WO2021038155A1

WO2021038155A1 - Modular multilevel converter for low-voltage application with optimized inductors and increased number of levels

Info

Publication number: WO2021038155A1
Application number: PCT/FR2020/051449
Authority: WO
Inventors: Igino TOIGO
Original assignee: Socomec
Priority date: 2019-08-30
Filing date: 2020-08-07
Publication date: 2021-03-04
Also published as: FR3100395A1; FR3100395B1

Abstract

An MMC (1, 31) with at least one branch (30), two outer modules (2, 3, 2, 33) and an inner module (4, 34), each module (1-4, 32-34) comprising two of the switching units (7, 8, 37, 38) each comprising a diode (12, 42) and a semiconductor transistor (IGBT) (13, 43), a first, a second and a third terminal (9, 10, 11, 39, 40, 41). Said at least one branch (30) comprises a first inductor (16, 46) and a second inductor (17, 47) both having a pole connected, respectively, to the third terminal (11, 41) of two different outer or inner modules (2, 3, 4, 5, 32, 33, 34). Said at least one branch (30) further comprises a third inductor (21, 51), said at least one branch (30) being designed to be electrically connected to a phase of a load passing through said third inductor (21, 51).

Description

Title of the invention: Modular multilevel converter for low voltage application with optimized inductances and an increased number of levels

Technical area

The invention relates generally to voltage source converters, and more particularly to a modular multilevel converter (MMC) for low voltage application.

Prior art

Multilevel converters have aroused great interest in this industry. They have different advantages such as lower harmonic distortion, smaller filter size, reduced electromagnetic interference (EMI), and higher efficiency. Additionally, low voltage semiconductor devices can be used to synthesize higher voltage levels.

Several topologies can provide alternating voltage (AC). The neutral point clamped (NPC), the flying capacitors (FC), the cascade H-bridge and the multi-level modular converter (MMC) are the basic topologies of multi-level inverters. The NPC topology and its variants are the most widespread and widely used in industrial applications for low voltage applications.

The MMC topology, introduced in the early 2000s for the high voltage direct current (HVDC) converter, has received special attention due to its many features. The most relevant features are: modularity, scalability and reliability.

Just like a two-level converter or a six-pulse line-switched converter, an MMC, i.e. a multi-level modular converter, is made up of six modules, each connecting an AC terminal to a DC terminal. However, where each module of the two-level converter is a high voltage switch consisting of a large number of Insulated Gate Bipolar Transistors (IGBTs), or equivalent semiconductors such as transistors, Mosfet SICs, etc., connected in series and acting as one, each module of an MMC is a separate controllable voltage source. IGBTs or other semiconductors are connected in series in order to have an equivalent device of higher voltage. Each MMC module includes a number of independent conversion submodules, each containing its own storage capacitor. In the most common form of the circuit, the half-bridge variant, each submodule contains two IGBTs connected in series across the capacitor, with the electrical node coupled to the two IGBTs and one of the two capacitor terminals pulled out to form two external connections. Depending on which of the two IGBTs of each submodule is activated, the capacitor is bypassed or connected to the circuit. Each submodule therefore acts as an independent two-level converter generating a voltage of zero or of a value U _sm (where Usm is the voltage of the capacitor of the submodule). With an appropriate number of submodules connected in series, the module can synthesize a stepped voltage waveform that closely approximates a sine wave and contains very low levels of harmonic distortion.

The MMC differs from other converter types in that current flows continuously through the six converter modules throughout the mains frequency cycle. The direct current divides equally into the three phases and the alternating current divides equally into the upper and lower modules of each phase.

A typical MMC for an HVDC application contains about 300 submodules connected in series in each valve and is therefore equivalent to a 301 level converter. Therefore, the harmonic performance is excellent and no filter is generally required.

Another advantage of the MMC is that it is not necessary to resort to pulse width modulation (PWM), so the power losses are much lower than that of the two-level converter, at about 1% per end. Finally, since direct serial connection of IGBTs is not required, IGBT gate controls do not need to be as sophisticated as those of a two-level converter.

The MMC has two main drawbacks. First of all, the control is much more complex than that of a two-level converter. Balancing the voltages of each of the submodule capacitors is a tall order and requires considerable computing power and high speed communications between the central control unit and the valve. Second, the submodule capacitors themselves are large and bulky. An MMC is considerably larger than a comparable level converter, although this can be compensated by the space saving due to the absence of filters.

Due to the main characteristics (modularity, scalability and reliability) offered by the MMC topology, MMCs are used in high and medium voltage applications. For low voltages, MMCs are generally not used in industrial applications.

The MMC topology could be arranged with a high number of modules in order to share the voltage using lower quality components. This concept could also be extended to low voltage applications, but given the complexity of the components available (mainly solid state switches) the reliability and target power of the converter must be taken into account. ²

In Figure 1 is illustrated a three-phase MMC 100, known in the state of the art, designed to be coupled to a power supply 101. Each branch 102 of the MMC 100 comprises two arms 104 (an upper arm and a lower arm) with two modules 106 by arm 104. Each module 106 comprises two switching units 108 coupled in series, and a capacitor 110 coupled in parallel with the assembly formed by the two switching units 108 in series. Each switching unit 108 is formed by a MOSFET transistor coupled in parallel with a diode.

The two upper and lower arms 104 of the same branch 102 are coupled to an output terminal 112 of the leg by means of two separate inductors 114, an upper inductor 114 coupled between the upper arm 104 and the output terminal 112 and a lower inductor 114 coupled between lower arm 104 and output terminal 112.

The three-phase MMC 100 illustrated in figure 1 represents the minimum installation to be able to use an MMC topology for a low voltage application.

The two arms 104 of a branch 102 operate practically in parallel. The upper current i _u is defined by the following equation 1:

[Math

equation 1

With I _dc the current coming from the power supply 101, _t the current coming from the output terminal 112, w the frequency of the signal supplied by the power supply 101 and f its phase shift.

The lower current is defined by equation 2:

[Math

equation 2

From equations 1 and 2 we can obtain the following equations 3 and 4:

[Math 3] i _u {t) - i _t (t) = I _out sin (wί + ø) equation 3

[Math equation 4

As can be seen, the upper current i _u (t) and the lower current h (t) have one term representing the output current i _out defined in Equation 3, and two common mode terms representing the circulating current defined by equation 4.

As defined by the following equation 5, the term I _dc depends only on the power managed by the inverter, which, in a three-phase inverter, corresponds to the total output power P _out

[Math

equation 5

The term I _2w cos (2wΐ + f) depends on the current control strategy used. As a general rule, the higher the circulating second harmonic current, the lower the value required for the capacitance of each module for the same voltage ripple. Even the value of inductors and capacitors influences the flow of second harmonic current.

Low voltage has been defined internationally as up to 1000 V _A c and 1500 V _DC - Indeed, in low voltage industrial applications, there are two main direct current buses (DC buses) used to operate an inverter. : - 800 Vnom 1000 Vmax

- 1200 V _nom 1500 V _max

The potentially usable semiconductors are:

- 650 V and 1200 V IGBTs

- 650V, 900V, or 1200V SIC MOSFETs,

- GAN MOSFETs.

But silicon MOSFETS which have a bias voltage V _dd greater than 200 V are not suitable because of the low recovery capacity of the diode.

Therefore, for low voltage applications, the optimal choice is to use 650V or 1200V components.

The minimum hold voltage of each switch V _sw in the MMC could be evaluated as follows by equation 6:

[Math 6] V _sw = 1.5

equation 6

Where N is the number of modules 106 per arm 104, V _n0 m is the nominal voltage of the DC bus and 1, 5 is a safety margin taking into account any possible voltage increase during operation, such as overvoltage peaks, voltage higher than rated voltage, etc.

The optimal number N of modules 106 per arm 104 to use is defined by equation 7:

[Math 7] N> 1.5

equation 7

The number N of modules 106 per branch 104 in a low voltage MMC is calculated with:

V _n0 m = 1200 V for N> 1.5 V _n0 m = 650 V for N> 1. 85

Consequently, for low voltage, the minimum and optimal number N of modules 106 per branch 104 is two modules 106.

Current solutions for inverters or multilevel rectifiers for low voltage in the industrial sector are the neutral point clamped inverter (NPC) or a derivative solution with a similar arrangement (TNPC or NPC2). A topology with a number of levels greater than three is possible and also applies to medium or high voltage applications, but its complexity and cost are so high that its use could only be justified under specific conditions and not for specific conditions. low voltage industrial applications.

The topology of NPCs suffers from a major drawback due to the parasitic inductance which can limit the operation when a high current or a large variation in current (dl / dt), due to high speed devices, are used in particular when four quadrant operation is required.

Due to the geometric connection of the different switches, the leakage inductance in the switching loops limits the maximum current and the time variation of the current (Dl / dt).

The main critical situations appear when the inverter manages a current of opposite sign to the voltage or when it is used as a rectifier.

The geometric connection and the minimization of switching loops make it necessary to use specially dedicated power modules for medium power applications, in which a segment is integrated in a single housing, while for high power applications this solution is applicable. , due to the dimensions of the components, but only with specific provisions or to actively control the current flow path.

This problem has little impact in applications using renewable energy inverters operating with a fairly high power factor (ooef ~ 1 to 0.9), but it is particularly relevant in applications relating to inverters or rectifiers.

Another disadvantage of NPC is that not all switches are used the same, because the external and internal switches do not have the same power dissipation due to the modulation strategy. In addition, the transactions between the upper and lower sections (zero crossing of the voltage) must be carefully synchronized, especially when the current is out of phase with the voltage. The standard MMC topology could solve the three previous problems, namely parasitic inductance, the fact that the switches are not used in the same way and the synchronization at zero crossing.

Because each module can be made with two switches and a capacitor, parasitic inductance can be minimized quite easily even by using a standard and inexpensive module assembly.

In addition, in an MMC topology, the switches are always modulated and not just half-wave. The amount of current flowing depends on the power but also on the modulation strategy (control of the flowing current) and the V _dc and V _out ratios. As a result, the switches dissipate the same amount of loss, resulting in more uniform temperature operation.

In addition, zero-crossing synchronization is no longer required thanks to continuous modulation which also improves the THD of the output waveform.

Unfortunately, the enormous amount of capacitance and, in turn, the energy stored in module capacitors, limits the use of classic MMC topology in low voltage applications.

As illustrated in Figures 2 and 3, which represent two electrical diagrams of two standard MMC branches 102 with two modules 106 per arm (figure 2 without neutral point and figure 3 with neutral point), the number of voltage levels applied to a load or an output filter for a three-phase converter is three (N + 1) if we consider phase to neutral, while it is five (2N + 1) if we consider phase to phase.

From the diagrams shown in Figures 2 and 3, it can immediately be seen that each capacitor 110 in the circuit operates independently of each other and that there is no ripple compensation in a three-phase system or compensation between the upper and lower arms. Due to the independence of the modules 106, the modulation requested for the synthesis of the output voltage could be applied with some freedom between the modules 106.

However, it has been proved that the use of a “phase shift carrier”, or “phase sift carrier” in English, is recommended for an MMC. This strategy modulation could be of two types: phase-shifted carrier with N carriers or phase-shifted carrier with 2N carriers, N being the number of modules.

In a phase-shifted carrier with N carrier, the modules of the same arm are driven by a PWM generated from the same signal using carriers shifted by 2p / N. The upper and lower arms are driven with the same PWM.

In a phase shifted carrier with a 2N carrier, the upper arm modules are driven with a PWM generated using carriers shifted by 2p / N. The modules located in the lower arm are driven by a PWM generated using carriers offset by 2p / N from each other but synchronized with a p / N offset from the upper arm.

The need to use relatively large capacities in MMC topologies is a major problem. Different ideas have already been proposed to reduce this problem by introducing topology modifications, as indicated in the IEEE journal article published in October 2015 and titled "A modified modular multilevel converter with reduced capacitor voltage fluctuation", or by controlling the current flowing through the converter arms, as described in the article in the IEEE journal published in 2018 and entitled "An enhanced steady-state model and capacitor sizing method for modular multilevel converters for HVDC applications".

However, the reductions in capacity obtained are not such as to present economic and size advantages over current solutions. In addition, no research or solution is offered in low voltage applications where the amount of capacitors is higher due to a higher I / V ratio for the power unit.

There is a need to find an MMC topology solution that offers smaller size, better cost, and lower losses, allowing better performance for low voltage MMC topology. The state of the art does not currently offer any solution for MMCs in low voltage applications where the size is not optimal due to the higher l / V ratio for the power unit.

Disclosure of the invention To this end, the present invention proposes an MMC topology which can be used equally well for medium and high voltage applications as for low voltage applications and which has an optimized dimension.

In a first object of the invention, there is proposed a multilevel modular converter comprising at least one branch intended to be connected to a phase, said at least one branch comprising at least a first and at least a second external modules and an internal module. , each external module and each internal module comprising:

- a first and a second switching unit each comprising a diode, a transistor, a first connector electrically connected to a first pole of the diode and a first pole of the transistor, and a second connector electrically connected to a second pole of the diode and a second pole of the transistor,

- a first terminal electrically connected to the first connector of the first switching unit,

- a second terminal electrically connected to the second connector of the second switching unit, and

a third terminal electrically connected to the second connector of the first switching unit and to the first connector of the second switching unit.

Said at least one branch comprising a first inductor having a pole connected respectively to the third terminal of an internal or external module and a second inductor having a pole connected to the third terminal of an internal or external module distinct from the internal or external module to which the first inductor is coupled.

According to a general configuration of the invention, said at least one branch comprises a third inductor, said at least one branch being configured to be electrically connected to a phase of a load via said third inductor.

In a first aspect of the multilevel modular converter, for one branch, that is to say for one branch or for each branch, the inductance value of the first inductor of the branch may be equal to the inductance value of the branch. second inductance of the branch and the inductance value of the third inductor of the branch can be equal to half of the inductance value of the first inductor of the branch.

This configuration concerning the inductance values of the three different inductors of a branch, makes it possible to have the same ripple in the current. In this way, the voltage variation in each period of pulse width modulation applied to the inductors is no longer V _dc / 2 but V _dc / 4, V _dc being the voltage at the input of the MMC. This is equivalent to increasing the number of converter levels by one, which further improves the performance of magnetic materials.

In a second aspect of the modular multilevel converter, said at least one internal module may further comprise a capacitor comprising a first pole electrically connected to the first terminal of the internal module and a second pole electrically connected to the second terminal of the internal module.

And the converter can also include a first external capacitor comprising a first pole electrically connected to the first terminal of a first external module and a second pole electrically connected to the second terminal of a first external module, and a second external capacitor comprising a a first pole electrically connected to the first terminal of a second external module and a second pole electrically connected to the second terminal of a second external module.

In a third aspect of the modular multilevel converter, the converter may further include a control unit configured to drive the first and second external modules with a modulation pattern calculated by a phase-shifted carrier with N carriers, N being an integer value, and for drive the internal module with a pulse width modulation (PWM) signal calculated by an n-phase carrier, where n is an integer value and the same error signal.

In a fourth aspect of the modular multilevel converter, the converter can include three branches, each configured to be connected to a different phase, and the first external capacitor can be electrically coupled in parallel to the first external module of the three branches, and the first external capacitor can be electrically coupled in parallel with the first external module of the three branches. second external capacitor can be electrically coupled in parallel to the second external module of the three branches.

In a fifth aspect of the modular multilevel converter, said at least one branch may include only a first external module, a second external module and an internal module, the third terminal of the first external module being electrically coupled to the first terminal of the internal module via a first inductor, the third terminal of the second external module being electrically coupled to the second terminal of the internal module via a second inductor, the third terminal of the internal module being configured to be connected to a phase of a load or a network to which the converter is configured to be connected, and the second terminal of the first external module being electrically connected to the first terminal of the second external module.

In this aspect, the converter may further comprise a first external capacitor comprising a first pole electrically connected to the first terminal of the first external module of said at least one branch and a second pole electrically connected to the second terminal of the first module of said at least one branch. at least one branch, and a second external capacitor comprising a first pole electrically connected to the first terminal of the second external module of said at least one branch and a second pole electrically connected to the second terminal of the second module of said at least one branch.

By connecting the second terminal of the first external module to the first terminal of the second external module, the first and second external capacitors are coupled together. In this way, the two capacitors can share the total voltage, act dynamically with a double capacitance value, and be common to the three branches when the converter is in a three phase converter configuration.

In addition, for any application of the modular multilevel converter, when the neutral must be supplied to the load, generally for use in an uninterruptible power supply (UPS), it is not necessary to add the two capacitors conventionally required to create the midpoint given that they are already integrated into the structure of the present invention, these two capacitors being formed by the two external capacitors. This is a big advantage, because a DC capacitor bank is always needed to handle the power flow, the phase and ripple supply unbalance and the external arm capacitors are a natural part of this capacitor bank.

The combination of the two internal modules of a conventional MMC topology into a single internal module such as in the topology of the present invention with a third terminal connected to the load makes it possible to compensate for the voltage ripple.

All these characteristics of the MMC according to the invention allow the MMC not only to be used for a low voltage application, but also to have a reduced overall size with a reduced number of capacitors and without complex processing.

In a sixth aspect of the modular multilevel converter, the output current of each branch is limited by the values of the various electrical elements of the converter, such as capacitances.

In a seventh aspect of the modular multilevel converter, the control unit is configured to control the current in said at least one branch by monitoring the current in the first external module and in the second external module.

Reducing the current component I _2w results in greater ripple of the capacity of the internal module with the demand for a higher capacitance value, but allows the optimization of current and dissipation in modules and switches.

Brief description of the drawings

The invention will be better understood on reading below, by way of indication but not limiting, with reference to the appended drawings in which:

[Fig. 1] FIG. 1, previously described, is a three-phase MMC topology with two modules per arm and three levels as known in the state of the art. [Fig. 2] FIG. 2, previously described, is an electrical diagram of a branch of a standard MMC with two modules per arm with N = 2 and without a neutral point, as is known in the state of the art.

[Fig. 3] FIG. 3, previously described, is an electrical diagram of a branch of a standard MMC with two modules per arm with N = 2 and a neutral point, as they are known in the state of the art.

[Fig. 4] FIG. 4 schematically represents a single-phase multilevel modular converter according to a first embodiment of the invention.

[Fig. 5] FIG. 5 schematically represents a three-phase multilevel modular converter according to a second embodiment of the invention.

[Fig. 6] Figure 6 schematically shows a single-phase multilevel modular converter according to a third embodiment of the invention.

Description of the embodiments

The present invention will be described in connection with particular embodiments and with reference to certain drawings, but the invention is not limited thereto, but only by the claims. The drawings described are only schematic and are not limiting. In the drawings, the size of some of the items may be exaggerated and not drawn to scale for illustration purposes. When the term "comprising" is used in this description and the claims, it does not exclude other elements or steps. When an undefined or defined article is used to denote a singular name, e.g. "a", "the", this includes a plural of that name, unless otherwise specified.

The term "comprising" as used in the claims should not be interpreted as being limited to the means listed below; it does not exclude other elements or steps. Thus, the scope of the expression "a device comprising means A and B" should not be limited to devices consisting only of components A and B. This means that, in relation to the present invention, the only relevant components of the device are A and B.

Furthermore, the terms first, second, third and the like in the description and the claims are used to distinguish like elements and not necessarily to describe a sequential or chronological order. It should be understood that the terms so used are interchangeable in appropriate circumstances and that the embodiments of the invention described herein are capable of functioning in sequences other than those described or illustrated herein.

FIG. 4 schematically represents a modular multilevel converter (MMC) according to a first embodiment of the invention.

The MMC 1 comprises three modules: a first external module 2, a second external module 3 and an internal module 4. Each external and internal module 2 to 4 comprises a first switching unit 7, a second switching unit 8, a first terminal 9, a second terminal 10 and a third terminal 11.

Each first and second switching unit 7 and 8 comprises a diode 12, an insulated gate bipolar transistor (IGBT) 13, a first connector 14 electrically connected to a first pole of the diode 12 and to a first pole of the IGBT 13 , and a second connector 15 electrically connected to a second pole of the diode 12 and to a second pole of the IGBT 13.

The first terminal 9 of a module 2 to 4 is electrically connected to the first connector 14 of its first switching unit 7. The second terminal 10 of a module 2 to 4 is electrically connected to the second connector 15 of its second switching unit 8. And the third terminal 11 of a module 2 to 4 is electrically connected to the second connector 15 of its first switching unit 7 and to the first connector 14 of its second switching unit 8. The third terminal 11 of the first external module 2 is electrically coupled to the first terminal 9 of the indoor module 4 via a first inductor 16.

The third terminal 11 of the second outdoor module 3 is electrically coupled to the second terminal 10 of the indoor module 4 via a second inductor 17. The third terminal 11 of the indoor module 4 is electrically coupled to a first pole of a third inductor 21 with a second pole configured to be connected to a phase of a load or network to which the MMC 1 is configured to be connected.

The second terminal 10 of the first external module 2 is electrically connected to the first terminal 9 of the second external module 3.

The internal module 4 comprises an internal capacitor 18, having an internal capacitance value C _m and comprising a first pole 180 electrically connected to the first terminal 9 of the internal module 4 and a second pole 185 electrically connected to the second terminal 10 of the internal module 4.

The MMC 1 also comprises a first external capacitor 19 having a first external capacitance value C _top and comprising a first pole 190 electrically connected to the first terminal 9 of the first external module 2 and a second pole 195 electrically connected to the second terminal 10 of the first external module 2, and a second external capacitor 20 having a second external capacitance value C _ottom and comprising a first pole 200 electrically connected to the first terminal 9 of the second external module 3 and a second pole 205 electrically connected to the second terminal 10 of the second external module 3.

The MMC 1 is coupled to an input voltage V _D c via the first terminal 9 of the first external module 2 and the second terminal 10 of the second external module 3.

In dotted lines is shown a coupling of the second pole 195 of the first external capacitor 19 and of the first pole 200 of the second external capacitor 20. This coupling only exists if the MMC 1 is in a configuration with a neutral point.

The MMC 1 illustrated in FIG. 4 further comprises a filter capacitor 22 coupled between the reference point in a voltage distribution system (ground, neutral, etc.) and an output terminal of the MMC 1 which corresponds to the second pole of the third inductor 21. This filtering capacitor 22 makes it possible to compensate for the ripple of the output choke current in order to filter the switching frequency and to keep only the fundamental of the output voltage (50 or 60Hz).

FIG. 5 schematically illustrates a three-phase MMC 31 according to another embodiment of the invention. The MMC 31 comprises three branches 30 each comprising three modules: a first external module 32, a second external module 33 and an internal module 34.

Each branch 30 of the three-phase MMC 31 is composed of its three modules 32 to 34 in the same way as the single-phase MMC 1 illustrated in FIG. 4. Thus, each external and internal module 32 to 34 comprises a first switching unit 37, a second switching unit 38, a first terminal 39, a second terminal 40 and a third terminal 41.

Each first and second switching unit 37 and 38 of a branch 30 comprises a diode 42, an insulated gate bipolar transistor (IGBT) 43, a first connector 44 electrically connected to a first pole of the diode 42 and a first pole of the IGBT 43 and a second connector 45 electrically connected to a second pole of the diode 42 and to a second pole of the IGBT 43.

The first terminal 39 of a module 32 to 34 of a branch 30 is electrically connected to the first connector 44 of its first switching unit 37. The second terminal 40 of a module 32 to 34 is electrically connected to the second connector 35 of its second switching unit 38. And the third terminal 41 of a module 32 to 34 is electrically connected to the second connector 45 of its first switching unit 37 and to the first connector 44 of its second switching unit 38. For each branch 30 , the third terminal 41 of the first external module 32 of the branch 30 is electrically coupled to the first terminal 39 of the internal module 34 of the same branch 30 via a first inductor 46.

For each branch 30, the third terminal 41 of the second external module 33 of the branch 30 is electrically coupled to the second terminal 40 of the indoor module 34 of the same branch 30 via a second inductor 47.

For each branch 30, the third terminal 41 of the internal module 34 of the branch 30 is electrically coupled to a first pole of a third inductor 51 provided with a second pole intended to be connected to a phase of a load or of a network to which the MMC 31 is intended to be connected. For each branch 30, the second terminal 40 of the first external module 32 of the branch 30 is electrically connected to the first terminal 39 of the second external module 33 of the same branch 30.

The internal module 34 of each branch 30 comprises an internal capacitor 48, having an internal capacitance value C _m and comprising a first pole 480 electrically connected to the first terminal 39 of the internal module 34 of its branch 30 and a second pole 185 electrically connected to the second terminal 40 of the internal module 34 of its branch 30.

The MMC 1 also includes a first external capacitor 49 and a second external capacitor 50. The first external capacitor 49 is coupled in parallel with the first external module 32 of the three branches 30, and the second external capacitor 50 is coupled in parallel with the second external module. 33 of the three branches 30. In other words, there is a first external capacitor 49 coupled in parallel to three first external modules 32 and a second external capacitor 50 coupled in parallel to three second external modules 33.

The first external capacitor 49 has a first external capacitance value C _top and comprises a first pole 490 electrically connected to the first terminal 39 of the first external module 32 of each branch 30 and a second pole 495 electrically connected to the second terminal 40 of the first external module 32 of each branch 30. The second external capacitor 50 has a second external capacitance value C _o ttom and comprises a first pole 500 electrically connected to the first terminal 39 of the second external module 33 of each branch 30 and a second pole 505 electrically connected to the second terminal 40 of the second external module 33 of each branch 30.

The MMC 31 is coupled to an input voltage V _D c via the first terminal 39 of the first external module 32 of each branch 30 and the second terminal 40 of the second external module 33 of each branch 30.

The MMC 31 can also include a filtering capacitor for each branch 30 coupled between the reference point of a voltage distribution system and the output terminal of the branch 30 of the MMC 31 which corresponds to the second pole of the third inductor 51. of branch 30. FIG. 6 schematically represents an MMC with two modules per arm, but according to a third embodiment of the invention. The solution can be applied to any number of modules for each arm. Although the embodiment illustrated in FIG. 6 comprises two modules per arm, therefore four per branch, the proposed solution is applicable for any number of modules in the arms.

The MMC 61 comprises three modules: a first external module 2, a second external module 3 and a first internal module 4 and a second internal module 5. The numerical references are kept identical to the first embodiment of the invention illustrated in FIG. 4 for identical elements. In this embodiment, the MMC 61 differs from the MMC 1 of the first embodiment in that it comprises symmetrical arms each having an external module and an internal module and therefore differs in that it comprises an additional internal module 5. In this third embodiment, the third terminal 11 of the first outdoor module 2 is electrically coupled directly to the first terminal 9 of the first indoor module 4 and the third terminal 11 of the second outdoor module 3 is electrically coupled directly to the second terminal. 10 of the second internal module 5.

The third terminal 11 of the first internal module 4 is coupled to a first pole of a first inductor 76, and the third terminal 11 of the second internal module is coupled to a first pole of a second inductor 77. The second poles of the first l The inductor 76 and the second inductor 77 are coupled together to a first pole of a third inductor 81, the second pole of the third inductor 81 being adapted to be connected to a phase of a load or grid at which the MMC 1 is designed to be connected.

In this third embodiment, the second terminal 10 of the first external module 2 is not electrically connected to the first terminal 9 of the second external module 3. Instead, there are two additional capacitors 91 and 92 respectively coupled between the reference point in a distribution system voltage and the first terminal 9 of the first external module 2 and between ground and the second terminal 10 of the second external module 3. The MMC 61 is coupled to an input voltage V _D c via the first terminal 9 of the first external module 2 and the second terminal 10 of the second external module 3.

The MMC 61 illustrated in Figure 6 further includes a filter capacitor 82 coupled between the reference point of a voltage distribution system and an output terminal of MMC 61 which corresponds to the second pole of the third inductor 81. This filtering capacitor 82 makes it possible to compensate for the ripple of the indusctance output current in order to filter the switching frequency and to keep only the fundamental of the output voltage (50 or 60 Hz). The invention applies to all types of power converters for medium to large power converters (inverter or rectifier) as well as for low voltage applications with semiconductors of appropriate ratings.

The MMC could be realized as a printed circuit solution for low power applications, for example, or as a hardwired solution for medium or high power applications.

Claims

[Claim 1] Modular multilevel converter (1, 31, 61) comprising at least one branch (30) intended to be connected to one phase, said at least one branch comprising at least a first external module (2, 32) and at least a second external module (3, 33) and at least one internal module (4, 5, 34), each external module and each internal module (2-5, 32-34) comprising:

- a first and a second switching units (7, 8, 37, 38) each comprising a diode (12, 42), an insulated gate bipolar transistor (13, 43), a first connector (14, 44) electrically connected to a first pole of the diode (12, 42) and a first pole of the transistor (13, 43), and a second connector (15, 45) electrically connected to a second pole of the diode (12, 42) and a second transistor pole (13, 43),

- a first terminal (9, 39) electrically connected to the first connector (14, 44) of the first switching unit (7, 37),

- a second terminal (10, 40) electrically connected to the second connector (15, 45) of the second switching unit (8, 38), and

- a third terminal (11, 41) electrically connected to the second connector (15, 45) of the first switching unit (7, 37) and to the first connector (14, 44) of the second switching unit (8, 38) , said at least one branch (30) comprising a first inductor (16, 46) having a pole connected respectively to the third terminal (11, 41) of an internal or external module and a second inductor (17, 47) having a pole connected to the third terminal (11, 41) of an internal or external module separate from the internal or external module to which the first inductor (16, 46) is coupled, characterized in that said at least one branch (30) comprises a third inductor (21, 51, 81), said at least one branch (30) being configured to be electrically connected to a phase of a load via said third inductor (21, 51, 81), for one branch (30), the inductance value of the first inductor (16, 46, 76) of the branch being equal to the inductance value of the second inductor (1 7, 47, 77) of the branch and the inductance value of the third inductor (18, 48, 78) of the branch being equal to half of the inductance value of the first inductor (16, 46, 76) of the branch.

[Claim 2] A modular multilevel converter (1, 31) according to claim 1, wherein said at least one internal module (4, 34) further comprising a capacitor (18, 48) having a first pole (180, 480) connected electrically to the first terminal (9, 39) of the internal module (4, 34) and a second pole (185, 485) electrically connected to the second terminal (10, 40) of the internal module (4, 34), and the converter (1, 31) further comprises a first external capacitor (19, 49) comprising a first pole (190, 490) electrically connected to the first terminal (9, 39) of the first external module (2, 32) and a second pole (195, 495) electrically connected to the second terminal (10, 40) of the first external module (2, 32), and a second external capacitor (20, 50) having a first pole (200, 500) electrically connected to the first terminal (9, 39) of the second external module (3, 33) and a second pole (205, 505) electrically connected to the second terminal (10, 40) of the second external module do (3,

33).

[Claim 3] Modular multilevel converter (1, 31) according to one of claims 1 or 2, further comprising a control unit configured to drive the first and second external modules (2, 32, 3, 33) with a pattern modulation calculated by a phase-shifted carrier with N carriers, N being an integer value, and to drive the internal module (4, 5,

34) with a pulse width modulation (PWM) signal calculated by an n-phase carrier, where n is an integer value and the same error signal.

[Claim 4] Modular multilevel converter (1, 31) according to one of claims 1 to 3, comprising three branches (30), each configured to be connected to a different phase, and the first external capacitor (49) is coupled electrically in parallel with the first external module (32) of the three branches (30), and the second external capacitor (50) is electrically coupled in parallel to the second external module (50) of the three branches (30).

[Claim 5] Modular multilevel converter (1, 31) according to one of claims 1 to 3, wherein said at least one branch (30) comprises only a first external module (2, 32), a second external module ( 3, 33) and an internal module (4, 34), the third terminal (11, 41) of the first external module (2, 32) being electrically coupled to the first terminal (9, 39) of the internal module (4, 34 ) via a first inductor (16, 46), the third terminal (11, 41) of the second external module (3, 33) being electrically coupled to the second terminal (10, 40) of the internal module (4, 34) via a second inductor (17, 47), the third terminal (11, 41) of the internal module (4, 34) being configured to be connected to a phase of a load or of a network to which the converter (1, 31) is configured to be connected, and the second terminal (10, 40) of the first external module (2, 32) being electrically connected to the first terminal (9, 39) of the second external module (3, 33), the converter (1, 31) further comprises a first external capacitor (19, 49) comprising a first pole (190, 490) electrically connected to the first terminal (9, 39) of the first external module (2, 32) of said at least one branch and a second pole (195, 495) electrically connected to the second terminal (10, 40) of the first external module (2, 32) of said at least one branch, and a second external capacitor (20, 50) comprising a first pole ( 200, 500) electrically connected to the first terminal (9, 39) of the second external module (3, 33) of said at least one branch and a second pole (205, 505) electrically connected to the second terminal (10, 40) of the second external module (3, 33) of said at least one branch.