transformer, according to the present invention, further comprises a secondary winding 2, at low voltage, and a voltage regulator circuit 3, which is associated with at least one adjustment winding. Preferably, the adjustment is achieved by means of an adjustment transformer 4 (booster), which comprises an auxiliary primary winding 5, which is the adjustment winding, and an auxiliary secondary winding 6, which is series-connected to the secondary winding 2.

The auxiliary primary winding 5 (adjustment winding) is directly connected to the power supply line and it is therefore in parallel to the primary winding 1.

The voltage regulator circuit 3 comprises two voltage regulators, designated by the reference numerals 7 and 8 respectively, which are series-connected to each other and are parallel-connected to the primary winding 1. The auxiliary primary winding 5 is parallel-connected to the regulator 8. The voltage regulators can be constituted, for example, by two thyristors in opposition having an adequate power handling capacity.

The current 11 flows in the primary winding 1, which is powered by the voltage VI. The current I2 flows in the secondary winding 2 and the adjusted voltage V2 is present at the terminals of the series circuit comprising the secondary winding 2 and of the auxiliary secondary winding 6.

In the first embodiment of the transformer, shown in Figure 1, the adjustment transformer 4 supplies an adjustment voltage, also of the sliced sinusoidal type.

Said adjustment voltage is added in phase, and/or in phase opposition, to the sinusoidal base voltage outside the main transformer (formed by the primary winding 1 and the secondary winding 2).

By acting on the gate terminal of the regulators 7 and 8 respectively, with the delay angle comprised between 0 and 180°, the adjustment voltage produced by the adjustment transformer 4 can be added to, or subtracted from, the sinusoidal base voltage supplied by the main transformer. This addition can be performed continuously, with a continuous sinusoidal form, for a delay angle equal to 0, or discontinuously, with a discontinuous sinusoidal form, for a delay angle greater than 0.

In this case, one obtains a voltage addition, which forms a rounded step and therefore the sinusoidal voltage produced by the main transformer has, at a given position along the sinusoid, a voltage step which is formed by the adjustment voltage supplied by the adjustment transformer 4.

The maximum, medium and minimum voltages are perfectly sinusoidal, while the intermediate voltages have a rounding, which depends on the delay angle with which one acts on the gate terminal of the regulators and on the extent of the adjustment voltage.

If the extent of the adjustment is divided into multiple steps, one obtains an increase in the number of positions with sinusoidal voltages, a continuous gradual adjustment from the sinusoidal position to the next one with an increasingly smaller rounding so as to avoid creating any appreciable harmonic content.

Figures la-lc plot the waveforms of the voltages in output from the transformer, wherein Vris is the resultant voltage, Vmx is the maximum voltage, Vmd is the medium voltage, Vmin is the minimum voltage and Vrg is the alternating

adjustment voltage, which can be added or subtracted. Figures la and lc illustrate the case in which the adjustment voltage Vrg is subtracted, while Figure lb illustrates the case in which the voltage is added. All three cases show the delay angle with which one acts on the terminals of the voltage regulators (the delay angle is designated by cor). Figure lb also plots the waveform of the current I, which is not in phase with the voltage V.

Figure 2 illustrates a second embodiment of the transformer according to the invention, in which identical reference numerals designate identical elements.

This applies to the subsequent figures as well.

With reference therefore to Figure 2, the difference between the second embodiment and the first embodiment of the transformer according to the invention consists of the fact that an excitation winding 10 is provided, which is series-connected to the pair of regulators 7 and 8. The circuit shown in Figure 2 is used, for example, in the case of high supply voltages.

Figure 3 illustrates a third embodiment of the transformer according to the invention, in which differently from the first and second embodiments there is a second pair of regulators and the voltage regulator circuit is designated, in this case, by the reference numeral 13. The second pair of regulators comprises the regulators 15 and 16 that are arranged in series to each other and in parallel to the first pair of regulators 7 and 8, so that the second pair of regulators is arranged in opposition to the regulators of the first pair.

Figure 4 illustrates a fourth embodiment of the transformer according to the invention, similar to the one shown in Figure 3, the difference being that as in Figure 2 there is an excitation winding 10 which is series-connected to the first pair of regulators 7 and 8.

One terminal of the excitation winding 10 is connected to the common node A between the regulator 7 and the regulator 15, and another terminal of the excitation winding 10 is connected to a common node B between the regulator 8

and the regulator 16, respectively.

The first and second embodiments illustrate transformers (figures 1 and 2), in which the adjustment voltage can added to or, as an alternative, subtracted from the sinusoidal base voltage produced by the main transformer. The third and fourth embodiments (figures 3 and 4) instead illustrate transformers in which the adjustment voltage can be alternatively added to, or subtracted from, the sinusoidal base voltage produced by the main transformer.

In the fifth embodiment of the transformer according to the invention, shown in Figure 5, the adjustment winding 5 is part of the winding whose voltage is to be adjusted. In practice, therefore, the adjustment winding 5 is series-connected to the main winding 1. The secondary winding, designated by the reference numeral 20 in this case, is a secondary winding both for the primary winding 1 and for the adjustment winding 5.

The voltage regulator circuit 4 comprises, in this embodiment, a first regulator 21, which is arranged in series between the primary winding 1 and the adjustment winding 5, and a second regulator 22. The second regulator 22 is connected, by means of the first terminal 221, between the first regulator 21 and the primary winding 1, and by means of the second terminal 222 to the other end of the adjustment winding 5, which can be connected to the ground.

Figure 6 illustrates a sixth embodiment of the transformer according to the invention. In this embodiment, the adjustment voltage can be added to, and subtracted from, the main voltage induced in the winding 1, differently from what occurs in the first embodiment (figure 1), in which the adjustment voltage can be only added or subtracted.

The difference between Figure 6 and Figure 5 consists of the different structure of the regulator circuit, now designated by the reference numeral 24. The regulator circuit 24 comprises a first pair of regulators 25 and 26, respectively connected to a first and a second end of the adjustment winding 5, so as to be

mutually parallel and connected to one end of the primary winding 1.

Two additional voltage regulators 27 and 28 are respectively connected between the first end of the winding 5 and the ground and between the second end of the winding 5 and the ground.

In practice it has been found that the transformer according to the invention fully achieves the intended aim, since it allows to perform an electronic adjustment of the voltage, adding and subtracting an adjustment voltage with respect to the sinusoidal base voltage obtained from the main transformer.

The transformer thus conceived is susceptible of numerous modifications and variations, all of which are within the scope of the inventive concept; all the details may furthermore be replaced with other technically equivalent elements.