FR3059855B1 - DC / DC CONVERTER FOR ELECTRICAL NETWORKS - Google Patents

Abstract

The invention relates to a DC / DC converter comprising: • a multi-level DC / AC converter whose output terminals (3a) are connected to input terminals of an inductive circuit (4, 7) in which an alternating current flows. a DC input DC input voltage (Vb) being applied between input terminals of the DC / AC converter, and • a multilevel voltage source AC / DC converter (52) having input terminals (3b) are connected to output terminals of the inductive circuit (4, 7), a DC output voltage (Vh) of the DC / DC converter being available between output terminals of the AC / DC converter, The DC / DC converter according to the invention is characterized in that the DC / AC converter is a DC / AC converter with multilevel current source (6).

Description

DC / DC converter for electrical networks

Technical Field and State of the Art The invention relates to a DC / DC converter such as that shown in FIG. 1. Such a converter comprises: • a DC / AC multilevel converter whose output terminals (3a) are connected to terminals input of an inductive circuit (4, 7) in which an alternating current flows, a DC input voltage (Vb) of the DC / DC converter being applied between input terminals of the DC / AC converter, and a multi-level voltage source AC / DC converter (52) whose input terminals (3b) are connected to output terminals of the inductive circuit (4, 7), a DC output voltage (Vh) of the DC converter / DC is available between output terminals of the AC / DC converter. The inductive circuit is in this example a three-phase transformer 4; the primary windings of the transformer 4 are connected to output terminals of the DC / AC converter and the secondary windings of the transformer 4 are connected to input terminals of the AC / DC converter.

Such a converter is generally used in power electronics to switch from a DC voltage level to a different DC voltage level.

Such a converter finds an application particularly in the field of electrical networks where one seeks to interconnect two DC networks that operate at different voltage levels. Such a converter also finds an application for connecting energy sources (such as wind turbines, fuel cells, solar panels,...) Producing electrical energy at a low level of DC voltage to a grid of electricity. transmission or distribution having the highest DC voltage possible to limit losses in lines. It is therefore a question of raising the output voltage of each source of energy taken individually to bring it to the voltage of the transmission or distribution network.

The AC / DC converters or DC / AC voltage source converters here are multilevel converters that have the advantage of being modular and extensible with particularly large voltage ranges and therefore M conversion ratios between the low voltage side and the high medium or high voltage side; this makes it possible to output medium or high voltages from low input voltages.

However, the larger the conversion ratio M is, the greater the current that the DC / AC converter 51 on the low voltage side has to bear is important: the current flowing on the low voltage side is substantially equal to M times the current flowing on the medium or high voltage side.

To support these large currents, the electronic components used to make the low voltage DC / AC converter can be sized larger (in terms of the current supported) than those used to make the high voltage side AC / DC converter. This solution is however not sufficient if a very high conversion ratio M is sought. It is also possible to associate several DC / AC converters with an input parallel voltage source to distribute the current on the low voltage side. But this solution is complex to implement in practice.

DESCRIPTION OF THE INVENTION The invention proposes a new DC / DC converter that does not have the drawbacks of the converters described above. For this purpose, the invention proposes a new DC / DC converter comprising: a multi-level DC / AC converter whose output terminals (3a) are connected to input terminals of an inductive circuit (4, 7) in which AC current flows, a DC input voltage (Vb) of the DC / DC converter being applied between input terminals of the DC / AC converter, and • a multi-level voltage source AC / DC converter (52) of which input terminals (3b) are connected to output terminals of the inductive circuit (4, 7), a DC output voltage (Vh) of the DC / DC converter being available between output terminals of the AC / DC converter.

A DC / DC converter according to the invention is characterized in that the DC / AC converter is a DC / AC converter with multilevel current source (6). Compared to a voltage source DC / AC converter, a current source DC / AC converter has the advantage of being more easily scalable to withstand much larger currents, as will be seen later in the description. detailed embodiments.

The converter according to the invention can be polyphase with N phases, N being an integer greater than or equal to one. In this case: • the current-source DC / AC converter (6) has N alternative output terminals (3a), • the AC / DC voltage source converter (52) has N input terminals (3b) alternatives and • the inductive circuit (4, 7) comprises N magnetic circuits.

In a converter according to the invention, the inductive circuit may be a transformer (4) with N coupled magnetic circuits (corresponding to an N-input primary coupled to a secondary N-outputs) or N individual inductances (7). A transformer has the advantage of supporting very different voltages at the primary and secondary levels. Conversely, the voltages at the input and at the output of an inductor are quite close, the voltage conversion is done in this case via the converter AC / DC multilevel voltage source.

According to one embodiment, in the DC / DC converter according to the invention, the DC / AC converter and the AC / DC converter are of the modular multilevel converter (MMC) type, made from cascaded elementary cells, each cell elementary component comprising controlled active components and passive components for energy storage.

Such DC / AC or AC / DC converters of the MMC type make it possible to better adjust the DC / DC converter's input and output voltages and currents by adjusting the number of elementary cells on either side of the inductive circuit. Indeed, at the terminals of the inductive circuit, the amplitude of the controlled alternating signals (the current in the case of the current source converter or the voltage in the case of the voltage source converter) is adjusted by adding or subtracting small steps current, or voltage depending on the case, each step being generated by an elementary cell, as will be seen better later in examples. In this way, at the terminals of the inductive circuit, the obtained AC current or voltage has stepped waveforms with very low harmonic distortion; the losses in the magnetic components are thus reduced as well as the size of the filters essential for the harmonic filtering.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will be better understood, and other characteristics and advantages of the invention will emerge in the light of the following description of examples of DC / DC converters according to the invention. These examples are given in a non-limiting manner. The description is to be read in conjunction with the accompanying drawings, in which: FIG. 1 shows a diagram of a known DC / DC converter; FIGS. 2 and 3 show embodiments of DC / DC converters according to the invention, FIGS. 4 to 9 show detailed embodiments of essential elements of the converters according to FIG. 2 or 3, and FIG. 10 shows the evolution of the current flowing at a particular point of a converter according to the invention.

Description of Embodiments of the Invention

As previously said, in FIG. 1 is shown schematically a known DC / DC converter comprising: a transformer 4; a DC / AC DC converter 51 / AC with voltage sources whose output terminals 3a are connected to a primary of the transformer 4, a voltage source AC / DC AC / DC converter whose input terminals 3b are connected to a secondary of the transformer 4.

When a continuous low voltage Vb is applied between the input terminals of the converter 51, a high voltage Vh is available between the output terminals of the converter 52.

FIG. 2 schematically shows a DC / DC converter according to the invention. With respect to the converter of FIG. 1, the voltage source converter 51 is replaced by a current source converter 6. Such a converter 6 behaves as an ideal current source with a very high input impedance; thus, between its output terminals, it can generate a current with a steep waveform of very large amplitude up to several kiloamperes. Also, in FIG. 2, the transformer is polyphase with N phases, N being an integer greater than or equal to 1. As a variant of the converter of FIG. 2, the transformer 4 can be replaced by N inductors 7 (FIG. .

FIG. 4 shows a detailed embodiment of a multi-level modular polyphase (N-phase) DC / AC converter 6 of the type known as the Modular Multilevel Converter (MCMMC): • the converter DC / AC 6 comprises M elementary current cells 8 connected in parallel, M being an integer, the input voltage Vb of the DC / AC converter being applied between an input terminal (8a) common to the M elementary current cells. and an output terminal 8b common to the M elementary current cells, each elementary current cell comprises N output terminals connected respectively to N input terminals of the inductive circuit 4, 7.

Each elementary current cell behaves as a controlled current source comprising a plurality of control branches and a plurality of inductive energy storage elements, and more precisely in the example of FIG. 4: each elementary cell of current of the DC / AC converter 6 comprises N current branches 9 connected in parallel to build each of the N phases of the polyphase converter, • a first inductive element 20a is connected between a first common terminal 9a of the N current branches and the terminal of common input 8a of the elementary cells 8, a second inductive element 20b is connected between a second common terminal 9b of the N common current branches and the second terminal 8b of the elementary cells. Each current branch comprises two unidirectional semiconductor switches in current and with bipolar voltage blocking capability, the two semiconductor switches being connected in series between the first common terminal 9a and the second common terminal 9b of the N current branches; a common point of the two semiconductor switches forms one of the N AC output terminals of the elementary current cell 8 and is connected to one of the N input terminals of the inductive circuit 4, 7; such a switch is generically represented in FIG. 4 by a diode associated in series with a control transistor; such a switch is controllable on opening and closing.

A capacitive impedance 10 is connected between each of the N output terminals of the DC / AC converter 6 and a converter ground.

When a DC voltage Vb is applied to the input terminals of the DC / AC converter, a continuous current Idc flows at the input of the converter and is distributed between the M elementary current cells 8; an alternating lake current is available on the N alternative output terminals of the DC / AC converter 6.

Of course, the DC / AC converter of FIG. 4 is only one exemplary embodiment; depending on the power to be transmitted and the voltages and currents that the active components can support, it is possible to increase the number N of branches 9 in each elementary cell 8 of the converter and / or the number M of elementary cells. More generally, it is possible to use any type of DC / AC converter with a multilevel current source.

The DC input current Idc between the input terminals of the current source DC / AC converter of FIG. 4 is distributed equitably between the M elementary current cells 8 connected in parallel. In this way, for the same direct current Idc, the current supported by each semiconductor switch individually is much smaller than the current that would support each semiconductor switch of a voltage source DC / AC converter. In addition, by adding more or less elementary current cells 8, the current supported by each semiconductor switch of the elementary cells 8 can be increased or reduced to be adapted to the nominal values supported by the selected semiconductor switches.

The DC / AC converter of FIG. 4 generates, at its output terminals 3a, an alternating current having a stepped waveform, such as that represented in FIG. 10. For this, the converter uses inductive elements such as the elements 20a, 20b of the elementary cells 8 as energy storage element. By controlling the semiconductor switches of the conversion branches, the stored energy in the form of a direct current in the inductive elements can be selectively transmitted to the output terminals 3a of the converter with a positive or negative polarity or, alternatively, a current of zero amplitude if the current is allowed to flow between the DC terminals of the converter without going through the AC terminals. By coordinating the operation of the N elementary cells 8, it is possible to obtain the desired stair current. In the example of FIG. 10, the 9 step stepped stream is obtained using M = 4 elementary cells; more generally, the number of levels is equal to 2 * M + 1. By way of example, for a converter according to FIG. 4 sized with M = 4 elementary cells to convert a power of 8MW, if a voltage Vb of 2kV is applied between the input terminals, then an Idc current of 4kA circulates at the input of the converter and is divided into 4 currents of lkA in the four cells of the converter; thus, the semiconductor switches constituting the elementary current sources 8 in each branch support a current limited to 1kA.

FIG. 5 shows a specific embodiment of a voltage source AC / DC converter according to FIG. 2 or 3, of the active converter type and more specifically of the multilevel modular converter type (VSMMC) of the English Voltage Source Modular Multilevel Converter. , generically known by the acronym MMC): the N-phase AC / DC converter (52) comprises N conversion branches connected in parallel, the output voltage (Vh) of the AC / DC converter being available between a first terminal 52a common to the N branches and a second terminal 52b common to the N branches, • each conversion branch comprises two conversion arms 12 connected in series between the first common terminal and the second common terminal; a midpoint between the two arms of a conversion branch is connected to an output terminal 3b of the inductive circuit 4; each conversion arm comprises a plurality of elementary cells 11 of voltage connected in series.

According to one embodiment, each elementary voltage cell behaves as a controlled voltage source comprising a plurality of control branches and a plurality of capacitive energy storage elements. In the example of FIG. 6, an elementary voltage cell 11 comprises a control branch comprising a pair of half-bridge connected semiconductor switches 11a, 11b and a capacitive element 11c connected in parallel to the pair of switches. In the example of FIG. 7, an elementary voltage cell comprises two control branches each comprising a pair of switches 11a, 11b, respectively 11d, 11c connected in full bridge and a capacitive element 11c connected in parallel on each branch of ordered.

FIG. 8 shows another embodiment of an AC / DC converter according to FIG. 2 or 3; here it is a passive converter of the diode rectifier type in configuration 2 * N pulses (N = 3 phases in FIG. 8), produced in the following manner: • the AC / DC converter 52 with N phases comprises N conversion branches connected in parallel, the output voltage (Vh) of the AC / DC converter being available between a first terminal 52a common to the N branches and a second terminal 52b common to N branches, • each conversion branch comprises two arms 13 of conversion mounted half-bridge: the two arms 13 are connected in series between the first common terminal 52a and the second common terminal 52b; a midpoint between the two arms of a conversion branch is connected to an output terminal 3b of the inductive circuit 4; each conversion arm comprises a plurality of diodes 14 connected in series.

The half-bridge diodes rectify the alternating current from the transformer 4 and provide a DC current on the output terminals 52a, 52b.

FIG. 9 shows a variant of the circuit of FIG. 8 in which the voltage source AC / DC converter is of the diode rectifier type in the 4 * N configuration (N = 3 phases in FIG. 9) produced in the following manner: The N-phase AC / DC converter 52 comprises N conversion branches connected in parallel, the output voltage (Vh) of the AC / DC converter being available between a first terminal 52a common to the N branches and a second terminal 52b common to the N branches, • each conversion branch comprises four rectifier arms 13 connected in series between the first common terminal 52a and the second common terminal 52b; The inductive circuit has a group of N primary windings and two groups of N secondary windings; the windings of the first group of secondary windings are star-connected and the windings of the second group of secondary windings are connected in a triangle; A mid-point between the first and second arms of a branch is connected to an output terminal 3bl of the first secondary winding and a midpoint between the third and fourth arms of a branch is connected to an output terminal 3b2 second secondary winding • the middle points between the second and third arms of all the branches are connected together, • each conversion arm comprises a plurality of diodes 14 connected in series.

The converter of FIG. 9 (configuration 12 taps) is equivalent to two converters according to FIG. 8 (configuration 6 taps), input (alternating side) each connected to a secondary of the transformer 4 and to the output (DC side) connected in series . With respect to the converter of FIG. 8, the converter of FIG. 9 produces a direct current having a much lower AC component, since there are 12 (4 * N) conduction sub-cycles of the rectifier arms instead of 6 (2 * N). At the output of the DC / DC converter, the final result is a direct current containing fewer harmonics than in the case of a diode rectifier converter type 2 * N pulses; thus the size and the cost of the harmonic filters conventionally used at the output of the DC / DC converter can be reduced.

Unlike an active voltage source converter 52, a diode rectifier bridge is a passive converter, which offers less flexibility than an active voltage source converter. For example, a diode rectifier type converter is not able to regulate the value of the output DC voltage Vh, and thus the output voltage Vh depends directly on the value of the input AC voltage. Also, a diode rectifier converter is a unidirectional diode rectifier has the advantage of being simpler, cheaper and more compact than an active voltage source converter; therefore, even if it is not suitable for all DC / DC conversion applications, it is still a good alternative to the active converter for some particular applications.

Of course, other embodiments of AC / DC high voltage side converter are possible, in particular depending on the power to be transmitted and the voltage and current that can withstand the electronic components used.

For example, the high voltage side AC / DC converters according to FIGS. 8, 9, whose conversion arms consist of diodes, are non-reversible: because of the non-reversibility of the diodes, it is not possible to reverse the input terminals and the output terminals of the DC / DC converter to produce a step-down converter. Thus, if a reversible DC / DC converter is desired, it will be preferred to make the AC / DC converter based on controlled transistors rather than diodes. Also, in all the detailed examples in connection with the drawings, the converters according to the invention have N = 3 phases. Of course, it is possible to realize single-phase converters as well as converters with N = 2, 4, 5, ... phases (more generally N an integer greater than 1).

NOMENCLATURE

I DC / DC converter 3a DC / AC converter outputs 51 3b AC / DC converter inputs 52 4 transformer (or more generally inductive circuit) 51 DC / AC low voltage converter with source of voltage 52 AC / DC high voltage converter with source Of voltage

52a, 52b output terminals of the AC / DC converter 6 DC / AC converter with current source 7 inductance (or more generally inductive circuit) 8 M elementary current cells 8a, 8b input terminal and output terminal of a cell current element 8 9 current branch 9a, 9b first terminal and second terminal of a current branch 9 10 capacitive impedance II elementary voltage cell 11a, 11b, 11d, 11c semiconductor switches 11c capacitive element 13 current rectifying arm 14 diodes 20a, 20b first and second inductive element of a current elementary cell 8

Claims (10)

A DC / DC converter comprising: a multi-level DC / AC converter whose output terminals (3a) are connected to input terminals of an inductive circuit (4, 7) in which an alternating current, a low current input DC voltage (Vb) of the DC / DC converter being applied between input terminals of the DC / AC converter, and • a multi-level voltage source AC / DC converter (52) having input terminals (3b ) are connected to output terminals of the inductive circuit (4, 7), a high DC output voltage (Vh) of the DC / DC converter being available between output terminals of the AC / DC converter, DC / DC converter characterized by the DC / AC converter is a DC / AC converter with a multilevel current source (6). 2. Converter according to one of the preceding claims comprising N phases, N being an integer greater than or equal to one, and wherein: • the current source DC / AC converter (6) has N alternative output terminals, • the voltage source AC / DC converter (52) has N AC input terminals, and • the inductive circuit (4, 7) comprises N magnetic circuits. 3. Converter according to the preceding claim wherein the inductive circuit is a transformer (4) with N coupled magnetic circuits or N individual inductances (7). 4. DC / DC converter according to one of the preceding claims wherein the DC / AC converter and the AC / DC converter are of the modular multilevel converter (MMC) type, made from cascaded elementary cells, each elementary cell comprising controlled active components and passive components for energy storage. 5. DC / DC converter according to one of the preceding claims wherein the DC / AC converter (5) comprises M elementary current cells (8) connected in parallel, M being an integer, the input voltage (Vb) of the DC / AC converter being applied between a common input terminal (8a) of the M elementary current cells and a common output terminal (8b) of the M elementary current cells, each elementary current cell acting as a source of controlled current comprising a plurality of control branches and a plurality of inductive energy storage elements, each elementary current cell comprising N output terminals connected respectively to N input terminals of the inductive circuit (4, 7). 6. DC / DC converter according to the preceding claim wherein each elementary current cell of the DC / AC converter (6) comprises N branches of current (9) connected in parallel, a first inductive element (20a) being connected between a first terminal (9a) common N current branches and the first terminal (8a) common elementary cells (8), a second inductive element (20b) being connected between a second terminal (9b) common N current branches and the second terminal (8b) common elementary cells (8), each current branch 9 comprising two semiconductor switches unidirectional current and with bipolar voltage blocking capability, the two semiconductor switches being connected in series between the first common terminal (9a) and the second common terminal (9b) of the N current branches, a common point of the two semiconductor switches forming one of the N output terminals of the cell elementary current element 8 connected to one of the N input terminals of the inductive circuit (4, 7). 7. DC / DC converter according to one of the preceding claims wherein the N-phase AC / DC converter (52) comprises N conversion branches connected in parallel, the output voltage (Vh) of the AC / DC converter being available between a first common terminal with N branches and a second common terminal with N branches, each conversion branch comprising two conversion arms (12) connected in series between the first common terminal and the second common terminal, a midpoint between the two arms; a conversion branch being connected to an output terminal of the inductive circuit, each conversion arm comprising a plurality of voltage elementary cells (11) connected in series and each elementary voltage cell acting as a controlled voltage source comprising a plurality of control branches and a plurality of capacitive energy storage elements. 8. DC / DC converter according to claim 7 wherein an elementary voltage cell comprises a pair of half-bridge connected semiconductor switches (11a, 11b) and a capacitive element (11c) connected in parallel to the pair of switches. . 9. DC / DC converter according to claim 7 wherein an elementary voltage cell comprises two pairs of switches (11a, 11b, 11d, 11c) connected in full bridge and a capacitive element (11c) connected in parallel on each pair of dc switches. 10. Converter according to one of claims 1 to 4 wherein the AC / DC converter is a diode rectifier bridge configuration 2 * N pulses or a diode rectifier bridge configuration 4 * N pulses.