FR3059855A1 - 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

Holder (s): SUPERGRID INSTITUTE Simplified joint-stock company.

Extension request (s)

Agent (s): INNOVATION COMPETENCE GROUP.

DC / DC CONVERTER FOR ELECTRICAL NETWORKS.

FR 3 059 855 - A1 (by) The invention relates to a DC / DC converter comprising:

a multilevel DC / AC converter, the output terminals of which (3a) are connected to the input terminals of an inductive circuit (4, 7) in which an alternating current flows, a direct input voltage (Vb) of the converter DC / DC being applied between input terminals of the DC / AC converter, and an AC / DC converter with multilevel voltage source (52) whose input terminals (3b) are connected to output terminals of the inductive circuit (4, 7), a direct 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 / DC converter AC is a multi-level current source DC / AC converter (6).

i

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 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 voltage (Vb) of the DC / DC converter being applied between input terminals of the DC / AC converter, and • an AC / DC converter with source of multilevel voltage (52) of which input terminals (3b) are connected to output terminals of the inductive circuit (4, 7), a direct output voltage (Vh) of the DC / DC converter being available between output terminals from 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 the output terminals of the DC / AC converter and the secondary windings of the transformer 4 are connected to the input terminals of the AC / DC converter.

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

Such a converter finds an application in particular in the field of electrical networks where it is sought to interconnect two direct current networks which 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 direct voltage to a network of transmission or distribution with the highest possible DC voltage to limit line losses. It is therefore a question of raising the output voltage of each energy source taken individually to bring it to the voltage of the transmission or distribution network.

The AC / DC or DC / AC converters with voltage source are here multilevel converters which have the advantage of being modular and extensible with particularly wide 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 obtain medium or high voltages at the output from low input voltages.

However, the greater the conversion ratio M, the greater the current which the DC / AC converter 51 must withstand on the low voltage side: 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 DC / AC converter on the low voltage side can be dimensioned larger (in terms of current supported) than those used to make the AC / DC converter on the high voltage side. This solution is however not sufficient if a very high conversion ratio M is sought, it is also possible to combine several DC / AC converters with voltage source in parallel at the input 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 which does not have the drawbacks of the converters described above. To this end, the invention proposes a new DC / DC converter comprising:

• a multilevel DC / AC converter whose output terminals (3a) are connected to input terminals of an inductive circuit (4, 7) in which an alternating current, a direct input voltage (Vb) of the DC / DC converter being applied between input terminals of the DC / AC converter, and • an AC / DC converter with multi-level voltage source (52) whose input terminals (3b) are connected to output terminals of the inductive circuit (4, 7), a direct 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 a multi-level current source (6). Compared to a DC / AC converter with voltage source, a DC / AC converter with current source has the advantage of being more easily scalable to support much larger currents, as will be seen better below 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 that case :

• the DC / AC converter with current source (6) has N alternative output terminals (3a), • the AC / DC converter with voltage source (52) has N alternative input terminals (3b), and • the inductive circuit (4, 7) comprises N magnetic circuits.

In a converter according to the invention, the inductive circuit can be a transformer (4) with N coupled magnetic circuits (corresponding to a primary with N inputs coupled to a secondary with N outputs) or N individual inductances (7). A transformer has the advantage of withstanding very different voltages in primary and secondary. Conversely, the voltages at the input and at the output of an inductor are fairly close, the voltage conversion is done in this case via the AC / DC converter with 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 multilevel modular converters (MMC) type, produced from elementary cells connected in cascade, each cell elementary comprising active controlled components and passive components for energy storage.

Such DC / AC or AC / DC converters of the MMC type make it possible to adjust the voltages and currents at the input and output of the DC / DC converter as best as possible, 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 alternating signals controlled (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 as the case may be, each step being generated by an elementary cell, as will be seen better below in examples. In this way, at the terminals of the inductive circuit, the alternating current or voltage obtained have stepped waveforms having a very low harmonic distortion; the losses in the magnetic components are thus reduced as well as the size of the filters essential for the filtering of the harmonics.

Brief description of the figures

The invention will be better understood, and other characteristics and advantages of the invention will appear in the light of the following description of examples of DC / DC converters according to the invention. These examples are given without limitation. The description should be read in conjunction with the accompanying drawings in which:

• Figure 1 shows a diagram of a known DC / DC converter • Figures 2 and 3 shows embodiments of DC / DC converters according to the invention, • Figures 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 said previously, in FIG. 1 is shown schematically a known DC / DC converter comprising:

• a transformer 4, • a 51 DC / AC DC / AC converter with voltage source whose output terminals 3a are connected to a primary of the transformer 4, • a 52 AC / DC AC / DC converter with voltage source whose input terminals 3b are connected to a secondary of transformer 4.

When a low direct 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.

In Figure 2 is shown schematically a DC / DC converter according to the invention. Compared to the converter of FIG. 1, the converter 51 with voltage source is replaced by a converter 6 with current source. Such a converter 6 behaves like an ideal current source with a very high input impedance; thus, between its output terminals, it can generate a current with a very large amplitude staircase waveform of 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 in FIG. 2, the transformer 4 can be replaced by N inductors 7 (fig. 3) .

FIG. 4 shows a detailed embodiment of a polyphase (N-phase) DC / AC 6 converter with current source, of the multilevel modular converter type (or CSMMC, from Current Source Modular Multilevel Converter):

• the DC / AC converter 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 cells elementary current 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 like 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 current cell 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 common input terminal 8a of the elementary cells 8, a second inductive element 20b is connected between a second common terminal 9b of the N current branches and the second common terminal 8b of the elementary cells. Each current branch comprises two unidirectional current semiconductor switches and with bipolar voltage blocking capacity, 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 alternating 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 represented generically in FIG. 4 by a diode associated in series with a control transistor; such a switch can be controlled 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 ground of the converter.

When a direct 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 among the M elementary current cells 8; AC alternating current is available at the N AC output terminals of the DC / AC converter 6.

Of course, the DC / AC converter of FIG. 4 is only an example of an embodiment; depending on the power to be transmitted and the voltages and currents that the active components can withstand, 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 continuous input current Idc between the input terminals of the DC / AC converter with current source 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 weaker than the current that each semiconductor switch would support from a DC / AC converter with voltage source. 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 that the selected semiconductor switches support.

The DC / AC converter of FIG. 4 generates, on 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 an energy storage element. By controlling the semiconductor switches of the conversion branches, the energy stored in the form of 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 zero amplitude current if current is allowed to flow between the DC terminals of the converter without crossing the AC terminals. By coordinating the operation of the N elementary cells 8, it is possible to obtain the desired staircase current. In the example of FIG. 10, the staircase current with 9 levels is obtained by using M = 4 elementary cells; more generally, the number of levels is equal to 2 * M + 1.

For example, for a converter according to figure 4 dimensioned with M = 4 elementary cells to convert a power of 8MW, if a voltage Vb of 2kV is applied between the input terminals, then a current Idc of 4kA flows at the input of the converter and is divided into 4 lkA currents in the four cells of the converter; thus, the semiconductor switches constituting the elementary sources 8 of current in each branch support a current limited to lkA.

FIG. 5 shows a specific embodiment of an AC / DC converter with voltage source according to FIG. 2 or 3, of the active converter type and more precisely of the multilevel modular converter type (VSMMC from the English Voltage Source Modular Multilevel Converter , generically known by the acronym MMC):

• 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 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 voltage cells 11 connected in series.

According to one embodiment, each elementary voltage cell behaves like 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 semiconductor switches 11a, 11b connected in half-bridge and a capacitive element 11c connected in parallel on 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.

Figure 8 shows another embodiment of an AC / DC converter according to Figure 2 or 3; here it is a passive converter of the diode rectifier type in 2 * N pulse configuration (N = 3 phases in FIG. 8), produced as follows:

• 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 the N branches, • each conversion branch comprises two conversion arms 13 mounted in a 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 diode half-bridges rectify the alternating current coming from the transformer 4 and supply a direct current on the output terminals 52a, 52b.

FIG. 9 shows a variant of the circuit of FIG. 8 where the AC / DC converter with voltage source is of the diode rectifier type in a 4 * N pulse configuration (N = 3 phases in FIG. 9) 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 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 connected in a star and the windings of the second group of secondary windings are connected in a triangle;

• a midpoint 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 of the second secondary winding • the midpoints 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 in Figure 9 (12-pin configuration) is equivalent to two converters according to Figure 8 (6-pin configuration), at the input (alternating side) each connected to a secondary of the transformer 4 and at the output (continuous side) connected in series . Compared to the converter of figure 8, the converter of figure 9 produces a direct current having a much weaker alternating component, since there are 12 (4 * N) sub-cycles of conduction 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 less harmonics than in the case of a diode rectifier converter of type 2 * N pulses; thus the size and the cost of the harmonic filters traditionally 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 a converter with active voltage source. For example, a diode rectifier converter is not capable of regulating the value of the output DC voltage Vh, and therefore the output voltage Vh depends directly on the value of the AC input voltage. Also, a diode rectifier converter is a unidirectional diode rectifier has the advantage of being simpler, less expensive and more compact than a converter with active voltage source; therefore, even if it is not suitable for all DC / DC conversion applications, it remains a good alternative to the active converter for certain particular applications.

Of course, other embodiments of the AC / DC converter on the high voltage side can be envisaged, in particular as a function of the power to be transmitted and of the voltage and the current which the electronic components used can support.

For example, the AC / DC converters on the high-voltage side according to FIGS. 8, 9, whose conversion arms consist of diodes, are non-reversible: due to the non-reversibility of the diodes, it is not possible to reverse the input and output terminals of the DC / DC converter to make a step-down converter. Thus, if a reversible DC / DC converter is sought, we will rather choose to make the AC / DC converter based on controlled transistors rather than diodes.

Also, in all the examples detailed in relation to the drawings, the converters according to the invention have N = 3 phases. Of course, it is possible to make single-phase converters as well as converters with N = 2, 4, 5, ... phases (ie more generally N an integer greater than 1).

NOMENCLATURE

I DC / DC converter

3a outputs of the DC / AC converter 51

3b inputs of the AC / DC converter 52

4 transformer (or more generally inductive circuit) low voltage DC / AC converter with voltage source High voltage AC / DC converter with voltage source

52a, 52b output terminals of the AC / DC converter DC / AC converter with current source 10 7 inductance (or more generally inductive circuit)

M elementary current cells

8a, 8b input terminal and output terminal of an elementary current cell 8 current branch

9a, 9b first terminal and second terminal of a current branch 9 15 capacitive impedance

II elementary voltage cell lia, 11b, lld, lie semiconductor switches

11c capacitive element

13 diode current rectifier arm

20a, 20b first and second inductive element of an elementary current cell 8

Claims (10)

1. DC / DC converter including: • a multilevel DC / AC converter whose output terminals (3a) are connected to input terminals of an inductive circuit (4, 7) in which an alternating current, a direct input voltage (Vb) of the DC / DC converter being applied between input terminals of the DC / AC converter, and • an AC / DC converter with multi-level voltage source (52) whose input terminals (3b) are connected to output terminals of the inductive circuit (4, 7), a direct output voltage (Vh) of the DC / DC converter being available between output terminals of the AC / DC converter, DC / DC converter characterized in that the DC / AC converter is a converter DC / AC with multi-level 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 in which: • the DC / AC converter with current source (6) has N alternative output terminals, • the AC / DC converter with voltage source (52) has N alternative input terminals, and • the inductive circuit (4, 7 ) includes 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 inductors (7) individual. 4. DC / DC converter according to one of the preceding claims, in which the DC / AC converter and the AC / DC converter are of the multilevel modular converters (MMC) type, produced from elementary cells connected in cascade, each elementary cell comprising controlled active components and passive components for energy storage. 5. DC / DC converter according to one of the preceding claims, in which the DC / AC converter (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 behaving 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 current branches (9) connected in parallel, a first inductive element (20a) being connected between a first terminal (9a) common of the N current branches and the first common terminal (8a) of the elementary cells (8), a second inductive element (20b) being connected between a second terminal (9b) common of the N current branches and the second terminal (8b) common of the elementary cells (8), each current branch 9 comprising two unidirectional semiconductor switches in current and with ίο bipolar voltage blocking capacity, 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 c elementary current ellule 8 connected to one of the N input terminals of the inductive circuit (4, 7). 5. DC / DC converter according to one of the preceding claims, in which 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 common to the N branches and a second terminal common to the 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 of a branch of 10 conversion being connected to an output terminal of the inductive circuit, each conversion arm comprising a plurality of elementary voltage cells (11) connected in series and each elementary voltage cell behaving like a controlled voltage source comprising a plurality of 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 15 of semiconductor switches (11a, 11b) connected in half-bridge and a capacitive element (11c) connected in parallel on the pair of switches. 9. DC / DC converter according to claim 7, in which an elementary voltage cell comprises two pairs of switches (lla, llb, lld, lie) connected in full bridge and a capacitive element (11c) connected in parallel on each pair of 'switches. 20 10. Converter according to one of claims 1 to 4 wherein the AC / DC converter is a diode rectifier bridge in configuration 2 * N pulses or a diode rectifier bridge in configuration 4 * N pulses. 1/4 3a 3b