FR3060238A1 - CONTROL METHOD APPLIED TO A MEDIUM VOLTAGE SPEED DIMMER - Google Patents

Abstract

The invention relates to a control method applied to a variable speed drive (10) of the medium voltage type, said variable speed drive comprising a plurality of switching arms, each switching arm comprising a plurality of power cells (cellk) connected in series on each phase. output signal, each power cell comprising at least one DC supply bus to which is applied a bus voltage (Vbusk) and a switching stage (INVk) controlled by control commands (Scellk) for generating an output voltage (Vcellk), each switching arm of the drive supplying an output voltage (Vout) to the electric motor. The method consists in estimating the bus voltage (Vbusk) of each power cell (cellk) from the output voltage (Vout) acquired for the said at least one output phase and the control commands (Scellk) applied to the cells. of power to obtain said measured output voltage.

Description

Technical field of the invention

The present invention relates to a control method applied to a medium voltage type speed controller for controlling an electric motor. The invention also relates to the control system implementing said method and the variable speed drive including said control system.

State of the art

A variable speed drive intended to control a medium voltage electric motor has a structure such as that described in US patent 5,625,545. This type of variable speed drive is connected to the electric motor by several output phases and includes a switching arm connected to each output phase. Each switching arm has several power cells connected in series. Each power cell comprises in particular a direct supply bus on which a bus voltage is applied and a switching stage controlled by control commands with a view to supplying an output voltage, called cell output voltage. Each switching arm supplies an overall output voltage on the output phase, this overall output voltage resulting from the cell output voltages generated by each power cell of the switching arm.

Such a drive also includes a central controller responsible for determining the control orders for the switching stages of each power cell in order to comply with a setpoint applied at the input, taking into account the current flowing in each output phase and the DC bus voltage. of each power cell. However, in its control loop, the central controller is based, for each power cell, on a preset average value of the DC bus voltage of each power cell. As a result, the voltage actually applied to the motor is permanently erroneous and does not conform to the expected value. One solution would be to measure the bus voltage in each power cell using a voltage sensor and send it back to the central controller for control. However, this solution is not satisfactory for the following reasons:

It requires a specific communication link for information transfer between each power cell and the central controller; It requires the use of sensors, which are bulky and expensive;

It requires synchronization of the measurements at the level of the power cells and of the output phases.

The object of the invention is therefore to propose a new solution for determining the bus voltage of each power cell, which is compact, inexpensive and which does not require the creation of a specific communication link between each cell. power and the central controller, nor a measurement synchronization between the power cells.

Statement of the invention

This object is achieved by a control method applied to a medium voltage type speed controller comprising a transformer, several output phases connected to an electric motor, said speed controller comprising several switching arms, each switching arm comprising several cells. of power connected in series on each output phase, each power cell comprising at least one continuous supply bus on which a bus voltage is applied and a switching stage controlled by control commands to generate an output voltage by cell, each switching arm of the variator supplying an output voltage to the electric motor, resulting from the sum of the output voltages of the power cells connected in series on the switching arm, said method being characterized in that it comprises steps from:

- Acquisition of the output voltage applied to the electric motor on at least one output phase;

Estimation of the bus voltage of each power cell from the output voltage acquired for said at least one output phase and from the control commands applied to said power cells to obtain said measured output voltage.

According to one feature, the method comprises a step of measuring the output voltage on said at least one output phase using a voltage sensor.

According to another particular feature, said estimation step is implemented taking into account the following expression:

„K = N„

Vout (t) - Σ Scella xVbus / ç k- \

In which :

N corresponds to the number of power cells k of the switching arm connected to said output phase of the variable speed drive, Vout (t) corresponds to the estimated output voltage on said output phase,

Scell _k corresponds to the command order applied to each power cell k of the switching arm connected to said output phase, Vbus _k corresponds to the bus voltage of a power cell k of the switching arm connected to said phase Release.

According to a first embodiment, said estimation step is implemented by applying a regulatory loop based on the following expression:

- [v /? ". S’] = iyy [SC] - (Vout (t) ~ Vout (t)) dt

In which :

Vbus corresponds to the bus voltage of a cell of said switching arm, [sc] corresponds to the matrix of control orders applied to the power cells of said switching arm,

Vout (r) corresponds to the output voltage generated on said output phase by said switching arm,

Vout (t) corresponds to the estimated output voltage on said output phase, _vo corresponds to a convergence gain.

According to a second embodiment, said estimation step is implemented by applying a regulatory loop based on the following expression:

- [ASC] · (\ Vout (t) -AVout (t)) dt

In which :

Vbus corresponds to the bus voltage of a cell of said switching arm, [ASC] corresponds to the matrix of variations in control orders applied to the power cells of said switching arm between a first instant and a second instant,

AVout (t) corresponds to the variation of the output voltage generated on said output phase by said switching arm between said first instant and said second instant,

AVout (t) corresponds to the variation of the estimated output voltage on said output phase between said first instant and said second instant, _vo corresponds to a convergence gain.

According to a third embodiment, said estimation step is implemented by applying a regulatory loop based on the following expression:

- [W / .s] = ιγ) [A5C] - (AVout (t) -AVout (t)) dt

In which :

Vbus corresponds to the bus voltage of a cell of said switching arm, [ASC] corresponds to the matrix of variations in control orders applied to the power cells of said switching arm between a first instant and a second instant,

AVout (r) corresponds to the variation of the output voltage generated on said output phase by said switching arm between said first instant and said second instant,

AVout (t) corresponds to the variation of the estimated output voltage on said output phase between said first instant and said second instant, determined during the transition from a first command order to a second command order of the cell, _vo corresponds to a convergence gain.

The invention relates to a control system for a medium voltage type speed controller comprising several output phases connected to an electric motor, said speed controller comprising several switching arms, each switching arm comprising several power cells connected in series on each output phase, each power cell comprising at least one continuous supply bus on which a bus voltage is applied and a switching stage controlled by control commands to generate an output voltage per cell, each arm for switching the variator supplying an output voltage to the electric motor, resulting from the sum of the output voltages of the power cells connected in series on the switching arm, said system being comprising:

A module for acquiring the output voltage applied to the electric motor on at least one output phase;

A module for estimating the bus voltage of each power cell from the output voltage acquired for said at least one output phase and control commands applied to said power cells to obtain said measured output voltage.

According to a particular feature, the system includes means for measuring the output voltage on said at least one output phase comprising at least one voltage sensor.

According to another particular feature, said estimation module is executed taking into account the following expression:

„K = N„

Vout (t) - Σ Scella xVbuSfr / <= l

In which :

N corresponds to the number of power cells k of the switching arm connected to said output phase of the variable speed drive, Vout (t) corresponds to the estimated output voltage on said output phase,

Scell _k corresponds to the command order applied to each power cell k of the switching arm connected to said output phase, Vbus _k corresponds to the bus voltage of a power cell k of the switching arm connected to said phase Release.

According to a first embodiment, said estimation module executes a regulation loop based on the following expression:

- [w? ". S] = V (9- [SC] - (Vout (t) -Vout (t)) dt

In which :

Vbus corresponds to the bus voltage of a cell of said switching arm, [sc] corresponds to the matrix of control orders applied to the power cells of said switching arm,

Vout (t) corresponds to the output voltage generated on said output phase by said switching arm,

Vout (t) corresponds to the estimated output voltage on said output phase, _vo corresponds to a convergence gain.

According to a second embodiment, said estimation module executes a regulation loop based on the following expression:

- ^ - [vbus] = [ASC] · (AVouf (f) -AVout (t))

In which :

Vbus corresponds to the bus voltage of a cell of said switching arm, [ASC] corresponds to the matrix of variations in control orders applied to the power cells of said switching arm between a first instant and a second instant,

AVout (r) corresponds to the variation of the output voltage generated on said output phase by said switching arm between said first instant and said second instant,

AVout (t) corresponds to the variation of the estimated output voltage on said output phase between said first instant and said second instant, _vo corresponds to a convergence gain.

According to a third embodiment, said estimation module executes a regulation loop based on the following expression:

- [vZ? M5] = vq- [A5C] - (AVout (t) -AVout (t)) dt

In which :

Vbus corresponds to the bus voltage of a cell of said switching arm, [ASC] corresponds to the matrix of variations in control orders applied to the power cells of said switching arm between a first instant and a second instant,

AVout (t) corresponds to the variation of the output voltage generated on said output phase by said switching arm between said first instant and said second instant,

AVoiiHi) corresponds to the variation of the estimated output voltage on said output phase between said first instant and said second instant, determined during the transition from a first command order to a second command command of the cell, _vo corresponds to a convergence gain.

The invention also relates to a medium voltage type speed controller comprising a transformer, several output phases connected to an electric motor, said speed controller comprising several switching arms, each switching arm comprising several power cells connected in series on each output phase, each power cell comprising at least one direct supply bus on which a bus voltage is applied and a switching stage controlled by control commands to generate an output voltage per cell, each switching arm of the variator supplying an output voltage to the electric motor, resulting from the sum of the output voltages of the power cells connected in series on the switching arm, said variator comprising a control system conforming to that defined above.

Brief description of the figures

Other characteristics and advantages will appear in the detailed description which follows, made in conjunction with the appended figures listed below:

FIG. 1A represents the architecture of a medium voltage variable speed drive and its associated control system;

FIG. 1B schematically represents the architecture of the first switching arm of the variator on an output phase connected to the electric motor;

FIG. 2 represents the architecture of a power cell used in the medium voltage variable speed drive;

FIGS. 3A and 3B show the waveforms of output voltage obtained by a switching arm with three power cells in series, respectively when all the bus voltages of the cells are identical to 1000V and when the bus voltages of the cells are variable;

FIG. 4 represents the functional architecture of a central controller used in the control system of the invention;

FIGS. 5A to 5C represent, in the form of functional diagrams, the different configurations of the observer block used in the invention;

FIGS. 6A and 6B represent diagrams illustrating the operating principle of the observer block as shown in FIG. 5C.

Detailed description of at least one embodiment

The invention applies to a medium voltage type variator 10, intended to control an electric motor and having an architecture such as that shown in FIG. 1.

With reference to FIGS. 1A to 3, this architecture has the following particularities:

A transformer 100 comprising a primary winding intended to be connected to a power source supplying an alternating voltage (AC) and several secondary windings.

Several output phases (referenced 1, 2, 3 in FIG. 1A) connected to the electric motor M, for example three output phases.

Several switching arms each connected to a separate output phase (1, 2, 3) and controlled to apply an output voltage to it.

Each switching arm comprises several power cells of identical topology connected in series with one another. In FIGS. 1A and 1B, the power cells are referenced cellik with i corresponding to the output phase to which the power cell is connected and k the reference of the power cell on the switching arm. In the following description, in general, each power cell is referenced cellk to express that it can be part of any switching arm.

- Each cellk power cell has an input connected to a separate secondary winding triplet of the transformer 100.

- Each power cell comprises a direct supply bus comprising at least one bus capacitor Cbus defining a direct bus voltage Vbusk and a switching stage INVk.

The switching stage INVk of each switching cell is a single-phase inverter (DC / AC type) composed of at least four power switches controlled to convert the DC bus voltage into a variable voltage.

The power switches are associated in pairs to form two separate switching arms, referenced arm R and arm L. Each arm is connected as an input to the DC voltage source formed by the bus capacitor Cbus and the midpoint of each arm forms an output terminal of the switching stage. The two switches of the same arm are always in complementary states.

- Each power cell advantageously comprises, at the input, a rectifier stage RECk of AC / DC type, for example formed by a diode bridge, connected by the input of the power cell to a secondary winding triplet (in the figure 2 three input phases). The rectifier stage RECk is responsible for converting the alternating voltage supplied by the power source into a direct voltage.

- Each cellk power cell is associated with a local controller LC which notably includes protection circuits, means of dialogue with a central controller MC and supply means.

A central controller MC is responsible for determining the Scellk control orders for the switches of each cellk power cell and transmits them to each local controller LC for application.

- Each local controller LC transmits to the central controller MC logical information on the operating state of the power cell with which it is associated.

The switching stage INVk of each power cell is controlled according to the control orders defined by the central controller

MC so as to generate an output voltage Vcellk between its two output terminals, designated cell_output voltage.

- Depending on the state of the power switches of the switching stage, the output voltage Vcellk of a power cell can take three distinct values which are + Vbus, -Vbus and 0.

The output voltage Vout supplied to the motor M on an output phase by a switching arm results from the sum of the cell output voltages supplied by the power cells of this switching arm (FIG. 1B).

A command shifted between the power cells makes it possible to generate a multilevel output voltage (see also FIGS. 3A and 3B).

The central controller MC defines the Scellk control orders to be applied to the power switches of the switching stage of each cellk power cell in order to comply with a set point to be applied to the motor M, this set point can be a speed and / or a couple. For this, it takes into account the current flowing in each output phase and a DC bus voltage value of each cellk power cell.

The central controller MC includes at least one microcontroller intended to execute software modules, in particular for implementing its various functional blocks, which will be described below in conjunction with FIG. 4.

Taking into account the architecture described above, the invention aims to propose a solution for estimating the DC bus voltage Vbusk of each power cell cellk of the medium-voltage variator as defined above. As described in the introductory part of the description, the central controller MC is generally based on a pre-established average value of DC bus voltage which is identical for all the power cells of the same switching arm. However, it turns out that this bus voltage is not always constant. The output voltage Vout applied by each switching arm to the motor is therefore found to be incorrect and not in accordance with the expected value. Furthermore, not knowing the bus voltage of each power cell does not ensure satisfactory control of the voltage during the phases of regeneration of the electrical energy resulting from the deceleration of the motor. FIG. 3A represents an output voltage waveform obtained by a switching arm with three cells in series, with an identical bus voltage of 1000V for each power cell. FIG. 3B represents a waveform for the same structure but with a variable bus voltage between each power cell. It is understood from these two figures 3A and 3B that it is imperative to take into account the variation of the bus voltage Vbusk of each power cell. The central controller MC must therefore be able to use a bus voltage value for each cell which is as precise as possible to ensure output voltages adapted to the setpoint.

For this, the invention uses the principle that the output voltage Vout applied to an output phase is continuously a combination of the bus voltages of each power cell and the control orders applied to the cells.

In the following description, the invention applies to a cell k of a switching arm of the variator. It should be understood that the output voltage Vout generated by this switching arm results from the switching of all the cells k connected in series on this switching arm. The power cell has a Vbusk bus voltage. The central controller MC is intended to implement the control method of the invention for estimating the bus voltage Vbusk of each cell k of the variator 10.

For each cellk power cell, we can note the cell_ output voltage Vcellk generated as a function of its bus voltage Vbusk and the control commands of the arms of its switching stage:

Vcel ^ ~ (S ^c -Lk ~ S ^c -Rk) * Vb ^us k ⁼ Scell _k * Vbus _k

With:

- Sc_x the control orders of the two arms L and R of the switching stage INVk of a power cell k and indicated Scellk for the power cell k;

Vbus _k the bus voltage of cell k;

The output voltage Vout of a switching arm of the drive, including N power cells k connected in series, is written:

k = N k = N

Vout (t) = ^ Vcel ^ t) = '^ Scell _k (t) * Vbus _k (t) k = lk = l

Therefore the output voltage Vout generated by a switching arm is directly dependent on the bus voltages Vbusk of the power cells of the arm and of the Scell command orders, applied to the power cells of the arm knowing that [Scel / = (Sc_L _k - Sc_R _k )] can only take the values -1,1 or 0.

For a three-phase application and for each output phase, we can write the simple voltages of the three arms (or phase-neutral compound voltages):

^V i (0 = ^ Sceliyt / Vbusyt) k = lk = N

V ₂ (r) ⁼ (t) * Vbus _2k (r) ffc = lk = N

V ₃ (i) = y \ Scell _3k (f) * Vbus _3k (f) k = l

According to the invention, the bus voltage Vbusk of each power cell is estimated individually. This estimation is advantageously carried out by the central controller MC which already has the information of the Scellk control orders for each power cell k and which must know the bus voltage of each power cell to generate each output voltage.

With reference to FIG. 4, the central controller MC presents the functional structure below. It includes:

A block B1 calculating voltages

The purpose of this block is to provide the reference voltages to be applied to the motor through an implementation by power electronics. It classically represents what is called the motor control law.

A block B2 management of neutral potential

The purpose of this block is to use the additional degree of freedom provided by the three-phase structure to maximize the amplitude of the output voltage, optimize power conversion criteria.

A standardization block B3

The purpose of this block is to transform the voltage value to be achieved into a percentage of cycle time to be achieved on the different output voltage levels of the power cells.

A B4 block of PWM generation (Pulse Width Modulation)

The purpose of this block is to carry out the control commands for the power switches of the power cells.

A B5 power control block

The purpose of this block is to protect power cells by controlling energy transfers between the mechanical system and the electrical system.

An OBS observer block

The purpose of this observer block is to provide observations (in the automatic sense) of the bus voltages of the power cells while these quantities are not being measured. These observations are based on the voltage measurements at the motor terminals, as well as the knowledge of the control commands sent to the power cells.

According to the invention, the observer block OBS can be implemented according to several distinct configurations which are described below.

The implementation of the observer block is described in detail in the case of a dimming switching arm, but the principle can be generalized to any number of arms.

The observer block advantageously includes one or more software modules executed by the central controller to implement the control method of the invention. The observer block is thus based at least on a module for acquiring the output voltage values Vout measured on each output phase of the variable speed drive and on a module for estimating the bus voltage of each power cell of the dimmer switching arm. The estimation module itself comprises several calculation and determination modules detailed below for the different regulation loops implemented according to the configuration of the observer block OBS which is adopted.

Block Watcher - 1 ^st configuration

This first configuration is based on the expression of the estimated output voltage as a function of the estimated bus voltages of the power cells (the character ^Λ to indicate the estimate aspect of the quantity):

„ ^{K = N} „

Vout (t) - © Scell _k xVbus _k k = l which is written in matrix form: Vout (t) = [5'C] ⁷ x [v7? Us]

With T which denotes the transposition operation; ^he

The two matrices are:

Scelf Vbus _x [sc] = [Wu / .s] = Seal _n Vbus _N

The following general expression comes:

- [vriu.s] = v <9 · [sc] · (Vout (t) -Vout (f)) dt

With _vo an adjustable gain of convergence between the estimated and real quantities.

This first configuration of the observer block is thus in the form of the regulation loop shown in its general form in FIG. 5A. In this loop, the observer block OBS is explained for a switching arm of the variator, intended to generate an output voltage Vout. The regulation loop has the following special features:

It comprises a module for determining the difference Vout (t) -Vout (t) between the measured output voltage generated by the switching arm and the estimated output voltage generated by the arm;

It includes a module for calculating the expression of - \ vb us] according to dt the expression defined above, taking into account the control orders applied to each cell k of the switching arm;

It includes an integration module in order to obtain the bus voltage

Λ estimated Vbus _k sought for each cell k;

The estimated output voltage Vout (t) of a switching arm is obtained from the control orders [SC] applied to the power cells cellk of this switching arm and the estimated bus voltage Vbus _k of each cell k the switching arm considered and obtained at the output of the regulation loop;

Block Watcher - ^2nd configuration

Likewise, this second configuration is also based on the expression of the estimated output voltage:

„ ^{K = N} „

Vout (t) - © Scell _k xVbus _k k = i which is written in matrix form: Vout (t) = [5'ΰ] ^Λ χ [ν7? Ί / .ν] with T which denotes the transposition operation ,

Seal _x Vbus _x [sc] = [Wn / .s] = Seal _n Vbus _N

In this configuration, the observation is no longer instantaneous but carried out between two instants t and Ts. We thus have:

EVout (t) = Vout (t) -Vout (t -Ts) = [SC (/) - SC (/ - rs)] ^r x [l ^ us (Y)]

With

ASC = SC (t) -SC (t-Ts)

AVout (t) = Vout (t) -Vout (t-Ts)

The following general expression comes:

- [ASC] · (AVout (t) - & Vout (t)) dt

With _vo an adjustable gain of convergence.

This second configuration of the observer block is thus in the form of the regulation loop shown in its general form in FIG. 5B. The loop thus has the following particularities:

It includes a module for determining the difference ASC between the control orders applied to the power cells at time t and the control orders applied to the power cells at time tTs;

It includes a module for determining the voltage variation AVout (r) between the output voltage measured at time t for the switching arm and the output voltage measured at time t-Ts for this same switching arm ;

It includes a module for determining the variation of the estimated output voltage AVout (t) from ASC and the estimated bus voltage

Λ

Vbus _k for each power cell and obtained at the output of the regulation loop;

It includes a module for determining the difference between AVout (t) ot AVout (r);

It includes a module for calculating the expression of - \ vb us] according to dt the expression defined above:

- [vfcu.s] = [A5C] - (AVout (t) -AVout (t));

dt

Block Watcher - ^3rd configuration

This third configuration is based on the principle developed for the second configuration above, that is to say:

AVout (j) = Vout (j) -Vout (t -Ts) = [SC (/) - SC (/ - Ts)] 5 <[w? Us (i)]

Most of the time, the order orders are constant, making the system unobservable.

Upon change of a control order, it is however possible to have a direct measurement of the bus voltage of the power cell cell _k for which the change of control order takes place.

Without loss of generality, considering a switching of the power cell k, we obtain at the time of the transition:

AV ont (r) = Vout (t) - Vout (t - Ts) = ± 1 x \ vb ^us k ( ^f ) J

In which the sign + or - depends on the type of transition (rising edge or falling edge).

The variation of the estimated output voltage AVout (t) is then directly impacted by the switching of the power cell k.

We are talking here about instantaneous value for Vout (t) and Vout (t-Ts).

But it is understood that:

Vout (t-Ts) can be any representation of the motor voltage between the last transition on the signal SC and the transition being processed. Vout (t) can be any representation of the motor voltage between the next transition on the signal SC and the transition being processed.

When switching cell k, the signal Vbus _k (t) is calculated by the following formula:

Vbus _k (t ') = ± 1 x [Vout (t) - Vout (t - Ts)]

In which the instant of calculation t ’can vary between t for the case of instantaneous values, and t + Ts for the case of values averaged over a sampling period Ts.

FIGS. 6A and 6B illustrate the principle of an instantaneous observation of the switching between the instants t-Ts and the instant t, respectively in the case where the variation of output voltage results from the difference in voltage measured on a transition to two determined instants and in the case where the voltage variation results from the voltage difference between two average values determined over a sampling period Ts. FIGS. 6A and 6B show each transition, in the case of a falling tension front and a rising edge (illustrated by the arrows).

This third configuration of the observer block is thus in the form of the regulation loop shown in its general form in FIG. 5C. It uses the same principles as that of the second configuration described above, by adding a module M1 for detecting the transition of control orders to determine the variation of the estimated output voltage AVomî (î).

The invention thus has many advantages. It allows, in an architecture such as that of a medium voltage variator with several power cells, to dispense with the use of voltage sensors at the level of each cell to determine the bus voltage. Thus, no communication solution is to be provided for the transfer of this information to the central controller.

Claims (13)

1. Control method applied to a medium voltage type speed controller (10) intended for controlling an electric motor and comprising a transformer (100), several output phases (1, 2, 3) connected to a motor electric (M), said variable speed drive comprising several switching arms, each switching arm comprising several power cells (cellk) connected in series on each output phase, each power cell comprising at least one direct supply bus on which is applied a bus voltage (Vbusk) and a switching stage (INVk) controlled by control commands (Scellk) to generate an output voltage (Vcellk) per cell, each switching arm of the drive supplying an output voltage (Vout) to the electric motor, resulting from the sum of the output voltages of the power cells connected in series on the switching arm, said method being characterized in that it includes steps to: - Acquisition of the output voltage (Vout) applied to the electric motor on at least one output phase; Estimation of the bus voltage (Vbusk) of each power cell (cellk) from the output voltage (Vout) acquired for said at least one output phase and control commands (Scellk) applied to said power cells for obtaining said measured output voltage. 2. Method according to claim 1, characterized in that it comprises a step of measuring the output voltage (Vout) on said at least one output phase using a voltage sensor. 3. Method according to claim 1 or 2, characterized in that said estimation step is implemented taking into account the following expression: „K = N„ Vout (t) - Σ Scella * Vbusk k- \ In which : N corresponds to the number of power cells k of the switching arm connected to said output phase of the variable speed drive, Vout (t) corresponds to the estimated output voltage on said output phase, Scell _k corresponds to the command order applied to each power cell k of the switching arm connected to said output phase, Vbus _k corresponds to the bus voltage of a power cell k of the switching arm connected to said phase Release. 4. Method according to one of claims 1 to 3, characterized in that said estimation step is implemented by applying a regulation loop based on the following expression: - [w? ". S] = v0 '[SC] - (Vout (t) -Vout (t)) dt In which : Vbus corresponds to the bus voltage of a cell of said switching arm, [sc] corresponds to the matrix of control orders applied to the power cells of said switching arm, Vout (t) corresponds to the output voltage generated on said output phase by said switching arm, Vout (t) corresponds to the estimated output voltage on said output phase, _vo corresponds to a convergence gain. 5. Method according to one of claims 1 to 3, characterized in that said estimation step is implemented by applying a regulation loop based on the following expression: - ^ - [vbus] = [ASC] · (AVout (t) -AVout (t)) In which : Vbus corresponds to the bus voltage of a cell of said switching arm, [ASC] corresponds to the matrix of variations in control orders applied to the power cells of said switching arm between a first instant and a second instant, AVout (t) corresponds to the variation of the output voltage generated on said output phase by said switching arm between said first instant and said second instant, AVout (t) corresponds to the variation of the estimated output voltage on said output phase between said first instant and said second instant, _vo corresponds to a convergence gain. 6. Method according to one of claims 1 to 3, characterized in that said estimation step is implemented by applying a regulation loop based on the following expression: - [W / .s] = ιγ) [A5C] - (AVout (t) -AVout (t)) dt In which : Vbus corresponds to the bus voltage of a cell of said switching arm, [ASC] corresponds to the matrix of variations in control orders applied to the power cells of said switching arm between a first instant and a second instant, AVW (r) corresponds to the variation of the output voltage generated on said output phase by said switching arm between said first instant and said second instant, AVout (t) corresponds to the variation of the estimated output voltage on said output phase between said first instant and said second instant, determined during the transition from a first command order to a second command order of the cell, _vo corresponds to a convergence gain. 7. Control system for a variable speed drive (10) of medium voltage type comprising several output phases (1, 2, 3) connected to an electric motor (M), said variable speed drive comprising several switching arms, each switching arm comprising several power cells (cellk) connected in series on each output phase, each power cell comprising at least one direct supply bus on which a bus voltage (Vbusk) is applied and a switching stage ( INVk) controlled by control commands (Scellk) to generate an output voltage (Vcellk) per cell, each switching arm of the drive supplying an output voltage (Vout) to the electric motor, resulting from the sum of the output voltages of the power cells connected in series on the switching arm, said system being characterized in that it comprises: A module for acquiring the output voltage (Vout) applied to the electric motor on at least one output phase; A module for estimating the bus voltage (Vbusk) of each power cell (cellk) from the output voltage (Vout) acquired for said at least one output phase and control orders (Scellk) applied to said cells of power to obtain said measured output voltage. 8. System according to claim 7, characterized in that it comprises means for measuring the output voltage (Vout) on said at least one output phase comprising at least one voltage sensor. 9. System according to claim 7 or 8, characterized in that said estimation module is executed taking into account the following expression: „K = N„ Vout (t) - Σ Scellk xVbuSk k- \ In which : N corresponds to the number of power cells k of the switching arm connected to said output phase of the variable speed drive, Vout (t) corresponds to the estimated output voltage on said output phase, Scellk corresponds to the command order applied to each power cell k of the switching arm connected on said output phase, Vbus _k corresponds to the bus voltage of a power cell k of the switching arm connected on said output phase exit. 10. System according to one of claims 7 to 9, characterized in that said estimation module executes a regulation loop based on the following expression: - [w? ". S’] = V (9- [SC] - (Vout (t) -Vout (t)) dt In which : Vbus corresponds to the bus voltage of a cell of said switching arm, [sc] corresponds to the matrix of control orders applied to the power cells of said switching arm, Vout (t) corresponds to the output voltage generated on said output phase by said switching arm, Vout (t) corresponds to the estimated output voltage on said output phase, _vo corresponds to a convergence gain. 11. System according to one of claims 7 to 9, characterized in that said estimation module executes a regulation loop based on the following expression: - ^ - [vbus] = [ASC] · (AVout (t) -AVout (t)) In which : Vbus corresponds to the bus voltage of a cell of said switching arm, [ASC] corresponds to the matrix of variations in control orders applied to the power cells of said switching arm between a first instant and a second instant, AVout (r) corresponds to the variation of the output voltage generated on said output phase by said switching arm between said first instant and said second instant, AVout (t) corresponds to the variation of the estimated output voltage on said output phase between said first instant and said second instant, _vo corresponds to a convergence gain. 12. System according to one of claims 7 to 9, characterized in that said estimation module executes a regulation loop based on the following expression: - [Ww.s] = vo · [aô'C] · (AVout (t) -AVout (t)) dt In which : Vbus corresponds to the bus voltage of a cell of said switching arm, [ASC] corresponds to the matrix of variations in control orders applied to the power cells of said switching arm between a first instant and a second instant, AVout (t) corresponds to the variation of the output voltage generated on said output phase by said switching arm between said first instant and said second instant, AVouKi) corresponds to the variation of the estimated output voltage on said output phase between said first instant and said second instant, determined during the transition from a first command order to a second cell command order, _vo corresponds to a convergence gain. 13. Medium voltage type variable speed drive (10) comprising a transformer (100), several output phases (1, 2, 3) connected to an electric motor (M), said variable speed drive comprising several switching arms, each switching arm comprising several power cells (cellk) connected in series on each output phase, each power cell comprising at least one direct supply bus on which a bus voltage (Vbusk) is applied and a switching stage ( INVk) controlled by control commands (Scellk) to generate an output voltage (Vcellk) per cell, each switching arm of the drive supplying an output voltage (Vout) to the electric motor, resulting from the sum of the output voltages of the power cells connected in series on the switching arm, characterized in that it comprises a control system in accordance with that defined in claims 7 to 12. 1/4