EP3275004B1 - Three-phase transformer for twelve-phase rectifier - Google Patents

Description

Technical area

The invention relates to the field of electrical engineering, and more particularly power rectifiers. It relates more specifically to a particular transformer structure used to supply two-phase rectifiers. It relates more particularly to a configuration which makes it possible to reduce the rate of harmonic distortion in current and having the minimum possible losses.

Invention background

In general, electrical energy conversion installations are increasingly demanding in terms of performance. Thus, it is generally asked of converters to deliver the most constant possible voltages or currents, while generating on the supply network, or more generally on the power supply source, the lowest possible harmonic disturbances .

Among the most common converters, rectifier systems are known which are supplied by a mains network supplying a three-phase voltage, and which generate a DC voltage, used to supply various loads. A classic configuration consists in using several three-phase full-wave rectifiers, whose outputs are connected in parallel on a common load, and whose inputs are supplied by a transformer delivering voltage systems phase shifted by an angle depending on the number of rectifiers connected in parallel.

A widely used implementation consists in using thyristor rectifiers forming three-phase Graetz bridges, the outputs of which are connected in parallel on a filtering capacitor, and therefore the three-phase inputs are connected to two series of outputs of a three-phase transformer. . More specifically, such a transformer comprises three primary windings and two series of three secondary windings. A first series of secondary windings is connected in a star while the second series is connected in a triangle, so that the voltage systems generated by these two systems are phase shifted by 30 °. In this way, and with a synchronized control of the two rectifiers, the first harmonics of order 5 and 7 generated by each of the rectifiers compensate for the primaries, so that the input current of the converter has harmonics whose first rank is of order 11.

To filter the output voltage, and to limit the intensity of the harmonic currents at the input, it is known to use inductors which can advantageously be installed in series with the primary windings. In this case, the transformer comprises a first main core, on which are wound the primary windings and the secondary windings, on three separate columns, each corresponding to a phase of the supply voltage. The transformer also includes an auxiliary core including three separate columns, on each of which is wound only a primary winding. An example of such an arrangement is described in the documents GB 1,325,938 and WO 2014/031959 .
To ensure good magnetic coupling between the primary windings and the two series of secondary windings, each primary winding is generally composed of a pair of two windings connected in parallel, each of these elementary windings being wound intimately with the secondary winding d 'one of the series. Consequently, the columns of the inductive core or auxiliary core support primary windings, which are traversed by phase-shifted currents. It follows that the magnetic flux within the column is not homogeneous, and varies between the upper part and the lower part of the same column. The Applicant has thus found that phenomena of magnetic leakage occur, with a looping of the magnetic field outside the circuit formed by the secondary core, and in particular through the conductors of the primary windings wound on the inductive core. These phenomena increase the losses and are therefore prejudicial for the efficiency of the transformer. They also increase the harmonic currents in the secondary windings and to a lesser extent in the primary windings.

This phenomenon is all the more marked as the power of the converter is high, and more precisely as the intensity of the current circulating in the windings is high. Indeed, to increase the intensity, it is necessary to use conductors in the form of strip which have a large section and a small thickness, to be relatively easily deformable and windable on the columns of the transformer, as opposed to solid conductors flat type. However, the strips have a large width and extend over a larger portion of the column of the inductive core. They are therefore subjected in a more marked manner to parasitic fields as mentioned above.

A problem which the invention therefore proposes to solve is that of limiting the rate of harmonic distortion in current (THDI), while reducing the losses present at the transformer, and this while maintaining an optimal compactness of the converter.

Statement of the invention

To do this, the Applicant has designed a three-phase transformer for two-phase rectifier including two three-phase full-wave rectifiers connected in series or in parallel depending on the application. This transformer includes a main magnetic core which includes three columns magnetically connected in parallel at their ends by two yokes. On each of these columns is wound a pair of primary windings connected in parallel, corresponding to a phase of the supply network, as well as two secondary windings. Three of these secondary windings, which are wound on separate columns, are connected in star form to the terminals of the transformer which are connected to a first full-wave rectifier. The other three secondary windings are connected in a triangle to the terminals of the transformer which are connected to the other rectifier. The transformer also includes an auxiliary core (or inductive core) which has three columns magnetically connected at their ends by two yokes for closing the magnetic field. On each of these columns are wound the two windings of one of the pairs of primary windings corresponding to a phase.

According to the invention, this transformer is characterized in that the auxiliary core has a middle magnetic portion connecting the midpoints of each column located between two primary windings of the same pair. In other words, the invention consists in providing the inductive auxiliary core in such a way that the magnetic fields generated by the two primary half windings close independently of one another, by additional portions forming an intermediate bridging of the magnetic circuit of the core inductive.

In other words, the transformer according to the invention comprises two magnetically autonomous inductive half-cores, physically assembled in a single frame. In this way, the magnetic leaks are limited, and it is observed that the losses of the transformer are much lower than those observed in a configuration where the characteristic middle portion is absent. These advantages are obtained without increasing the overall size of the transformer, since the characteristic bridging portion is located in a zone of separation of the two primary half-windings, which must be far enough apart to avoid coupling phenomena between secondary windings on the main core.

Advantageously in practice, the secondary windings of the same column are wound on one of the primary windings of the same column, to ensure optimal coupling, while avoiding parasitic couplings between secondary.

The elimination, or at the very least the very strong limitation of magnetic leaks at the level of the inductive auxiliary core, makes it possible to use conductors for the primary and / or secondary windings which can be produced from metallic strip , thus authorizing the use of the transformer with particularly high intensities.

Advantageously in practice, to determine the inductance value, provision may be made for the columns of the auxiliary core to have air gaps, typically produced by the separation between different sections stacked to form the columns of the inductive auxiliary core.

Brief description of the figures

• la figure 1 est un schéma électrique simplifié montrant un convertisseur alternatif continu incluant un transformateur conforme à l'invention
• la figure 2 est une vue en perspective sommaire d'un transformateur censé de représenter l'invention, mais où la portion magnétique médiane du noyau auxiliaire n'est pas montrée.
• les figures 3 et 4 sont des vues en perspective sommaire respectivement du noyau principal et du noyau auxiliaire du transformateur de la figure 2
• la figure 5 est une vue en coupe selon le plan V-V' de la figure 2.

The manner of carrying out the invention, as well as the advantages which result therefrom, will emerge clearly from the description of the embodiments which follow, with the support of the appended figures in which:

• the figure 1 is a simplified electrical diagram showing a continuous AC converter including a transformer according to the invention
• the figure 2 is a summary perspective view of a transformer supposed to represent the invention, but where the middle magnetic portion of the auxiliary core is not shown.
• the figures 3 and 4 are summary perspective views of the main core and the auxiliary core of the transformer respectively figure 2
• the figure 5 is a sectional view along the plane VV 'of the figure 2 .

detailed description

As already mentioned, and as illustrated in the figure 1 , the invention relates to a transformer 10 , forming part of a converter 1 ensuring an AC-DC conversion. More specifically, the transformer 10 is connected at the input to a three-phase voltage source 2. At the output, the transformer 10 is connected to two three-phase full-wave rectifiers 3 , 4 whose outputs are connected in parallel on a filtering capacitor 5 allowing to deliver a DC voltage between the output terminals 6 , 7 .

More specifically, the transformer 10 comprises six primary windings 11 - 16 , connected in pairs 11 , 14 ; 12 , 15 ; 13 , 16 , at one of the phases 201 , 202 , 203 of the three-phase power source 2. In addition, the transformer 10 also includes six secondary windings 21 - 26 each coupled with one of the primary windings 11 - 16 . The secondary windings 21 - 26 are divided into two series. The first series of secondary windings 21 , 22 , 23 is connected in a triangle to the terminals 31 , 32 , 33 of the transformer so as to deliver a system of compound voltages. The second series of secondary windings 24 , 25 , 26 is connected to the common point 28 in star configuration, and connected to the terminals 34 , 35 , 36 of the transformer to which the second rectifier 4 is connected, so as to deliver a system of simple voltages .

Complementarily, from an electrical point of view, the transformer 10 comprises six input chokes 41 - 46 each connected on the one hand to a primary winding 11 - 16 , and on the other hand to one of the phases 201 , 202 , 203 from the power source 2. These primary inductors are formed by the primary windings and by the magnetic leakage generated by the auxiliary inductive core. The primary windings enclosing both the auxiliary core and the main core, the latter being itself surrounded by the secondary windings, it is the insertion of the auxiliary core between primary and secondary which increases and controls the magnetic leakage, and therefore constitutes the primary inductance.

The invention relates in particular to the mechanical constitution of the transformer 10 , which is in particular illustrated in the figure 2 . More specifically, the transformer 10 comprises a main core 50 and an auxiliary inductive core 60. These two cores 50 , 60 receive the windings 71-76 which form on the one hand the primary windings 11-16 and on the other hand the inductances 41 - 46. The main core also receives the windings 81 - 84 , which form the secondary 21 - 26 of the transformer.

As illustrated in figure 3 , the main core 50 has three columns 51 , 52, 53 which receive the windings 71 - 76 and the secondary windings 81 - 86. These three columns 51 - 53 are connected at the top and bottom by portions forming the yokes 54 , 55 . of course, this core can be made according to the rules of art to optimize performance in terms of yield, especially regarding the choice of materials used, puff up, and assembly of the different elements that form columns and cylinder heads, which can be monolithic or formed by the assembly of separate parts. Thus, for small powers, it is conceivable to use magnetic circuits of the EI type, namely that the 3 columns and one of the cylinder heads are assembled within a part of general E shape, the latter being closed by a single cylinder head in the form of I. From a practical point of view, the powers being able to be important, the 3 columns are generally individualized and closed by two cylinder heads.

As illustrated in figure 4 , the secondary core, or inductive core, 60 is also composed of three columns 61 , 62 , 63 , which are magnetically connected by their upper and lower ends by yokes 64 , 65 closing the magnetic circuits. The different columns can be realized by separate sections 66 , 67 , between which a gap 68 is defined making it possible to adjust the value of the inductance formed by the winding produced around the column. The rules of the art are applied to determine the values of these air gaps so as to obtain inductance rates, corresponding to short-circuit impedances of the order of 20%, making it possible to obtain a THDI of the around 5%.

According to the invention, the auxiliary core 60 includes a part 69 which bridges the midpoints of the columns 61 , 62 , 63. This intermediate part or median core 69 thus defines two distinct zones of the inductive core 60 in which the transformer primaries dedicated to a given three-phase elementary rectifier. So, as illustrated in the figure 5 , it is observed that the windings 74 are wound around the column 51 of the main core 50 and of the column 61 of the inductive core 60. On the other hand, the winding 84 which constitutes the secondary 24 of the transformer is wound only around the column 51 of the main core 50.

The characteristic part 69 of the inductive frame plays an important role in the reduction of magnetic leaks in the auxiliary core 60. More precisely, as mentioned above, the two windings 71 , 74 which are wound around the upper portion 91 and the portion respectively bottom 92 of column 61 are traversed by separate currents. Indeed, the currents flowing through these two windings have harmonic components, and in particular harmonics of rank 5 and 7 which are in phase opposition, because they are generated by the two rectifiers 3 , 4 , which are themselves same supplied by voltage systems phase shifted by 30 °. Consequently, the magnetic fluxes generated by the currents passing through these windings are different between the upper part 91 and the lower part 92 of the column 61. The presence of the characteristic part 69 makes it possible to channel the leakage flux corresponding to the difference between the magnetic fluxes present in the upper 91 and lower parts of the same column. As a result, the neighboring conductors are traversed by a negligible leakage flow, and are therefore the seat of less losses by the Joule effect. In addition, the harmonic components in particular of order 5 and 7 are less disturbed, and their cancellation because they are in phase opposition from one winding to the other is more effective, with an improvement in the harmonic distortion rate. while running.

Of course, the configuration illustrated in the figure 4 , in which the part 69 forming the median core is unique could be available in different versions, and in particular be produced by two separate parts, so as to form two magnetically independent half inductive core.

It appears from the above that the transformer according to the invention has multiple advantages, in particular that of significantly reducing the losses linked to the current filtering function produced by the incorporation of input inductors. The reduction of these losses, by channeling the different fluxes generated by this inductance system, makes it possible to limit the Joule losses within the conductors forming the inductors, and in addition to reduce the harmonics in current at the primary. This improvement in efficiency and performance is achieved by retaining optimum compactness of the transformer, which is particularly appreciable for applications requiring a limited footprint.

Claims (4)

1. Three-phase transformer (10) adapted to be connected to a twelve-phase rectifier including two full-wave three-phase rectifiers (3,4) connected in parallel or in series, comprising:

   - a main magnetic core (50) comprising three columns (51, 52, 53) connected magnetically in parallel at their ends by two yokes (54, 55), and on each of which are wound:

   - a pair of primary windings (11, 14; 12, 15; 13, 16) connected in parallel,

   - two secondary windings (21-26),

   three of the secondary windings (21-23) wound on separate columns being Delta connected to terminals (31-33) of the transformer adapted to be connected to a first full wave three-phase rectifier (3), the other three secondary windings (24-26) being connected in star configuration to terminals (34-36) of the transformer adapted to be connected to the second full wave three-phase rectifier (4),

   - an auxiliary core (60) comprising three columns (61, 62, 63) connected magnetically at their ends by two field closure yokes (64, 65), one of said pairs of primary windings (11, 14; 12, 15; 13, 16) being wound on each of the columns, and comprising a median magnetic portion (69) connecting the midpoints of each column (61, 62, 63) located between two primary windings of the same pair (11, 14; 12, 15; 13, 16).
2. Transformer according to claim 1 in which each primary winding of a same column is wound on one of the secondary windings of said column.
3. Transformer according to claim 1, in which the primary and/or secondary windings are made from a metal strip.
4. Transformer according to claim 1 in which the columns of the auxiliary core (60) comprise air gaps (68).

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