Classifications

• • H—ELECTRICITY
  • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
  • H02P—CONTROL OR REGULATION OF ELECTRIC MOTORS, ELECTRIC GENERATORS OR DYNAMO-ELECTRIC CONVERTERS; CONTROLLING TRANSFORMERS, REACTORS OR CHOKE COILS
  • H02P9/00—Arrangements for controlling electric generators for the purpose of obtaining a desired output
  • H02P9/14—Arrangements for controlling electric generators for the purpose of obtaining a desired output by variation of field
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F03—MACHINES OR ENGINES FOR LIQUIDS; WIND, SPRING, OR WEIGHT MOTORS; PRODUCING MECHANICAL POWER OR A REACTIVE PROPULSIVE THRUST, NOT OTHERWISE PROVIDED FOR
  • F03D—WIND MOTORS
  • F03D7/00—Controlling wind motors 
  • F03D7/02—Controlling wind motors  the wind motors having rotation axis substantially parallel to the air flow entering the rotor
  • F03D7/04—Automatic control; Regulation
• • H—ELECTRICITY
  • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
  • H02K—DYNAMO-ELECTRIC MACHINES
  • H02K19/00—Synchronous motors or generators
  • H02K19/16—Synchronous generators
  • H02K19/26—Synchronous generators characterised by the arrangement of exciting windings
  • H02K19/28—Synchronous generators characterised by the arrangement of exciting windings for self-excitation
• • H—ELECTRICITY
  • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
  • H02P—CONTROL OR REGULATION OF ELECTRIC MOTORS, ELECTRIC GENERATORS OR DYNAMO-ELECTRIC CONVERTERS; CONTROLLING TRANSFORMERS, REACTORS OR CHOKE COILS
  • H02P9/00—Arrangements for controlling electric generators for the purpose of obtaining a desired output
  • H02P9/08—Control of generator circuit during starting or stopping of driving means, e.g. for initiating excitation
• • H—ELECTRICITY
  • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
  • H02P—CONTROL OR REGULATION OF ELECTRIC MOTORS, ELECTRIC GENERATORS OR DYNAMO-ELECTRIC CONVERTERS; CONTROLLING TRANSFORMERS, REACTORS OR CHOKE COILS
  • H02P9/00—Arrangements for controlling electric generators for the purpose of obtaining a desired output
  • H02P9/14—Arrangements for controlling electric generators for the purpose of obtaining a desired output by variation of field
  • H02P9/26—Arrangements for controlling electric generators for the purpose of obtaining a desired output by variation of field using discharge tubes or semiconductor devices
  • H02P9/30—Arrangements for controlling electric generators for the purpose of obtaining a desired output by variation of field using discharge tubes or semiconductor devices using semiconductor devices
• • H—ELECTRICITY
  • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
  • H02P—CONTROL OR REGULATION OF ELECTRIC MOTORS, ELECTRIC GENERATORS OR DYNAMO-ELECTRIC CONVERTERS; CONTROLLING TRANSFORMERS, REACTORS OR CHOKE COILS
  • H02P9/00—Arrangements for controlling electric generators for the purpose of obtaining a desired output
  • H02P9/14—Arrangements for controlling electric generators for the purpose of obtaining a desired output by variation of field
  • H02P9/26—Arrangements for controlling electric generators for the purpose of obtaining a desired output by variation of field using discharge tubes or semiconductor devices
  • H02P9/30—Arrangements for controlling electric generators for the purpose of obtaining a desired output by variation of field using discharge tubes or semiconductor devices using semiconductor devices
  • H02P9/305—Arrangements for controlling electric generators for the purpose of obtaining a desired output by variation of field using discharge tubes or semiconductor devices using semiconductor devices controlling voltage
• • H—ELECTRICITY
  • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
  • H02P—CONTROL OR REGULATION OF ELECTRIC MOTORS, ELECTRIC GENERATORS OR DYNAMO-ELECTRIC CONVERTERS; CONTROLLING TRANSFORMERS, REACTORS OR CHOKE COILS
  • H02P9/00—Arrangements for controlling electric generators for the purpose of obtaining a desired output
  • H02P9/48—Arrangements for obtaining a constant output value at varying speed of the generator, e.g. on vehicle
• • H—ELECTRICITY
  • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
  • H02K—DYNAMO-ELECTRIC MACHINES
  • H02K7/00—Arrangements for handling mechanical energy structurally associated with dynamo-electric machines, e.g. structural association with mechanical driving motors or auxiliary dynamo-electric machines
  • H02K7/18—Structural association of electric generators with mechanical driving motors, e.g. with turbines
  • H02K7/1807—Rotary generators
  • H02K7/1823—Rotary generators structurally associated with turbines or similar engines
  • H02K7/183—Rotary generators structurally associated with turbines or similar engines wherein the turbine is a wind turbine
  • H02K7/1838—Generators mounted in a nacelle or similar structure of a horizontal axis wind turbine
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
  • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
  • Y02E10/00—Energy generation through renewable energy sources
  • Y02E10/70—Wind energy
  • Y02E10/72—Wind turbines with rotation axis in wind direction

Definitions

• the present invention relates to the production of a single or multiphase alternating current, for example three-phase, and more particularly the conversion of mechanical energy, obtained for example by means of a wind turbine, into electrical energy.
• the invention thus relates to electromechanical assemblies comprising an alternator, in particular of high power, typically greater than or equal to 1 MW, and a converter, the alternator comprising a rotor rotated, for example by a wind turbine, and more particularly generators. synchronous with wound rotor.
• the training can also take place by means of a tidal generator, hydraulic or tidal.
• An alternator comprises, in a manner known per se, an inductor winding, generally at the rotor, fed with direct current either by rings and brushes, or by an exciter, so as to generate, in an armature winding, generally at the stator, a AC voltage.
• variable speed operation is possible thanks to a variable speed drive.
• a rectifier makes it possible to deliver a DC voltage
• an inverter makes it possible to deliver a fixed frequency AC voltage corresponding to that of the network, with a unit power factor.
• the nominal power of the alternator determines the maximum power that the wind turbine can supply.
• the converters used are dimensioned for all the power exchanged between the alternator and the network. They therefore represent a significant cost, have significant losses, and can cause disturbances that affect the performance and quality of energy delivered.
• the use of such converters which are necessarily oversized for the entire nominal power of the generator, can lead for example up to 3% of the nominal power losses.
• the rectifier used may be a pulse width modulation controlled rectifier (PWM).
• PWM pulse width modulation controlled rectifier
• the reactive power transfer can be controllable, but the active power transfer remains the same as when using a simple rectifier having a diode bridge.
• the stator In the case where a dual-feed asynchronous machine is used, the stator is connected directly to the network while the power electronics interface is inserted between the rotor and the network thus allowing a sliding variation range and therefore the speed of rotation of the order of 30%.
• the stator of the machine being directly connected to the network, it can also undergo strong variations of current during disturbances of the network.
• the application EP 1 187 307 A2 discloses an electric machine comprising a rotor with claw poles, delivering on a converter.
• a claw pole rotor is characteristic of a small power machine with low power.
• No. 5,083,039 A discloses a cage asynchronous machine and the regulation operates on the parameters i d and i q to control the torque or the voltage.
• US Patent 6,239,996 B1 relates to a machine for charging automotive batteries.
• US Patent 6,437,996 B1 aims to switch to constant DC voltage to decrease the reactive power to transport energy with the least loss over several kilometers.
• the invention aims to meet all or part of the aforementioned needs.
• an electromechanical assembly operating at a variable speed, in particular at a speed, power or variable power factor comprising:
• a synchronous generator with wound rotor supplied with direct current by a voltage Vf, in particular by means of a exciter or a direct excitation by ring and collector, and delivering an output voltage Us,
• a converter comprising a rectifier for rectifying the output voltage Us of the alternator, the rectifier being able to be pulse-width modulated, or diodes and possibly followed by a DC / DC converter,
• the assembly may include a controller configured to act on the voltage Vf so as to maintain the output voltage Us of the alternator to a voltageimposée value U ff r e f.
• the supply voltage Vf of the wound rotor can be determined to minimize the difference between the output voltage Us of the alternator and the reference voltage U is ffref-
• the reference voltage U ref is ff can be selected so as to change, in particular to maximize the yield of the electromechanical assembly.
• U reference voltage ff ref is preferably calculated so as to minimize at least one of the losses of the following list: iron losses, which are composed of eddy current losses and hysteresis losses, effect losses Joule rotor, Joule losses to the stator, conductive losses of the converter, converter switching losses.
• the evolution of the voltage Us as a function of the frequency is a straight line in the case of constant U / f control.
• This evolution of the output voltage Us as a function of the frequency can be in the invention other than a straight line, U / f not being constant in particular over the range of rotational speeds from the lowest rotational speed (for example above 500 rpm) to the nominal speed (for example above 1500 revolutions per minute) where the power of the alternator tends to grow strongly.
• the frequency of the output voltage of the alternator as a function of the frequency can be a succession of straight segments with a slope which increases slightly by one. segment to another with the speed of rotation.
• the curve giving the evolution of the output voltage and therefore of the voltage setpoint as a function of the frequency may have two portions joined by a very pronounced setback connecting to a plateau where the voltage remains constant.
• a conventional regulation where U / f is constant up to U maximum also has a plateau where U remains constant, but without recess.
• Us can be lower than the voltage typically encountered in constant U / f regulation, up to a certain speed of rotation, where the voltage then becomes maximum.
• the electromechanical assembly according to the invention makes it possible to provide the same advantages in terms of efficiency as an alternator with permanent magnets, without the drawbacks associated with the presence of these permanent magnets.
• the assembly according to the invention makes it possible to improve the cost of the assembly and the ease of maintenance.
• the assembly according to the invention can be easily de-energized, unlike a permanent magnet alternator.
• the invention makes it possible to optimize the efficiency of the coiled rotor alternator and the associated converter operating at variable speed by modifying the voltage across the coiled rotor. The voltage is thus controlled via the excitation of the rotor.
• the assembly according to the invention is less expensive to manufacture.
• a fault for example a fault in the network
• the assembly according to the invention may thus make it possible to remedy deficiencies in the electrical network, which may make it possible to meet certain regulatory requirements.
• variable speed operation optimizes the energy captured by the wind turbine. Preferably, it is systematically sought to operate at optimum speed, that is to say that for a given operating regime, that is to say a wind speed given, it is desired that the power supplied is maximum.
• the alternator according to the invention makes it possible to supply electrical energy with a variable frequency, which must be adapted to that of the network, which is obtained via the power converter, comprising the rectifier as well as a inverter connected via a continuous bus.
• the speed of rotation of the wind turbine is limited to a maximum value to preserve its integrity.
• the reference voltage U ref is ff may depend on the operation of the whole system.
• U reference voltage ff ref may be dependent on at least one of speed, power, power factor, the thermal state of the machine, which may for example be known by means of temperature sensors.
• Power factor means the characteristic of an electrical receiver that is for an electric dipole, supplied with a variable current regime over time (sinusoidal or not), equal to the active power consumed by this dipole divided by the product of the rms values of current and voltage (apparent power). It is always between 0 and 1. In particular, if the current and the voltage are sinusoidal functions of the time, the power factor is equal to the cosine of the phase shift between the current and the voltage. The power factor is then commonly called "cos ⁇ ".
• This optimum voltage is preferably chosen as the reference voltage and can be calculated for each application.
• U a reference voltage value can for example select to ff ref of several prerecorded values, depending on the application and / or operating speed, in particular a rotational speed of the rotor.
• the assembly may comprise, for example a pre-recorded table of precomputed values of several reference voltages U ref ff each suited to a given operating speed. Automatically, switching from one operating mode to another causes the change of the reference voltage.
• the voltage V r to generate the necessary excitation in the exciter can be slaved to the effective output voltage U ff is supplied by the alternator.
• the excitation of the wound rotor of the alternator can be direct, by ring and collector.
• the voltage V f is directly controlled from the reference voltage U ref is voltage U ref is voltage imposée ff.
• the assembly may comprise a regulator controlling electronic switches of the rectifier.
• the control of the rectifier can be controlled by the phase, the power factor of the machine, the direct current intensity I D and the intensity of the quadrature current I Q , which can be used to control the electronic switches of the rectifier.
• the output current of the rectifier can supply the DC bus.
• the rectifier can be controlled so as to maintain a constant bus voltage.
• the assembly may comprise for this purpose a regulator, which makes it possible to slave a current reference of the regulator controlling the switches of the rectifier as a function of a measured voltage Ud c measured across the DC bus and a reference voltage Ud. c r e f.
• This current reference can be a reference value of the intensity of the current in quadrature and / or direct.
• the invention also relates to a generator, including a wind turbine, comprising an assembly as defined above.
• the wind turbine may comprise a multiplier for increasing the speed of a mechanical shaft of the assembly, driven in rotation by blades of the wind turbine, for example three blades.
• the subject of the invention is also a process for converting mechanical energy, in particular of wind energy, into electrical energy, in particular by means of an assembly as defined above, in which the rotor of the rotor is supplied with direct current.
• the aim is to minimize the difference between the output voltage of the alternator Us and a reference voltage U preffected.
• FIG. 1 schematically represents a wind turbine produced according to the invention
• FIG. 2 schematically illustrates the configuration of the synchronous alternator of the wind turbine of FIG. 1,
• FIG. 3 diagrammatically represents the operation of the alternator and its associated converter
• FIG. 4 is a schematic view of the control chain of the exciter
• FIG. 4a is a block diagram illustrating the operation of the regulator
• FIG. 5 shows the efficiency of the assembly according to the invention compared to other known assemblies
• FIGS. 6 to 15 schematically represent how the regulation (s) according to the invention can be modeled
• FIG. 16 represents the load curve for a wind turbine, and the curves giving the output voltage of the alternator as a function of the speed
• FIG. 17 shows in more detail the output voltage curve as a function of speed
• Figure 18 shows the efficiency as a function of the rotor speed of the alternator.
• FIG. 1 illustrates a wind turbine 1 according to the invention, comprising a nacelle to which are fixed blades 2, for example three blades, intended to be driven in rotation by the wind.
• the nacelle is fixed at the top of an unrepresented mast.
• a multiplier 5 makes it possible to increase the speed of a mechanical shaft 6 of the wind turbine.
• the nacelle houses a synchronous generator 10 and a converter 20 for converting wind energy received by the mechanical shaft 6 into electrical energy and supplying a network 9.
• the alternator outputs for this purpose a U output voltage ff, for example a three-phase voltage, the three phases being conventionally designated U, V and W.
• the alternator comprises an exciter 1 1 having, at the stator 12, an exciter inductor winding supplied with direct current so as to generate a current in an exciter armature winding of the exciter rotor 13. alternating, which is then rectified by a rectifier bridge 14 to supply rectified current I F the main inductor winding of the coiled rotor 15 of the alternator and generate a current in the main armature stator 16 of the alternator.
• Each phase of the main armature may comprise one or more windings.
• a synchronous exciter 11 constituted by an inverted alternator is used, in which the excitation circuit is placed on the stator 12 and supplied with direct current at a voltage V r via a voltage regulator 18, so as to generate a fixed magnetic field.
• the rotor 13 of the exciter 11 comprises a system of three-phase windings whose currents are rectified by the diode bridge rectifier 14 which is rotated to supply the main inductor.
• the exciter is mounted on the mechanical shaft 6 and driven at the same speed as the main rotor.
• the converter 20 comprises a rectifier 21 to pulse width modulation (PWM), which transforms the alternating voltage U and the current I ff s delivered by the alternator alternating DC voltage and DC current.
• PWM pulse width modulation
• the rectifier 21 is connected by a DC bus 22 comprising a capacitor C to an inverter 23 which recovers this DC voltage and this DC current.
• the control of the inverter is adjusted to adjust and obtain at the output of the inverter a signal with an amplitude and a frequency adapted to those of the network 9.
• the pulse width modulation rectifier 21 and the inverter 23 are decoupled from each other via the continuous bus 22.
• the passage through the DC bus 22 allows the inverter 23 to regulate amplitude and frequency.
• the rectifier 21 used can be bi-directional in power. Sinusoidal currents can be obtained by rejecting harmonics at high frequencies with adequate control. To this end, is regulated at 28, based on the reference intensity of the direct current I Dr ef and the current quadrature lQ ref with a decoupling algorithm 29, the current by controlling the switches of the rectifier 21 so as to control the phase shift ⁇ between the voltage and the current of the stator 16 of the alternator, which may allow a modification of the power factor cos ⁇ .
• the regulation of the phase shift at 24 makes it possible to reduce the stator losses by the Joule effect, as a function of a reference phase shift cp re f and of the measured quadrature intensity I rm surge, the joules losses stator and conduction of the converter being minimum for a power factor of 1. The overall efficiency of the assembly is thus satisfactory.
• regulators As an example of regulators, it is possible to use standard market regulators, among which may be mentioned: Leroy Somer models, for example D600, R449, BASLER DECS 100, 200 models, ABB Unitrol 1000 models, etc.
• Leroy Somer models for example D600, R449, BASLER DECS 100, 200 models, ABB Unitrol 1000 models, etc.
• power converter modules regulation in voltage and / or cos ⁇
• ABB ACS800 type for example of the ABB ACS800 type.
• FIGS. 6 to 15 describe in greater detail an example of regulations according to the invention.
• the rectifier 21 is in the PWM type developed example and it can be modeled as shown in FIG. 6. In the example developed, the rectifier 21 is bidirectional in power. An adequate control of the rectifier can make it possible to obtain sinusoidal currents, with rejection of harmonics at high frequencies, in the alternator.
• a control of the phase shift ⁇ between the voltage and the current of the stator of the alternator can be carried out, allowing a modification of the power factor cos ⁇ of the machine.
• phase shift ⁇ can also make it possible to act on the losses by Joule effect, the latter being minimum for a coscp equal to 1.
• the rectifier 21 can participate in the regulation of the voltage U dc of the DC bus 22, this regulation being able to avoid oversizing the inverter 23.
• the components 100 of the rectifier 21 are considered as perfect switches having the following Boolean behavior:
• the basic structure of the rectifier 21 is shown in FIG.
• the switches 100 are considered ideal and bidirectional current.
• the matrix representing the configuration of the rectifier 21 is defined as follows:
• T1, T2 and T3 are three switches respectively complementary to the switches T4, T5 and T6, as shown in Figure 7.
• This matrix MC evolves in real time depending on the control law.
• the alternator being star-coupled and connected to the input of the rectifier 21, it is possible to calculate the potentials across each phase of the main armature winding 16 to the stator. For this, we define by "n" the star coupling point of the alternator and we consider identical impedances for each phase and balanced voltages.
• the rectified current i red becomes
• the load R representing the inverter and the network is in the developed example chosen to have a DC voltage U dc of 800 V under the nominal power 3.15 MW.
• Resistor R can be used to simulate load impacts and to see the behavior of the system.
• the bus voltage U dc is related to the load power and the resistance by:
• R is chosen equal to 0.2 ⁇ .
• the voltage U dc must in the example developed be regulated to the value of 800 V to ⁇ 5%.
• the value of the capacitor C of the DC bus 22 can, as a first approximation, be determined by making the assumption that the capacitor must be able to provide 10% of the rated load current.
• the voltage of the DC bus 22 should not, in the example developed, not vary more than 5% from its nominal value.
• the active power of the machine thus depends on the quadrature current IQ.
• the active power of the machine corresponds to the power of the continuous bus 22.
• the current-related equations are then in the form of a first order system that can be regulated via a PI corrector.
• the signals Vd and V q are sent in a block allowing the passage DQ - abc, then on the stage of creation of the commands of the switches 100 of the rectifier 21.
• FIG. 11 shows the diagram of the voltages in the Park reference frame in motor convention:
• Fig. 12 shows the voltage diagram in the Park locator in motor convention with coscp equal to 1.
• An approach shown in Fig. 12 can be used to derive the internal phase shift angle for unit power factor operation.
• Phase shift control can be modeled using the Matlab-Simulink ® software. Such modeling is shown in Figure 13.
• FIG. 14 represents in the form of a block diagram a slave system in which the value U dc is slaved to a reference value U dc ref ⁇
• C (p) denotes the transfer function of the regulator PI in the Laplace domain.
• C (p) K p . ⁇ + -)
• W n is the natural pulsation and z is the damping coefficient.
• the output voltage of the stator 16 can be slaved with a regulator PI.
• the bandwidth of the current servo is in the example developed higher than that of the voltage, for example a ratio of 10 to 100.
• Matlab-Simulink ® software of the PWM control in 28 of the inverter 21 will now be described with reference to Figure 15 an example of modeling using Matlab-Simulink ® software of the PWM control in 28 of the inverter 21.
• the sampling frequency used is 5kHz in the example developed.
• the input signals mod a, mod b, mod c are directly sent by the decoupling algorithm. At the output, the control signals of the switches 100 are recovered.
• Such PWM control has the advantage of a constant switching frequency, which is fixed by the frequency of the carrier.
• the quadrature current IQ is used to regulate the continuous bus 22.
• the direct current I D is used to regulate the phase shift ⁇ .
• the excitation current Ip can make it possible to regulate the level of the excitation, this current being used as input data of an algorithm for optimizing the efficiency of the conversion chain. This search for minimizing the losses by the excitation voltage can be done by a systematic study or by a deterministic or stochastic algorithm.
• the voltage U is supplied by the alternator 10 is used to slave with the regulator 18 the voltage V r to generate the excitation required in the exciter 1 1, as shown in Figure 4, so that that the difference calculated with the reference voltage U is pre-imposed is minimum.
• FIG. 4a Another example of implementation of the regulator 18 is illustrated in FIG. 4a.
• a PID-type regulator followed by a setpoint filter is used.
• Modeling by means of a Park model of the assembly according to the invention makes it possible, compared to a Park model of a synchronous generator with permanent magnets, to obtain the yields A, B, B 'illustrated in FIG. Figure 5, given as a function of the speed of rotation of the alternator in revolutions per minute.
• the inverter portion 23 and network 9 can be likened to a resistive load R ch , as shown in FIG.
• the efficiency B of the assembly according to the invention is optimized, being very close to that A of a synchronous alternator with permanent magnets.
• Performance B ' is obtained with an assembly according to the invention used without performance optimization strategy (U ff voltage is constant).
• the efficiency optimization strategy used is to maximize the yield by minimizing a function representing all system losses.
• the losses of the machine can be modeled as follows:
• Losses in the converter in the literature, several works deal with the modeling of the losses of the power converters. For simplicity, losses in the converter can be estimated at constant losses due to hashing and losses proportional to the square of the current Is.
• the energy optimization makes it possible to bring a yield gain which is all the more interesting that the powers involved are important, the latter being of the order of 1 to several MW, for example from 1 to 11 MW, and given the annual production time.
• the optimization of the yield is very effective, which can be particularly interesting when one seeks to maximize the power extracted from the wind when this one is low.
• the direct and quadrature currents can be used to regulate the DC bus voltage, the torque, the power output and the phase shift ⁇ .
• the excitement can be used to maximize yield and manage overspeed.
• Figure 16 shows the evolution of the power of a wind turbine in kW as a function of the rotational speed of an alternator (curve P). The speed is expressed in revolutions per minute.
• FIG. 16 and FIG. 17 also show the evolution of the voltage as a function of the rotational speed for a conventional machine controlled according to the state of the art at constant U / f (curve A), and for an example of a machine controlled according to the invention (curve V).
• Optimized control voltage U was calculated based on power and speed to minimize losses.
• it is advantageously chosen to balance iron losses and Joule losses in order to minimize the sum thereof.
• the curve giving the output voltage (and thus the reference voltage) as a function of the rotational speed for an optimized voltage may be different from a straight line for speeds lower than the nominal speed, contrary to the case where the control performs with constant U / f up to the rated speed.
• the evolution of the voltage as a function of the speed with U optimized according to the invention may present a recess due to the sudden increase of the voltage to reach the maximum voltage. of operation, which results in a recess at the nominal speed on the curve, at 1600 revolutions per minute in the illustrated example.
• FIG. 18 shows the yield obtained by virtue of the invention (curve B) and the efficiency without the invention at constant U / f (curve C), as a function of the speed expressed in revolutions per minute.
• FIG. 18 illustrates the improvement of the yield obtained thanks to the invention. It can be seen that the gain (G) in efficiency is greater at low speed, the wind turbine being more efficient in weak wind.

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