High galvanic isolation power transformer

Abstract

High galvanic isolation power transformer. Power transformer (10) comprising a first closed magnetic core (12) having first (12A) and second (12B) columns, a second closed magnetic core (14) having first and second columns (14B) and separated from the first closed magnetic core by an insulating screen (16), a first winding (22) surrounding the first column of the first closed magnetic core, a second winding (24) surrounding the second column of the second closed magnetic core, and an additional closed winding (26) surrounding both the second column of the first closed magnetic core, the first column of the second closed magnetic core, and the insulating screen. Figure for the abbreviation: Fig. 1.

Description

Description Title of the invention: High-insulation power transformer galvanic technical field

[0001] — The present invention relates to the field of power conversion electrical systems in aircraft, as well as for generating electrical power for power grids on board as well as for the electric propulsion of aircraft and it concerns just as much the high power electrical conversion from High Voltage Direct Current (HVDC) to Low Voltage Direct Current (LVDC) in electric propulsion than the conversion of small electrical power to power solid type switching elements MOSFET in the edge network. Previous technique

[0002] = Figure 3 illustrates a standard single-phase transformer 30 consisting of a body or closed magnetic core 32 and at least two copper windings 34, 66 constituting the primary and secondary windings of the transformer.

[0003] — The closed magnetic body 32 can be of various shapes, but it is often consisting of a standardized sheet metal assembly with two columns joined in pairs cylinder heads (as opposed to the three-column armored single-phase transformer), and Depending on the configuration adopted, the windings passing through the central window 38 can to be arranged on each column, on either side of the closed magnetic body as illustrated, or one on top of the other for example.

[0004] The copper windings that form the coils 32, 34 are made of wires copper of various constructions, such as single-strand wires, multi-strand wires, Litz's son, flat surfaces, etc.

[0005] As is known, the current passing through the copper windings of the primary The transformer generates a magnetic flux within the closed magnetic body. magnetic flux in turn generates a current in the copper windings of the se- secondary. It is by this principle that electrical energy is transmitted from the primary to the secondary. secondary.

[0006] — The value of the magnetic flux © is proportional to the square of the current value passing through the primary winding and the number of turns Np of this winding primary. The value of the secondary current is inversely proportional to the value of the magnetic flux through the secondary winding, and the number of turns Ns of the secondary winding. It follows that the value of the secondary current is directly proportional to the value of the primary current, to the ratio of the numbers of primary and secondary turns constituting the transformation ratio of the trans- trainer. To ensure a minimum level of galvanic isolation, the copper windings are all coated with resin, forming an electrical insulator essential for preventing short circuits between the turns. When it is necessary to reinforce this galvanic isolation, an insulator is added between the copper windings and the closed magnetic core, and between the primary and secondary windings according to their arrangement. This standard single-phase transformer design is generally satisfactory. However, in the aeronautical field, it is not without drawbacks. Indeed, if there is any problem with a winding, for example a short circuit that melts the winding in question as well as the insulators that kept the winding and the magnetic core separate, then an electrical contact is established between the faulty winding and the magnetic core. And since the magnetic core is generally also electrically conductive, this fault can be found on the other side of the galvanic isolation barrier, breaking this galvanic isolation and leading to the possibility of fault propagation, which can be particularly detrimental to electric propulsion or the aircraft's onboard electrical system.This problem also arises when the magnetic body is made of ferrite, as this material is not necessarily a good insulator against the high voltages required for electric or hybrid aircraft propulsion. It is also known that to enhance insulation, a conductive screen, known as a Faraday screen, can be added between the primary and secondary windings and connected to the electrical ground. However, such a transformer, called a separation or isolation transformer, significantly reduces the central winding window and also presents manufacturing difficulties. Description of the invention The main objective of the present invention is therefore to limit, or even eliminate, the risk of electrical fault propagation between the primary and secondary windings of an electrical transformer in an aeronautical environment. Another objective is to limit parasitic capacitive elements inherent in the transformer's construction, which, in power switching applications, can generate electromagnetic interference. These goals are achieved by a power transformer comprising a first closed magnetic core having a first and a second column, a second closed magnetic core having a first and a second column and separated from the first closed magnetic core by an insulating screen, a first winding surrounding the first column of the first closed magnetic core, a second winding surrounding the second column of the second closed magnetic core, and an additional closed winding surrounding both the second column of the first closed magnetic core, the first column of the second closed magnetic core and the insulating screen. Thus, with this particular structure, it is possible to increase galvanic isolation without adding a conductive screen, and by eliminating a potential conductive path for the propagation of electrical faults, to reduce the risk of a fault propagating and turning into a catastrophic event for the aircraft. Preferably, the additional closed winding consists of a single turn. Advantageously, the additional closed winding consists of a single sheet of copper. Preferably, the insulating screen is a simple plate of an insulating material such as epoxy resin or polypropylene or even a Kapton® film. Advantageously, closed magnetic cores exhibit one of the following configurations: C and C, C and I, L and inverted L. The invention also relates to voltage converters and an aircraft electrical distribution network comprising at least one power transformer as mentioned above. Brief description of the drawings Other features and advantages of the present invention will become apparent from the description given below, with reference to the accompanying drawings which illustrate an example of an embodiment without being limiting in any way and on which: [Fig.1] [Fig.1] illustrates a cross-sectional view of a power transformer according to the invention, [Fig.2] [Fig.2] details the central part of the transformer from [Fig.1], and [Fig.3] [Fig.3] shows a cross-sectional view of a conventional single-phase transformer. Description of the implementation methods The principle of the invention is based on the fundamental principle of a transformer that transfers electrical energy between a primary winding and a secondary winding via magnetic coupling, but with an additional intermediate winding that loops back on itself and around an insulating screen, creating an electrically isolated magnetic coupling that prevents the propagation of electrical faults. One might think that this intermediate winding would cause the transformer to lose a significant portion of its electromagnetic properties. On the contrary, an advantage can be gained by incorporating the magnetic circuit of the secondary winding into this central loop. A power transformer 10 according to the invention is shown in cross-section in [Fig. 1]. It consists of two magnetic cores 12, 14 of two standard single-phase transformers mounted on either side of an insulating screen 16 so as to form a core with two winding windows 18, 20. Each magnetic core conventionally comprises two columns 12A, 12B; 14A, 14B joined by two yokes, thus constituting two C-shaped transformer halves (see [Fig. 2]), although this configuration is not mandatory; a C and I configuration (as in the left part of [Fig. 1]) or an L and inverted L configuration (as in the right part of [Fig. 1]) is also possible. These two-part configurations are primarily intended to facilitate the winding assembly. A first winding, or primary winding 22, is wound around the first column 12A of the first transformer half-body. This winding is intended to be supplied with a primary current Ip from a source (typically a battery or a DC power supply). A second winding, or secondary winding 24, is wound around the second column 14B of the second transformer half-body. This secondary winding is intended to deliver a secondary current Is for use (typically in a DC / DC or DC / AC converter). According to the invention, an additional central winding 26, intended to carry a current Ic, surrounds both the second column 12B of the first transformer half-body, the first column 14A of the second transformer half-body, and the insulating screen 16 that separates these two columns, with which it is in contact.Depending on the environmental, thermal and / or vibrational constraints to which the product will be subjected, as well as the potential costs, this contact could be achieved by simple gluing or by bolting, for example. Figure 2 shows in more detail this central winding 26 closed on itself (therefore short-circuited) and holding together with the insulating screen 16 the two transformer half-bodies 28A, 28B. The insulating screen is advantageously made of an epoxy resin or polypropylene plate or any other insulating material such as a Kapton® film for example. A person skilled in the art will be able to size the central winding, taking into account the desired integration and the transformer's feasibility. Considering the conservation of magnetic flux, the ratio of the number of turns between the windings is inversely proportional to the currents flowing through these turns: Ip*Np=Ic*Nc and Ic*Nc=Is*Ns. If they consider the Joule losses in these windings, it will be preferable to have as many turns as possible in the central winding to reduce the current, since Joule losses are proportional to the square of the current. Conversely, if they consider the industrial aspects of manufacturing this winding, they will It is preferable to have few turns because it avoids any wasted space in the winding windows (which allows for a low winding area to usable copper area ratio), and in practice, arranging a winding with many turns is not easy. At the extreme, this central winding will be formed from a single copper foil surrounding both the second column 12B, the insulating screen 16, and the first column 14A. The operation of the invention is as follows. The current Ip flowing in the primary winding 22 of the transformer generates a magnetic flux Gp in the closed magnetic core 12 of the first transformer. This magnetic flux, in turn, generates the current Ic in the central copper winding 26 surrounding the insulating screen 16. This current Ic, flowing through the short-circuited winding, then generates a new magnetic flux Is in the closed magnetic core 14 of the second transformer. This, in turn, generates the current Is flowing in the secondary winding. It is by this principle that electrical energy is transmitted in an isolated manner from the primary to the secondary. The magnetic flux in the secondary is identical to the magnetic flux in the primary if the two magnetic cores have the same properties (mechanical and magnetic).The value of the secondary current Is is then directly proportional to the value of the primary current Ip, to the ratio of the number of turns primary Np and secondary Ns constituting the transformation ratio of the power transformer. By placing an insulating screen 16 between the two magnetic cores 12, 14, the direct electrical connection between the primary and secondary windings, created by the magnetic core itself or by the specific arrangement of the primary and secondary windings, is broken, and the Faraday screen of the prior art is eliminated. Since the central winding is electrically isolated, it is not subject to disturbances leading to the degradation of its own insulation, and the galvanic isolation of the power transformer is maintained. The invention finds application in the on-board distribution network of aircraft for transferring low voltage power to sensors, for example in the case of a 28V to 28V voltage converter in a low voltage zone mechanically segregated from a high voltage zone or a 15V to 15V voltage converter used to power static switching switches, of MOSFET or IGBT type, which are implemented in power inverters or static power switches. The control section of these power switches is generally located in a low-voltage area, and to power and transfer the control information to these switches, always with the aim of limiting the risk of electrical propagation from high voltage to low voltage, it is advantageous to use a transformer. power of invention. In the case of redundant systems, particularly propulsion systems, for safety reasons, 800V (HVDC network) to 28V (LVDC network) converters require strong insulation to prevent high voltage fault propagation to low voltage.

Claims

Demands

1. DC power transformer (10) comprising a first core ma- closed genetics (12) having first (12A) and second (12B) columns, a second closed magnetic core (14) having first (14A) and second (14B) columns and separated from the first nucleus ma- closed genetic material by an insulating screen (16), a first winding (22) surrounding the first column of the first closed magnetic core, a second winding (24) surrounding the second column of the second nucleus closed magnetic field, and an additional closed winding (26) surrounding both the second column of the first magnetic core closed, the first column of the second closed magnetic core and the insulating screen.

2. Power transformer according to claim 1, wherein The additional closed winding (26) has a single turn.

3. Power transformer according to claim 1, wherein the additional closed winding (26) consists of a single copper sheet.

4. Power transformer according to claim 1, wherein the insulating screen (16) is a simple plate of epoxy or poly- resin propylene.

5. Power transformer according to claim 1, wherein the insulating screen (16) is a film of Kapton® or any other material insulating.

6. Power transformer according to claim 1, wherein the closed magnetic cores (12, 14) exhibit one of the configurations following: C and C, C and J, L and inverted L.

7. Voltage converter comprising a power transformer according to any one of claims 1 to 6.

8. Aircraft electrical distribution network comprising at least one power transformer according to any one of the claims there6.