Lc interfacing filter incorporating a damping circuit

Abstract

The invention relates to an LC interfacing filter comprising a parallel capacitor and a series inductor having eddy-current magnetic losses, the inductor (10) being configured so that the eddy-current magnetic losses are predominant with respect to the hysteresis losses, the inductor consisting of a tube (12) made of magnetic material and around which an electrical conductor (14) is wound, with an insulating sheet (16) separating the tube of magnetic material from the electrical conductor.

Description

[0001] Description

[0002] Title of the invention: LC interface filter incorporating a damping circuit

[0003] Technical Field

[0004] The present invention relates to the field of differential mode filtering in high power electrical supply networks, including in particular energy converters.

[0005] Prior art

[0006] When a power converter (switching or not), acting as a load or as a generator, is to be connected to a power supply network, it is necessary to insert an interface filter which will aim to reduce current ripples to a level lower than that specified by the power supply network standard and therefore to preserve its quality in terms of voltage ripples. The filter must also be designed in such a way as not to compromise the stability of the power supply network.

[0007] Typically, this filter is based on a parallel capacitor and a series inductor, the level of attenuation of the harmonic content of the current to be rejected depending directly on the ratio of the impedances of these two passive elements L and C. However, such an LC filter is unstable and, in practice, cannot be used without the addition of a damping circuit that is sized to achieve a certain damping ratio, defined according to the needs of the application. The damping ratio is a dimensionless quantity characterizing the evolution and decay over time of the oscillations of a physical system.

[0008] It should be noted that the addition of the damping circuit affects the impedance ratio, thus requiring correction of the values ​​of L and / or C, if one wishes to keep the initially defined attenuation level constant. It should also be noted that conventional damping circuits are poorly suited to high voltage direct current (HVDC) networks whose impedance seen by the filter is not constant (switchable sources and / or loads for example) and for which the only remedy is then to multiply the size of the filters extremely significantly in order to make the variations in network impedance as low as necessary in relative value.

[0009] It is known to realize the damping circuit of an LC filter according to several topologies classified essentially according to two types of structures: parallel structures where the damping circuit acts on the impedance of the parallel capacitor and series structures where the damping circuit acts on the impedance of the series inductor. These topologies can also be classified according to the order of the filter (the number of additional L or C elements).

[0010] Second-order parallel or series structures have the disadvantage of requiring, in addition to the L and C elements, an additional power resistor whose dissipation will be prohibitive and will most often require specific cooling, and third-order structures have the disadvantage of having a low damping ratio, typically less than 0.2 if one does not want to significantly affect the sizing of the filter. Indeed, a damping ratio of 0.5 implies adding, for damping purposes, a capacitor or an inductor which is 3 times the value of the initial L or C element of the filter, thus multiplying by 4 the size and therefore the weight of the filter with its damping circuit.

[0011] Furthermore, the lower the damping ratio is chosen (< 0.2), the greater the precision constraint on the filter components to guarantee this damping level. The tolerance required on the value of the components can then become a serious problem and must be taken into account when this choice of low damping is made to avoid the mass penalties that accompany the highest damping values.

[0012] Statement of the invention

[0013] The main aim of the present invention is therefore to propose a filter incorporating damping which overcomes the aforementioned drawbacks. Another aim of the invention is to obtain a filter having a constant damping rate over a wide frequency range.

[0014] These aims are achieved by an LC interfacing filter comprising a parallel capacitor and a series inductor comprising magnetic losses by eddy currents, characterized in that the inductor is configured so that the magnetic losses by eddy currents are in the majority compared to the losses by hysteresis and in that the inductor consists of a tube of magnetic material around which an electrical conductor is wound, an insulating sheet separating the tube of magnetic material from the electrical conductor.

[0015] Thus, by integrating the filter damping function within the filter inductance itself, we can do away with the usual use of power resistors traditionally used to form the damping circuit.

[0016] Preferably, the electrical conductor is wound inside the magnetic material tube instead of around it.

[0017] According to another embodiment, the LC interfacing filter comprising a parallel capacitor and a series inductor comprising magnetic losses by eddy currents, is characterized in that the inductor is configured so that the magnetic losses by eddy currents are in the majority compared to the losses by hysteresis and in that the inductor is made up of several tubes made of magnetic materials placed side by side and around which is wound an electrical conductor, an insulating tube or an insulating sheet separating the tubes made of magnetic material from the electrical conductor.

[0018] Preferably, the number of tubes of magnetic material is between 2 and 100.

[0019] Advantageously, the tubes made of magnetic material are identical.

[0020] Preferably, the tubes of magnetic material are arranged around the coiled electrical conductor instead of being inside it.

[0021] Advantageously, the tube(s) made of magnetic material each have a wall with slots or orifices.

[0022] Preferably, the tube(s) made of magnetic material each have a circular, square or polygonal profile. Advantageously, the magnetic losses by eddy currents are created by the skin effect if the aim is to obtain a constant damping rate over a wide frequency range.

[0023] Preferably, the magnetic material of the magnetic material tube(s) is ordinary magnetic steel if the frequency range where the damping is to be located is less than about 1 MHz.

[0024] Brief description of the drawings

[0025] Other characteristics and advantages of the present invention will emerge from the description given below, with reference to the appended drawings which illustrate an exemplary embodiment thereof without any limiting character and in which:

[0026] [Fig. 1] Figure 1 illustrates the electrical diagram equivalent to an LC filter with damper according to the invention,

[0027] [Fig. 2] and [Fig. 2A] Figures 2 and 2A show a first example of the embodiment of the inductance of the filter of Figure 1,

[0028] [Fig. 3] Figure 3 shows a second example of the realization of the inductance of the filter of Figure 1,

[0029] [Fig. 4] Figure 4 shows a third example of the realization of the inductance of the filter of Figure 1,

[0030] [Fig. 5] Figure 5 shows the variation, as a function of frequency, of the equivalent series inductance of the inductor of Figure 2, and

[0031] [Fig. 6] Figure 6 shows the variation, as a function of frequency, of the equivalent series resistance of the inductance of Figure 2,

[0032] [Fig. 7] Figure 7 shows the variation, as a function of frequency, of the damping ratio of the inductance of Figure 2.

[0033] Description of the embodiments

[0034] The principle of the invention is based on obtaining the damping of the LC filter by creating controlled and predictable magnetic losses at the level of the magnetic circuit of the inductance of the filter, so as to guarantee by design the damping rate of the filter. Thus and as illustrated in Figure 1, the damping circuit is not achieved by the addition of one or more power resistors associated or not with other passive components, capacitor in the case of parallel damping, or inductance in the case of series damping, these power resistors generating mass and volume and often requiring specific cooling for the power circuits, but by the magnetic losses (represented by its electrical equivalent Rmag in parallel with the inductance L) that it is possible to create at the level of the magnetic circuit of the inductance L of the LC filter.

[0035] To create magnetic losses, it is known to act on eddy current losses and / or hysteresis losses. However, in order to avoid excessive sensitivity to the fine characteristics of materials that may be subject to significant manufacturing dispersions, the magnetic circuit of the inductor is designed to make eddy current losses largely in the majority compared to hysteresis losses (typically 10 to 100 times higher) and to make them evolve in a substantially constant manner over a wide frequency range, typically between 100Hz and several hundred kHz, or even 1MHz. To do this and address this dual problem, eddy current losses are created by the skin effect.

[0036] The skin effect, also called the skin effect, is an electromagnetic phenomenon which causes current to flow only on the surface of conductors at high frequencies. This electromagnetic phenomenon, which exists for all conductors carrying alternating currents, causes the current density to decrease as one moves away from the periphery of the conductor. More precisely, the higher the frequency f, the more the current sheets induced in the magnetic tubes are driven towards their outer skin.

[0037] Thus, it is demonstrated that the equivalent thickness of current circulation at the level of the outer skin can be defined by the expression: root(Kmaterial / f), the current sheets encountering a resistance evolving in: l / [root(Kmaterial / f)].

[0038] At the same time, the equivalent flux seen by the coil decreases, resulting in a law of decrease in the equivalent inductance parallel to l / root(f). The damping ratio (ksi) is then constant over a very wide frequency range and therefore very insensitive and subject to variations in the value of the components interacting with the inductance L, as demonstrated by the following formula:

[0039] With ksi the damping ratio, Rdamping the damping resistance, Kl and K2 being two coefficients of proportionality depending on the magnetic material, the geometric shapes and the number of turns.

[0040] Figures 2-2A illustrate with a front view and a side view a first example of embodiment of the inductor L according to the aforementioned principle. The inductor 10 is made of a tube of magnetic material 12, typically of ordinary magnetic steel whose resistivity and permeability are chosen according to the desired damping level and frequency range, on which is wound an electrical conductor 14 of copper or aluminum, and an insulating sheet 16, typically a Kapton® plastic film, separating the metal tube from the electrical conductor. At each of the two ends 14A, 14B of this electrical conductor is fixed a metal lug 18A, 18B forming one of the two terminal terminals (input and output electrical connections) of the inductor.It will be noted that, in an alternative embodiment not illustrated, the electrical conductor may be wound inside the tube of magnetic material instead of around it and the tube of magnetic material may have a wall with slots or holes.

[0041] A second example of embodiment of the inductance L according to the aforementioned principle is illustrated in Figure 3. The inductance 20 is this time made up of several tubes of magnetic material (here eight tubes 22A - 22H without this number being limiting, a maximum number of 100 tubes being possible) identical or not and placed side by side in an insulating tube 26 on which the electrical conductor 24 is wound. The insulating tube, typically a tube made of PVC (polyvinyl chloride), polypropylene or epoxy resin for example, acts as a separator for the aforementioned insulating sheet. At each of the two ends 24A of the electrical conductor is fixed a metal lug 28A forming one of the two terminal terminals (input and output electrical connections) of the inductance.In the example illustrated, the tubes of magnetic material are arranged side by side along the internal circumference of the insulating tube, leaving a central part empty of metal tubes which, by the possibility it offers of air circulation inside the PVC or polypropylene tube, has the advantage of facilitating the cooling of these heated metal tubes following the damping effect deliberately obtained by the eddy currents.

[0042] Figure 4 illustrates a third embodiment of the inductor L according to the aforementioned principle. The inductor 30 is here made up of four tubes of magnetic material 32A - 32D, identical as illustrated or not, placed in parallel side by side and around which is wound the electrical conductor 34, an insulating sheet 36, typically a plastic film, separating the metal tubes from the electrical conductor. At each of the two ends 34B of this electrical conductor is fixed a metal lug 38B forming one of the two terminal terminals (input and output electrical connections) of the inductor. In this embodiment, the tubes of magnetic material of circular profile and which are inscribed in a square are adjacent and do not leave room for another tube between them. However, it could be otherwise if these tubes had a different profile, for example square or polygonal.

[0043] Whatever the chosen embodiment, the diameter (or cross-section) of the tube(s) made of magnetic material, the thickness of their wall, their length, and the material used for the tube constituting the magnetic core, are all parameters that can be used to adapt and optimize the inductance to the desired application. An important parameter is the wall thickness, which must be calculated so that the skin effect and therefore the damping is at the desired level in the desired frequency range where the LC filter must operate.

[0044] It will be noted that, in an alternative embodiment not illustrated, the tubes of magnetic material can be arranged around the wound electrical conductor instead of being inside the latter and the walls of the tubes of magnetic material can then include slots or orifices.

[0045] A prototype was made according to the geometric configuration of Figure 2 based on the following parameters: a tube 100mm long, 25.4mm in diameter, 1.5mm thick and weighing 85g, made of ordinary magnetic steel with a resistivity of 1.383 E -007 and initial relative magnetic permeability of 131; and a coil made up of 34 joined turns of a 2mm diameter copper conductor, for a coil weight of 125g.

[0046] Figures 5, 6 and 7 show respectively, as a function of frequency, the equivalent series inductance, the equivalent series resistance and the damping ratio obtained with this prototype of magnetic loss inductance. We can clearly see the root variability of the frequency of the equivalent series inductance and the equivalent series resistance, and the desired stability of the damping ratio from 100Hz to more than 100kHz.

[0047] To determine these parameters, it is essential to have dedicated calculation resources in order to be able to adapt and optimize it for the intended application.

[0048] To do this, we will refer to Maxwell's equations to arrive at the following two basic relationships, that of the reactive power Q exchanged with the magnetic circuit, and that of the active power P supplied to this same magnetic circuit:

[0049] [Math

[0050] [Math

[0051] The coefficients Qcoeff and Pcoeff are calculated based on the geometry of the magnetic circuit, the characteristics of the magnetic material, and the penetration depth of the frequency-dependent current. For the most common shapes, such as a ring, these equations use the Bessel-Kelvin functions.

[0052] Based on: Q = Ls*w*I 2 , and P = Rs*I 2, the person skilled in the art will easily determine the equivalent series loss inductance and resistance and will confirm by these calculations the curves obtained by the previous tests. Thus, with the invention, it is possible to replace most of the conventional LC filters with a damping circuit by a magnetic loss inductance as described above, with the following advantages:

[0053] - a single component replaces three, namely the inductance L of the LC filter, the power resistor and the inductance of the damping circuit, which implies less wiring,

[0054] - a considerable weight saving: for example, for a converter of several tens of kW, associated with a conventional damping circuit, the weight reaches 2 x 1.05 kg (for a damping of 0.2), compared to 2 x 0.35 kg for the solution with damping by magnetic losses (for a damping of 0.27 to 0.4),

[0055] - a lack of sensitivity to the precision of components as well as to manufacturing dispersion,

[0056] - a lack of sensitivity to variations in network impedance due to a constant magnetic loss inductance damping coefficient over very wide frequency ranges,

[0057] - easier cooling due to a winding arranged on the outer wall,

[0058] - low manufacturing cost due to the raw materials used: ordinary magnetic steel, copper (or aluminum), insulation and very simple manufacturing and assembly guaranteeing very high robustness, and

[0059] - a capacity to withstand high current overloads, the saturation level in terms of N*I can be raised very high, which is interesting in the event of a short circuit on the equipment itself and in the event of a short circuit upstream of the electrical network. Indeed, in the latter case, it is mainly the inductance of the input filter which limits the discharge current of the input capacitor in the short circuit. The risk, then, would be to destroy the protection device if the current reached too high levels (8000A to 10000A) following the saturation of this inductance. Maintaining the inductance at high current makes it possible to limit the discharge current peak. The absence of saturation is an indirect consequence of the geometric choices of the inductance. The absence of rebounds in the phase of decrease of the short-circuit current is, on the other hand, directly linked to the self-damping capacity.

Claims

Claims

1. LC interfacing filter comprising a parallel capacitor and a series inductor comprising eddy current magnetic losses, characterized in that the inductor (10) is configured so that the eddy current magnetic losses are at least 10 times greater than the hysteresis losses and in that the inductor consists of a tube of magnetic material (12), an electrical conductor (14) being wound around or inside said tube of magnetic material, an insulating sheet (16) separating the tube of magnetic material from the electrical conductor.

2. LC interfacing filter comprising a parallel capacitor and a series inductor comprising eddy current magnetic losses, characterized in that the inductor (20, 30) is configured so that the eddy current magnetic losses are at least 10 times greater than the hysteresis losses and in that the inductor consists of several tubes of magnetic materials (22A - 22H; 32A - 32D) placed side by side, an electrical conductor (24, 34) being wound around or inside said tubes of magnetic materials, an insulating tube (26) or an insulating sheet (36) separating the tubes of magnetic material from the electrical conductor.

3. An LC interfacing filter according to claim 2, wherein the number of magnetic material tubes is between 2 and 100.

4. An LC interfacing filter according to claim 2 or claim 3, wherein the tubes of magnetic material are identical.

5. An LC interfacing filter according to any one of claims 1 to 4, wherein the tube(s) of magnetic material each have a wall with slots or holes.

6. An LC interfacing filter according to any one of claims 1 to 5, wherein the tube(s) of magnetic material each have a circular, square or polygonal profile.

7. An LC interfacing filter according to any one of claims 1 to 6, wherein the eddy current magnetic losses are created by the skin effect.

8. A power supply network comprising an LC interfacing filter according to any one of the preceding claims.

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