DE102020127328A1 - Multilevel converter for converting electrical energy - Google Patents

Abstract

The invention relates to a method for resetting the status of components (10) of a vehicle to a basic setting.
Multilevel converter for converting electrical energy, with one connection for supplying electrical energy to be converted (input) and another connection for delivering the converted electrical energy (output), comprising a level generation unit and a converter unit that are set up to a specific number of voltage levels at the output, wherein the level generation unit is formed from at least one submodule, wherein the at least one submodule is formed by an electrical circuit comprising a first branch with an active switching element connected in series with at least one partial voltage source and a second branch with a switching element connected in parallel to the first branch.

Description

The present invention relates to a multilevel converter for converting electrical energy, with one connection for supplying electrical energy to be converted and another connection for outputting the converted electrical energy. The invention also relates to the use of such a converter according to the invention.

The embodiments described herein relate generally to the field of direct current (DC) to alternating current (AC) conversion, or direct voltage to alternating current voltage, and vice versa, more specifically multilevel converters (inverters and rectifiers).

With the increasing use of gate-controlled electronic instrumentation technologies, a variety of power converters have emerged. For example, various types of power converters and converters are used in a variety of industrial applications. In particular, multilevel converters offer high power quality, high efficiency, reduced harmonic deviation (THD) of the output voltage, improved electromagnetic compatibility and high-quality voltage waveforms close to a sinusoidal shape.

Such power conversion is particularly useful for directly linking renewable energy systems such as wind turbines or solar systems to a utility grid. In general, a wind turbine includes a rotor having a plurality of rotor blades that converts wind energy into mechanical torque that drives one or more electrical generators. Due to the growing number of renewable energy systems that are connected directly to a supply grid without intermediate storage, the inconstant feeding into the supply grid has a major impact on the grid voltage and grid frequency. It is therefore necessary to control the voltage profile and the frequency at the output of the generator set. Therefore, one or more voltage converters are coupled to the generator to ensure an appropriate voltage and frequency of the supply network.

Converters with a small number of output voltage levels, for example two levels, have disadvantages in terms of output AC voltage and current quality, low efficiency, high voltage drop and reduced power factor. These disadvantages can be compensated for with multilevel converters by being able to provide higher output levels. However, the number of power electronics components used increases significantly as the output levels increase.

Conventional multilevel converters can be classified into three main groups. In cascading H-bridge converters (CHB), in so-called Flying Capacitor Converters (FC) and so-called Neutral Point Clamped Converters (NPC). These multilevel converters are formed by a specific arrangement of power components and capacitances as voltage sources. The voltage generated at the output has a stepped signal form due to the different switching times of individual circuit breakers. Depending on the number of circuit breakers, the output voltage increases or decreases through the addition of individual connected voltages from the capacitances contained in the circuits. There is a need for a large number of power switches, DC power sources and gate control circuitry to provide the required number of voltage levels. This disadvantageously increases power dissipation, size and weight of the device and decreases efficiency. Last but not least, the manufacturing and operating costs of a conventional multilevel converter are increased due to the additional requirement for power electronics components.

The object of the invention is now to overcome the disadvantages of the prior art and to provide a topography for a multilevel converter which enables improved energy conversion while minimizing the power electronic components used.

The object is solved by the subject matter of the independent patent claim. Preferred refinements and further developments of the invention result from the features mentioned in the respective dependent subclaims, the drawings and the associated description.

Accordingly, the proposed topologies can be adapted for different industrial applications. Some advantageous applications are, for example, the connection of renewable energy sources, preferably wind power or solar power, to a power grid. in particular improved DC/AC conversion of renewable energy sources using multilevel converters for medium and high voltage applications. Furthermore, the proposed topologies can be used advantageously for vehicle electrical applications, in particular for controlling AC motors by converting battery DC voltage/current into high-quality AC voltage/current. Furthermore, the proposed topologies can be advantageously used to reduce the harmonics of the output voltage in applications where a low total harmonic distortion (THD) is required. Furthermore, the proposed topologies can be used advantageously for active filtering and for reactive power compensation.

A first aspect of the present invention relates to a multilevel converter for converting electrical energy, with a connection for supplying electrical energy to be converted (input) and another connection for delivering the converted electrical energy (output), comprising a level generation unit and a converter unit, which are set up to generate a specific number of voltage levels at the output, the level generation unit being formed from a first submodule and at least one further submodule connected in series with the first submodule, a submodule being formed by an electrical circuit comprising a first branch with an active switching element connected in series with at least one partial voltage source and a second branch with a switching element connected in parallel to the first branch.

The voltage level at the output can preferably be increased by the multilevel converter being supplemented with further submodules according to the invention. The multilevel converter according to the invention has a single-phase design, but can preferably also be formed as a single phase of a multi-phase converter. In principle, the converter according to the invention makes it possible to significantly reduce the effort involved in converting, since direct energy coupling can be provided on the output side to a power supply network without the need for additional coupling devices, such as known energy coupling devices that are based on the use of multiple inverters and Isolation transformers are based. Furthermore, with the proposed switching topography, the number of active switching elements to be controlled is advantageously reduced and the serial modular design offers the possibility of simply increasing the voltage level by combining different submodule types according to the invention, while at the same time minimizing the need for switching elements to be controlled. This not only significantly reduces the effort involved in converting, which increases efficiency and output quality, but also, and above all, costs, installation space and control complexity are reduced.

The output is preferably connected to an energy supply network. As a rule, these are energy supply networks that are operated at 50 Hz or 60 Hz with AC voltage. Furthermore, an effective voltage between two phases of the power supply network of 400 volts is often used. In such a case, it is therefore preferable to also provide a maximum voltage of 400 volts at the output of the converter.

More preferably, the multilevel converter can be used in a medium-voltage network, i.e. with an operating voltage greater than 1 kV, or in the field of high-voltage transmission, i.e. with an operating voltage greater than 100 kV, provided that the components are switch-resistant and meet the requirements.

Each of the submodules preferably comprises a first branch with an active switching element connected in series with at least one partial voltage source and a second branch with a switching element connected in parallel with the first branch. In particular, it is preferred that the switching element of the second branch is formed by a passive switching element, in particular by a diode. The diode is preferably arranged in the forward direction, so that the second branch of the submodule forms a passive bypass for the first branch and when the active switching element in the first branch is blocked, the partial voltage present at the partial voltage source cannot contribute to the voltage levels at the output.

Alternatively, the switching element of the second branch of a submodule can also be formed by an active switching element. The switching element in the first submodule is preferably formed by an active switching element. This ensures that there is no path in the circuit that does not have at least one actively controlled switching element. This advantageously ensures the switching quality for all possible output voltage levels.

The actively switched switching elements are connected to a control unit, which controls the switching elements in a suitable manner in order to implement the desired conversion process with the associated voltage levels. Basic control methods with regard to the conversion of energy by means of a multilevel converter between a DC voltage and an AC voltage are basically known to the person skilled in the art.

An active switching element within the meaning of the disclosure is preferably a controllable electronic switching element, in particular a controllable electronic semiconductor switch, for example a transistor, a metal-oxide-semiconductor field-effect transistor (MOSFET), a thyristor, combination circuits thereof, a gate-turn-of thyristor (GTO) , an isolated gate bipolar transistor (IGBT), in particular an IGBT with a blocking diode connected in antiparallel, or the like. An active switching element is particularly preferably formed by a discrete IGBT without a blocking diode connected antiparallel. The switching elements can preferably be controlled by a control unit and are more preferably designed as high-frequency switches.

It is also preferred that the total input voltage is provided by a DC voltage source and a partial voltage that is formed by dividing the total input voltage is present at the at least one partial voltage source. The converter is therefore preferably operated and controlled as an inverter between a DC voltage circuit at the input and an AC voltage circuit at the output.

A partial voltage source is preferably formed by a capacitor. The capacitor can be formed by a film capacitor, a ceramic capacitor, but also by an electrolytic capacitor or the like suitable for frequency applications. The capacitor of the partial voltage source can preferably also be formed by a combination of several individual capacitors. The same partial voltage is preferably present at the different partial voltage sources, the sum of the different partial voltages corresponding to the total voltage present at the input. This ensures that the distances between two adjacent voltage levels at the output are identical.

The converter unit is preferably formed by an H-bridge circuit. In a particularly preferred embodiment, the H-bridge circuit of the converter unit is formed from four active switching elements. Such a converter unit is consequently set up to change the polarity of the specific number (N _levels ) of voltage levels generated by the level generation unit in a second half-cycle of the voltage curve. In addition to the polarity change, the converter unit provides the zero levels for both the positive and negative half cycles. By way of example, a multilevel converter according to the invention with a level generation unit that generates four voltage levels supplies a total of nine separate operating states with four positive voltage levels, four negative voltage levels and one zero level at the output. The active switching elements of the H-bridge circuit can likewise be formed by the controllable electrical semiconductor switches described. However, due to the way the H-bridge circuit works, transistors without parallel blocking diodes, such as a discrete IGBT without an anti-parallel diode, are ruled out.

A preferred embodiment of a submodule type comprises in the first branch of the submodule two partial voltage sources connected in series with an active switching element arranged between them. This submodule type is preferably used for the other submodules that connect to the first submodule. In an alternative preferred type of submodule, the active switching element is not arranged in series between the two partial voltage sources, but rather in series before or after the two voltage sources. Such a submodule behaves like a submodule with a partial voltage source with an applied double partial voltage.

A multilevel converter of this type is particularly preferably characterized in that a number N _levels of voltage levels at the output is given by N _levels =2n+1, where n is the number of partial voltage sources.

In a particularly preferred embodiment, the level generation unit comprises a first submodule with a first partial voltage source connected in series with an active switching element in the first branch and a further active switching element in the second branch connected in parallel, with further submodules each having two partial voltage sources connected in series, with the first submodule and the further submodules are connected via a second partial voltage source connected in series between them. This structure preferably forms a symmetrical multilevel converter with an even number of partial voltage sources. The second partial voltage source connected in series ensures that there is no zero level at the output of the level generation unit, but a first voltage level, even if the switches of the submodules block the current flow and only the second bypass branch is active.

An asymmetrical multilevel converter with an odd number of partial voltage sources is preferably formed in that a last submodule of the further submodules in the first branch has only one partial voltage source.

In a further preferred aspect of the invention, the multilevel converter is used for directly coupling a renewable energy source to an AC voltage supply network, the input of the multilevel converter being connected to a DC voltage connection of the renewable energy source and the output of the multilevel -Converter is connected to a mains connection of the AC voltage supply network. In particular, the input can be connected to a direct current output line of a DC/DC converter of an electrical generator of a wind turbine or directly to photovoltaic systems.

The features and combinations of features mentioned above in the description, as well as the features and combinations of features mentioned below in the description of the figures and/or shown alone in the figures, can be used not only in the combination specified in each case, but also in other combinations, without going beyond the scope of the invention leaving. The invention is therefore also to be considered to include and disclose embodiments that are not explicitly shown and explained in the figures, but that result from the explained embodiments and can be generated by separate combinations of features. Versions and combinations of features are also to be regarded as disclosed which therefore do not have all the features of an originally formulated independent claim. Furthermore, embodiments and combinations of features, in particular through the embodiments presented above, are to be regarded as disclosed which go beyond or deviate from the combinations of features presented in the back references of the claims.

The terms used, such as "first", "second", "other" and "last" are not to be construed as being associated with any particular arrangement, quantity or importance, but are only used to distinguish individual elements from one another.

• 1 eine schematische Darstellung von Schaltdiagrammen verschiedener Grundtypen von erfindungsgemäßen Submodulen;
• 2 eine schematische Darstellung von Schaltdiagrammen verschiedener Levelerzeugungseinheiten bestehend aus erfindungsgemäßen Submodulen;
• 3 eine schematische Darstellung eines Schaltdiagramms für erweiterte Levelerzeugungseinheiten;
• 4 eine schematische Darstellung eines Schaltdiagramms eines erfindungsgemäßen 9-Level Multilevel-Umwandlers.
• 5 ein Diagramm das einen Zeitausschnitt einer Spannung am Ausgang des Multilevel-Umwandlers darstellt;
• 6 ein Diagramm das einen Zeitausschnitt der totalen harmonischen Distorsion (THD) einer Ausgangsspannung darstellt;
• 7 ein Diagramm das einen Zeitausschnitt von eingeleitetem Strom und Versorgungsnetzspannung darstellt;
• 8 eine schematische Darstellung einer Energieeinspeisekonfiguration von Erneuerbaren Energiequellen in ein Versorgungsnetz zeigt;
• 9 eine schematische Darstellung von PV-Modulen und PV-Strängen zeigt;
• 10 eine schematische Darstellung eines PV-Strangs als Teilspannungsquelle zeigt;
• 11 eine schematische Darstellung eines Multilevel-Umwandlers mit Teilspannungsquellen aus 10 zeigt; und
• 12 eine schematische Darstellung der Ausgangsspannung und der Spannungslevel zeigt.

The invention is explained below in exemplary embodiments with reference to the associated drawings. Show it:

• 1 a schematic representation of circuit diagrams of different basic types of submodules according to the invention;
• 2 a schematic representation of circuit diagrams of different level generation units consisting of submodules according to the invention;
• 3 a schematic representation of a circuit diagram for extended level generation units;
• 4 a schematic representation of a circuit diagram of an inventive 9-level multilevel converter.
• 5 a diagram representing a time section of a voltage at the output of the multilevel converter;
• 6 a diagram showing a time segment of the total harmonic distortion (THD) of an output voltage;
• 7 a diagram representing a time segment of the current introduced and the mains voltage;
• 8th shows a schematic representation of an energy feed configuration of renewable energy sources in a supply network;
• 9 shows a schematic representation of PV modules and PV strings;
• 10 shows a schematic representation of a PV string as a partial voltage source;
• 11 a schematic representation of a multilevel converter with partial voltage sources 10 indicates; and
• 12 shows a schematic representation of the output voltage and the voltage levels.

A multilevel converter 10 according to the invention comprises a level generation unit 11 and a converter unit 12 . The level generation unit 11 is formed by a plurality of submodules 13 . 1 13 shows the circuit diagram of exemplary configurations of submodules 13. A first submodule type (d) is formed by two parallel branches, with a partial voltage source Udn being arranged in series with an active controlled switching element San in a first branch. The partial voltage source can advantageously be formed by a capacitor. The actively controlled switching element San is preferably an IGBT, in particular using discrete IGBTs without an anti-parallel diode. The first branch is connected in parallel to a second branch with a passive switching element, for example a diode Dn.

The partial voltages or partial voltage sources are marked with C, U or V interchangeably in the figures.

A further submodule type (e) is formed by two parallel branches, with two series-connected partial voltage sources Udn and Udn-1 being arranged in the first branch with an active switching element San arranged between them.

The submodule type (f) also has two partial voltage sources in the first branch, whereby two partial voltage sources connected directly in series can be regarded as one partial voltage source with the value 2Udn.

A schematic representation of circuit diagrams of different level generation units 11 consisting of submodules 13 according to the invention is based on FIG 2 out.

In the topology (a) of a first exemplary level generation unit 11, a first submodule 13' is provided, which is essentially of the submodule type (d), although in the parallel second branch instead of the diode, i.e. a passive switching element, there is also one from a control unit controlled active electrical circuit breaker Sc is provided. However, the first branch is identical to a series-connected partial voltage source Ud1, which is formed in particular by a capacitor, and an active switching element Sb. A further (second) submodule 13" of submodule type (e) is connected via a series-connected second partial voltage source Ud2 . This includes the partial voltage sources Ud3 and Ud4. Such a topology of a level generation unit 11 can provide 4 voltage levels. In combination with an H-bridge converter 12, a 9-level multilevel converter is thus formed. A slightly modified variant of the topology (a ) is in the topology (b) the 2 shown. The further (second) submodule 13" is identical to the further (second) submodule 13" of topology (a). The first submodule 13' corresponds to the submodule type (d) with a diode as a passive power element in the parallel second branch of the circuit. However, a further active switching element Sc is provided in the switching direction of the output connection. A switching variant similar to the topology (a) is shown in c), where the topology in c) can provide 5 voltage levels and can therefore be used for an 11-level multilevel converter.

The topologies can be easily expanded with additional 13" submodules (see 3 ), which also increases the number of voltage levels that can be generated. The multilevel converters can be expanded both symmetrically and asymmetrically. For a symmetrical structure, the last extended submodule 13''' is formed by a submodule type with two partial voltage sources Udn, Udn-1, for example submodule type e). 1 , so that the total number of partial voltage sources is even. For an asymmetrical structure, the last extended submodule 13"' is formed by a submodule type with a partial voltage source Udn, for example submodule type d). 1 , so that the total number of partial voltage sources is odd.

The number of actively controlled switching elements required is reduced to a minimum compared to conventional multilevel converters and results for the level generation unit from Table 1 for the in 4 used symmetrical topology (a), c)) from (n/2)+1=3, where n is the number of partial voltage sources used. In addition, there are the four active switches S1, S2, S3 and S4 of the converter unit 12, so that the multilevel converter described only requires 7 switches. Advantageously, the number of switches in the converter unit 12 does not continue to increase, even if more levels are generated. Table 1: Number of actively controlled circuit breakers required. level generation unit Last submodule type d); asymmetrical Last submodule type e); symmetrical a) (n+3)/2 (n/2)+1 b) (n+3)/2 (n/2)+1 c) (n/2)+1 (n+3)/2

4 Figure 12 shows an exemplary multilevel converter configured to provide nine output levels. A topology of type a) is required as the level generation unit 11 2 intended. A direct voltage U _DC across the entire level generation unit 11 is present as the total voltage at the input connection 14 . The sum of the individual partial voltages corresponds to the total voltage UDC = U _d1 +U _d2 +U _d3 +U _d4 . The level generation unit 11 delivers 4 voltage levels at the output. An H4 bridge converter circuit 12 with four active power switches S1, S2, S3 and S4 is designed to switch on both half cycles of the output frequency. The switches S1 and S4 switch in the positive half-cycle and the switches S2 and S3 switch in the negative half-cycle of the AC voltage of the supply network connected at the output. The supply network is connected to the multilevel converter via nodes A and B. A number N _levels of voltage levels at the output results from N _level =2n+1, where n is the number of partial voltage sources U _dn . Table 2: Switching states of the active switches for generating the output level. output voltage sat Sb sc S1 S2 S3 S4 +4ud 1 1 0 1 0 0 1 +3Ud 1 0 1 1 0 0 1 +2Ud 0 1 0 1 0 0 1 +1ud 0 0 1 1 0 0 1 0 0 0 0 1 0 1 0 0 0 0 0 0 1 0 1 -1ud 0 0 1 0 1 1 0 -2Ud 0 1 0 0 1 1 0 -3Ud 1 0 1 0 1 1 0 -4Ud 1 1 0 0 1 1 0

Table 2 shows how the seven switching elements must be controlled in order to generate nine voltage levels at the output.

In a preferred embodiment, a renewable energy source is used to supply voltage to the input 14 of the multilevel converter. For example, the connection for supplying electrical energy to be converted, as in 4 as can be seen, fed by a DC voltage output of a DC/DC converter 16 of one or more renewable energy sources. This can be, for example, generators of one or more wind turbines or other types of electrical energy sources, such as solar power plants.

As a result of simulations of the multilevel converter topology using the PSCAD/EMTDC software, the 5 until 7 obtain. 5 shows a diagram in which a voltage curve at the AC voltage connection (output) is shown by means of a graph. The abscissa is a time axis where time is given in seconds. The output voltage in V is plotted on the ordinate. It can be seen that an AC voltage with an amplitude of 400 V at a frequency of 50 Hz is shown. The different, total of nine voltage levels are also clearly visible. 6 shows a diagram in which the percentage of the harmonic deviation (Total Harmonic Distribution, THD) of the output voltage is represented by means of a graph. The abscissa is a time axis, in where the time is given in seconds. The THD is plotted in percent on the ordinate. It can be seen that the THD stabilizes at a value below 2.1%, which is a sign of high signal quality. 7 shows a progression of the current fed in in A and the mains voltage in V over time in seconds. It can be clearly seen that the current and mains voltage fed in have a stable frequency and are in phase. This is a sign of a good power factor close to unity.

One of the proposed topologies is selected below to illustrate the application of connecting a renewable energy source (solar power / wind power) to a utility grid. The network embedding of an exemplary photovoltaic system (PV) as an energy source is described in detail.

A grid-connected renewable energy configuration is exemplified in 8th shown. In this configuration, a power injection system usually consists of three parts. A first part is a DC/DC converter 17 intended to extract and regulate the DC voltage of the renewable energy source, a PV array 18, or a wind turbine 19. A second part is a DC/AC converter for converting DC voltage/current into AC voltage/current. A third part is an LCL filter 20 to prepare converted voltage and current for supply to the utility grid 16 . In systems in which a multilevel converter is used as a DC/AC converter in the second part, the output voltage approaches a sinusoidal curve due to a large number of generated voltage levels. Therefore, in such a case, the LCL filter 20 can be omitted or replaced with a simple L filter. A proposed topology of a multilevel converter 10 can, as already shown in FIG 4 evident, in the second part of the configuration of 8th come into use.

As in 9 shown schematically, a panel of PV modules 22 is formed by an array of individual PV cells 21 . PV cells 21 use sunlight as an energy source and generate direct current (DC) electricity. An interconnection of several PV module panels 22 is also called a PV string 23 . A PV string 23 arrangement can be formed in two different ways: as a parallel string arrangement in 1) or as a series string arrangement in 2). Assuming a PV string 23 supplies DC voltage/current of 60 V and 6 A, then the entire parallel arrangement supplies 1) 60 V and 20 A and the series arrangement supplies 2) 400 V and 6 A.

For an exemplary use of a proposed multilevel converter 10 in a PV power plant with high-voltage generation, the individual PV strings 23 are not connected together to form arrangements in order to provide 400 V, for example, as described above, but rather the individual PV strings 23 of the PV -Kraftwerks are used as individual voltage source inputs for the level generation unit 11. In 10 an exemplary PV string 23 is shown that supplies an output voltage of 100 V.

11 shows an example of a 9-level generator analogous to 4 . in the in 11 shown use, are different (four total) individual voltage sources according to the 10 used as partial voltage sources Cd1, Cd2, Cd3 and Cd4. Each of the partial voltage sources supplies 100 V as a partial voltage. The resulting output voltage is in 13 shown. The output voltage is shown schematically in V. It can be seen that an AC voltage with an amplitude of 400 V is output. The different, total of nine voltage levels can also be seen.

It is understood that the specific features of the embodiments described in detail are not to be understood as limiting, but also embodiments with not all features, additional features, combinations and equivalents are included within the scope of the invention.

reference list

10

multilevel converter

11

level generation unit

12

converter unit

13

submodule

13'

first submodule

13''

another submodule

13'''

last submodule

14

connection (input)

15

connection (output)

16

supply network

17

DC converter

18

Generator of the electrical energy to be converted (PV)

19

Wind Turbine / Generator

20

LCL filter

21

PV cells

22

PV module

23

PV string

Claims (11)

Multilevel converter (10) for converting electrical energy, with a connection (14) for supplying electrical energy to be converted (input) and another connection (15) for delivering the converted electrical energy (output), comprising a level generation unit (11) and a converter unit (12) which is set up to generate a specific number of voltage levels at the output (15), the level generation unit (11) being formed from at least one submodule (13), the at least one submodule (13) being formed by an electrical circuit is formed comprising a first branch with an active switching element connected in series with at least one partial voltage source and a second branch with a switching element connected in parallel with the first branch. multilevel converter (10) according to claim 1 , characterized in that the total input voltage (U _DC ) is provided by a direct current voltage source and wherein a partial voltage is applied to the at least one partial voltage source, which is formed by a decomposition of the total input voltage (U _DC ). multilevel converter (10) according to claim 1 , characterized in that the converter unit (12) is formed by an H-bridge circuit. multilevel converter (10) according to claim 3 , characterized in that the H-bridge circuit of the converter unit (12) is formed from four active switching elements (S1, S2, S3, S4). multilevel converter (10) according to claim 1 , characterized in that the first branch of the sub-module (13) comprises two partial voltage sources connected in series with an active switching element arranged between them. multilevel converter (10) according to claim 1 , characterized in that a number N _levels of voltage levels at the output (15) is given by N _level =2n+1, where n is the number of partial voltage sources. multilevel converter (10) according to claim 1 , characterized in that the level generating unit (11) comprises a first submodule (13') with a first partial voltage source connected in series with an active switching element in the first branch and a further active switching element in the second branch connected in parallel, with further submodules (13" ) each with two partial voltage sources connected in series, with the first submodule (13') and the further submodules (13") being connected via a second partial voltage source (U _d2 ) connected in series between them. multilevel converter (10) according to claim 6 , characterized in that a last sub-module (13''') of the further sub-modules (13'') has only one partial voltage source in the first branch. Multilevel converter (10) according to one of the preceding claims, characterized in that an active switching element is formed by a high-frequency switch. multilevel converter (10) according to claim 8 , characterized in that the active switching elements are formed by discrete IGBT switches without an anti-parallel diode. Multilevel converter (10) according to one of Claims 1 until 9 for directly coupling a renewable energy source to an AC voltage supply network (16), the input (14) of the multilevel converter (10) being connected to a DC voltage connection (17) of the renewable energy source (18) and the output (15) of the multilevel converter (10) is connected to a mains connection of the AC voltage supply network (16).

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