ELECTRIC SYSTEM CONTROL

Abstract

Methods for supplying a power from a photovoltaic panel to an electric motor; a computer-readable storage medium, a computer program product, a processor and a control circuit including instructions allowing carrying out the methods; and a variable speed drive adapted to implement the methods.

Claims

1. A method for supplying a power from a photovoltaic panel to an electric motor, the photovoltaic panel, PV panel, being connected to a variable speed drive, VSD, driving the electric motor, the method comprising: controlling the electric motor to produce at least one voltage variation at an output voltage of the PV panel; monitoring a power supplied by the PV panel during a latest voltage variation of the at least one voltage variation; identifying a voltage having maximized the power supplied by the PV panel during the latest voltage variation of the at least one voltage variation; defining a target voltage such that applying a maximum power point tracking, MPPT, technique from the target voltage controls the output voltage of the PV panel to reach the identified voltage; controlling the electric motor to reach the target voltage at the output voltage of the PV panel; and starting the MPPT technique from the target voltage.

2. The method according to the preceding claim 1, wherein controlling the electric motor to produce at least one voltage variation at the output voltage of the PV panel comprises: controlling the electric motor to produce a first voltage variation corresponding to an evolution of the output voltage of the PV panel from a first voltage to a second voltage, wherein the first voltage is determined based on: a current voltage at an output voltage of the PV panel, or an output voltage of the PV panel when a speed of the electric motor is at a minimum point; and wherein the second voltage is determined based on a minimum operating voltage allowing the variable speed drive to control the electric motor by producing an alternating voltage supplying the electric motor.

3. The method according to claim 2, wherein controlling the electric motor to produce the first voltage variation comprises determining a first voltage reference in a control law of the electric motor, the first voltage reference corresponding to the second voltage; and wherein the first voltage reference is used in the control law to determine a first speed reference for the electric motor.

4. The method according to claim 2 or 3, wherein controlling the electric motor to produce at least one voltage variation at the output voltage of the PV panel comprises: controlling the electric motor to produce a second voltage variation, successive to the first voltage variation, corresponding to an evolution of the output voltage of the PV panel from the second voltage to a third voltage; and wherein the third voltage is determined based on the output voltage of the PV panel when the speed of the electric motor is at the minimum point.

5. The method according to claim 4, wherein controlling the electric motor to produce the second voltage variation comprises determining a second voltage reference in a control law of the electric motor, the second voltage reference corresponding to the third voltage; and wherein the second voltage reference is used in the control law to determine a second speed reference for the electric motor.

6. The method according to claim 4, wherein controlling the electric motor to produce the second voltage variation comprises determining a minimum speed reference for the electric motor in a control law of the electric motor; and wherein the minimum speed reference corresponds to the speed of the electric motor at a minimum point.

7. The method according to claim 1, wherein controlling the electric motor to reach the target voltage at the output voltage of the PV panel comprises determining a third voltage reference in a control law of the electric motor, the third voltage reference corresponding to the target voltage; and wherein the third voltage reference is used in the control law to determine a third speed reference for the electric motor.

8. The method according to claim 1, wherein the target voltage is greater than the identified voltage.

9. The method according to claim 1, wherein the method further comprises: monitoring the power supplied by the PV panel during the control of the electric motor to reach the target voltage; and if the power supplied by the PV panel during the control of the electric motor to reach the target voltage becomes greater than the power associated to the identified voltage, the method further comprises: redefining the target voltage as the current output voltage of the PV panel, thereby starting the MPPT technique from the current voltage.

10. The method according to claim 1, wherein a new iteration of the method is started when a timer started at a latest iteration of the method becomes greater than a first threshold.

11. The method according to claim 1, wherein a new iteration of the method is started when a gap between a maximum voltage and a minimum voltage of the output voltage of the PV panel in a first predetermined time window is greater than a second threshold.

12. The method according to claim 1, wherein a new iteration of the method is started when a gap between a maximum current and a current of an output current of the PV panel in a second predetermined time window is greater than a third threshold.

13. The method according to claim 1, wherein the target voltage belongs to a voltage range of the output voltage of the PV panel comprising a single maximum of power of the latest voltage variation and being free of any minimum of power of the latest voltage variation; and wherein the single maximum of power of the latest voltage variation corresponds to the maximum of power associated to the identified voltage.

14. A non-transitory computer-readable storage medium comprising instructions which, when executed by a controller, cause the controller to carry out the method of claim 1.

15. A variable speed drive, adapted to carry out the method of claim 1.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0004] FIG. 1 schematically illustrates an example of a Power/Voltage characteristic of a PV panel.

[0005] FIG. 2 schematically illustrates an example of a system according to the present disclosure.

[0006] FIG. 3 schematically illustrates another example of a system according to the present disclosure.

[0007] FIG. 4 schematically illustrates still another example of a system according to the present disclosure.

[0008] FIG. 5 schematically illustrates an example of a method according to the present disclosure.

[0009] FIG. 6a schematically illustrates another example of a Power/Voltage characteristic of a PV panel.

[0010] FIG. 6b schematically illustrates yet another example of a Power/Voltage characteristic of a PV panel.

[0011] FIG. 7 schematically illustrates another example of a method according to the present disclosure.

[0012] FIG. 8 schematically illustrates another example of a method according to the present disclosure.

[0013] FIG. 9a schematically illustrates yet another example of a Power/Voltage characteristic of a PV panel.

[0014] FIG. 9b schematically illustrates yet another example of a Power/Voltage characteristic of a PV panel.

[0015] FIG. 9c schematically illustrates yet another example of a Power/Voltage characteristic of a PV panel.

[0016] FIG. 10 schematically illustrates yet another example of a method according to the present disclosure.

[0017] FIG. 11 schematically illustrates an example of a functioning diagram of a control law of an electric motor according to the present disclosure.

[0018] FIG. 12a schematically illustrates yet another example of a method according to the present disclosure.

[0019] FIG. 12b schematically illustrates yet another example of a method according to the present disclosure.

[0020] FIG. 13 schematically illustrates another example of a functioning diagram of a control law of an electric motor according to the present disclosure.

[0021] FIG. 14 schematically illustrates yet another example of a method according to the present disclosure.

[0022] FIG. 15 schematically illustrates yet another example of a method according to the present disclosure.

[0023] FIG. 16 schematically illustrates yet another example of a method according to the present disclosure.

DETAILED DESCRIPTION

[0024] In an electric system comprising a photovoltaic panel (PV panel) supplying an electric motor, the current delivered by the PV panel varies due to its irradiance conditions.

[0025] In order to benefit from a maximum of power available, a maximum power point tracking (MPPT hereinafter) technique may be used to control an output voltage of the PV panel in order to reach a voltage associated with a maximum of power delivered by the solar panel.

[0026] The MPPT technique is a technique which continuously controls, by increment or decrement, the output voltage of the PV panel while monitoring the power delivered by the PV panel. As long as the power is rising when incrementing (or respectively decrementing) the voltage, the MPPT technique continues to increment (or respectively decrement) the output voltage. When the power starts to reduce, the MPPT technique inverses the direction of the control. That is, if the control was incrementing (or decrementing) the voltage, the control starts to decrement (respectively increment) the voltage when the power starts to decrease. Hence, the MPPT technique converges toward a maximum of power. The inventors have cleverly noticed that since the MPPT technique does not scan the whole power/voltage characteristic of the PV panel before being implemented, this technique leads to stabilize the voltage at the first maximum of power met by the technique, which may be a local maximum of power and not at a global maximum of power.

[0027] The FIG. 1 represents a power/voltage characteristic of a PV panel partially shaded and will be used to describe different scenarios of MPPT techniques, and especially to show why the voltage may be stabilized at a local maximum power point, depending on a voltage at which the MPPT technique is started. The x-axis represents the output voltage of the PV panel while the y-axis represents the power generated by the PV panel.

[0028] In a first scenario where the MPPT technique is started at point A, that is, when the output voltage of the PV panel is around 660V, the latter voltage will be controlled (decremented) until be stabilized around 610V, at a point corresponding to a first local maximum power point LMPP.sub.1. Indeed, when the voltage will be decremented below 610V, the power will be reduced, such that the MPPT technique will inverse the control (pass from voltage decrement to voltage increment), such that the voltage will be incremented to 610 V, and once the voltage will be greater than 610V, the power will be reduced, and the control will once again be inverted (pass from voltage increment to voltage decrement), until be stabilized. Hence, the voltage will converge around 610V at a point corresponding to the first local maximum power point LMPP.sub.1. In a second scenario where the MPPT technique is started at point B, that is, when the output voltage of the PV panel is around 520V, the voltage will be controlled (incremented this time) until be stabilized at the same voltage (around 610V) associated to the first local maximum power point LMPP.sub.1. In a third scenario where the MPPT technique is started at point C, that is, when the output voltage of the PV panel is around 480V, the voltage will be controlled (decremented) until be stabilized around 425V, which corresponds to the global maximum power point, GMPP. In a fourth scenario where the MPPT technique is started at point D, that is, when the output voltage of the PV panel is around 240V, the voltage will be controlled (incremented) until be stabilized around 425V, which corresponds to the GMPP. Finally, in a fifth scenario where the MPPT technique is started at point E, that is, when the output voltage of the PV panel is around 80V, the voltage will be controlled (incremented) until be stabilized at a 140V associated to a second local maximum power point LMPP.sub.2. Only the third and the fourth scenario leads the MPPT technique to stabilize the output voltage of the PV panel to a voltage maximizing the power supplied by the PV panel.

[0029] The inventors have also noted that controlling the output voltage of the PV panel in an electric system comprising a PV panel directly supplying a variable speed drive (VSD) driving an electric motor represents a challenge. A PV panel directly supplying a VSD means that the PV panel is directly connected to the VSD. That is, the PV panel is not connected to a DC/DC converter (DC stands for Direct-Current) which is then connected to the VSD to supply the VSD with a constant voltage. In case of a system with a DC/DC converter, which is an expensive system, the output voltage of the PV panel can be controlled by the DC/DC converter, without any impact in the electric motor control. Indeed, the voltage output of the DC/DC converter connected to the VSD is maintained stable such that modifying the voltage output of the PV panel does not produce any change in the control of the electric motor implemented by the VSD. However, when the VSD driving the electric motor is directly connected to the PV panel, the control of the electric motor by the VSD represents a challenge.

[0030] The inventors therefore propose a solution which may allow implementing a global maximum power point tracking technique, GMPPT, in an electric system comprising a PV panel directly connected to a VSD driving an electric motor. The solution presented by the present disclosure allows improving the power supplied by the PV panel to the electric motor compared to a MPPT technique.

[0031] With reference to FIGS. 2 to 4, it is now described an example of system 1 in which a method according to any one of the examples described in the present disclosure may be implemented.

[0032] The system 1 comprises a photovoltaic panel 2 connected to a variable speed drive 3. As mentioned above, the VSD 3 is directly connected to the PV panel 2. That is, there is no DC/DC converter between the PV panel 2 and the VSD 3.

[0033] A PV panel should be understood in this disclosure as any electronic or electrical unit able to convert light (photons) into an electrical current.

[0034] A variable speed drive 3 should be understood in this disclosure as an electronic, electrical, virtual or software implemented control unit for an electric motor. A variable speed drive may control an electric motor 4 using a control law determining a speed reference for the electric motor and applying a determined voltage to the electric motor according to the speed reference.

[0035] The VSD 3 is driving an electric motor 4. An electric motor 4 should be understood in this disclosure as any kind of electric motors which may be driven by a variable speed drive. In the illustrated non-limitative examples of systems 1, the electric motor 4 comprises three windings.

[0036] More specifically, the output of the PV panel 2 is connected to a DC-link 32 of the VSD 3. A DC-link 32 may be defined as a circuit configured to stabilize the voltage of a direct power bus. The DC-link 32 especially comprises a positive bus 32+ and a negative bus 32. In some examples, the DC-link presents a C-Less topology, for example a small film capacitor having an order of magnitude of 15 uF/kW. In other examples, the DC-link may comprise at least one of: [0037] an electrolyte capacitor having an order of magnitude of 100 uF/kW; or [0038] DC-link inductors.

[0039] The variable speed drive 3 also comprises an inverter 33, connected to the DC-link 32, and configured to cut off the voltage supplied by the DC-link into a variable voltage to control the electric motor 4.

[0040] In some examples, the inverter 33 may comprise a leg for each respective winding of the electric motor 4. In the illustrative examples of systems 1, the inverter 33 therefore comprises 3 legs. Each leg is connected on one side to a positive bus 32+ and on another side to a negative bus 32 of the DC-link 32.

[0041] The inverter 33 may comprise two switches 330 by leg. Each leg of the inverter comprises a top switch 330t and a bottom switch 330b. A connection between a leg and a winding of the electric motor 4 may be located between the top switch 330t and the bottom switch 330b as illustrated in FIG. 4. Top and bottom are used here to differentiate two switches in the illustrated example and may not necessarily imply that such switches are effectively in a top/bottom configuration.

[0042] Each switch 330 comprises two components. A first component is an Insulated Gate Bipolar Transistor, IGBT, and a second component is a freewheeling diode FD connected in parallel with the IGBT.

[0043] In some examples, the inverter 33 comprises a control circuit 331 allowing controlling a leg of the inverter 33, and advantageously each leg of the inverter 33. The control circuit 331 especially allows firing the IGBTs in due time based on a control law of the electric motor. The control circuit 311 is especially configured to send an electric signal (for instance a trigger pulse) to the gate of the IGBT for firing the IGBT, i.e. in order to switch the IGBT from a blocking state to a passing state. The control circuit 311 may be configured to operate at least a part of any examples of the method 100 hereby described. The control circuit 311 may especially be configured to implement any example of a control law of the electric motor presented hereby.

[0044] More specifically, the control circuit 311 may comprise a processor PROC and a memory MEM. The processor PROC may implement Pulse Width Modulation, PWM, signals based on the control law of the electric motor. The control law performed by the processor PROC makes it possible to determine the voltage to be applied to the output phases intended to be connected to the electric motor 4 to be controlled. The processor PROC may be configured to operate at least a part of any examples of the method 100 hereby described.

[0045] The memory MEM may correspond to a non-transitory machine-readable or computer readable storage medium. The memory MEM may be encoded with instructions executable by a controller such as the controller PROC. The memory MEM may comprise instructions to operate the controller PROC to perform at least a part of the examples of the method 100 hereby described. The memory MEM according to this disclosure may be any electronic, magnetic, optical or other physical storage device that stores executable instructions. The memory MEM may be, for example, Random Access Memory (RAM), an Electrically Erasable Programmable Read Only Memory (EEPROM), a storage drive, an optical disk, and the like. The controller PROC has therefore access to the information stored in the memory MEM.

[0046] In some examples, the system 1 may comprise a voltmeter (not shown) for acquiring voltage measurements of the DC-link 32 (which corresponds to the output voltage of the PV panel 2). In these examples, the voltmeter measures a voltage applied between the positive bus 32+ and the negative bus 32 of the DC-link 32. A voltmeter may for example be included in the variable speed drive 3.

[0047] In some examples, the system 1 may comprise an ammeter (not shown) for acquiring current measurements of the DC-link 32 (which corresponds to the output current of the PV panel 2). The ammeter may for example be included in the variable speed drive 3.

[0048] It is now described with reference to FIG. 5 an example of computer-implemented method 100 for supplying a power from a PV panel 2 to an electric motor 4, the photovoltaic panel 2 being connected to a VSD 3 driving the electric motor 4. As previously explained, the examples of method 100 describes in the present disclosure may be applied in any one of the examples of systems 1 described in the present disclosure, and may especially be applied at least in part by the control circuit 311 of the VSD 3.

[0049] It should be noted that the example of methods 100 illustrated in the figures are merely an illustration of examples of process representing, by means of blocks, the various operations that may be included in the process and described in the remainder of the document. As such, this illustration does not reflect any seriality between operations. In other words, the operations described with reference to the methods 100 illustrated in the figures are not necessarily implemented one after the other, and may in particular be implemented in a different order from the ones shown in the figures, or be implemented in parallel, unless when a given operation needs an output from another operation to be implemented. Similarly, it is not necessary for each operation to be implemented once before that a same operation could be performed a second time. The frequency of implementation of each operation is specific to it and is not necessarily linked to the implementation of the other operations.

[0050] One of the objectives of the examples of method 100 is to transfer as much power as possible from the PV panel 2 to the electric motor 4, in view of the current conditions of irradiance of the PV panel 2. This objective will be achieved by finding and reaching an output voltage of the PV panel 2 associated with a maximum of power of a Power/Voltage characteristic of the PV panel 2.

[0051] As illustrated in block 110, the method 100 comprises controlling the electric motor 4 to produce at least one voltage variation V at an output voltage of the PV panel 2. In an example of system 1 wherein the VSD 3 is directly connected to the PV panel 2, the inventors have cleverly found out that a way to control the output voltage of the PV panel (to reach a maximum power) is to control the electric motor, for example by controlling a speed of the electric motor.

[0052] As illustrated in block 120, the method 100 comprises monitoring a power supplied by the PV panel 2 during a latest voltage variation V of the at least one voltage variation V. Monitoring a power supplied by the PV panel 2 during the latest voltage variation V allows determining a Power/Voltage characteristic of the PV panel 2 during the latest voltage variation V of the at least one voltage variation V. We understand that when there is only one voltage variation V, the latest voltage variation V corresponds to the unique voltage variation V. In the other cases, the latest voltage variation V of the at least one voltage variation V corresponds to the voltage variation V of the at least one voltage V which have been implemented, in time, at a last position. In some examples, the power supplied by the PV panel may also be monitored during all the voltage variations V of the at least one voltage variation V.

[0053] A power supplied by the PV panel 2 during a voltage variation V may be determined based on at least one of the following pairs of parameters: [0054] a speed and a couple of the electric motor 4; or [0055] an output current and an output voltage of the PV panel 2.

[0056] Hence, measurements or estimations of the parameters are made during the latest voltage variation V for monitoring the power supplied by the PV panel 2 during said latest voltage variation V. It should be noted that a speed and a couple of the electric motor 4 are required for the control of the electric motor, such that they are already determined by the VSD. With respect to the output current and output voltage of the PV panel 2, they respectively correspond to the input current and input voltage of the DC-link, which are already determined by the VSD to produce the alternating voltage applied to electric motor 4. That is, the power monitoring does not require adding any elements to the VSD for determining these parameters.

[0057] In some examples, the input voltage and the input current may be determined based on measurements respectively acquired by the voltmeter and by the ammeter of the VSD 3. Alternatively, the input current of the DC-link may be determined based on a power supplied by the PV panel 2 and based on the input voltage of the DC-link.

[0058] A first example of a latest voltage variation V represented in a Power/Voltage characteristic of a PV panel is illustrated in FIG. 6a. The latest voltage variation V corresponds here to the variation from a voltage V.sub.a (around 640V) to a voltage V.sub.b (around 170V). A second example of a latest voltage variation V represented in a Power/Voltage characteristic of a PV panel is illustrated in FIG. 6b. In FIG. 6b, the latest voltage variation corresponds to the variation from a voltage Ve (around 680V) to a voltage V.sub.a (around 445V). In these figures, a portion of a Power/Voltage characteristic of the PV panel of FIG. 1 are illustrated since the latest voltage variation V of these two examples does not cover the full range of the voltage variation illustrated in FIG. 1. In other words, only a portion of the Power/Voltage characteristic of FIG. 1 can be determined from the latest voltage variation V of FIGS. 6a and 6b.

[0059] As illustrated in block 130, the method 100 comprises identifying a voltage V.sub.Pmax having maximized the power supplied by the PV panel during the latest voltage variation V of the at least one voltage variation. Here, the method 100 focuses on the latest voltage variation V of the at least one voltage variation considering that the Power/Voltage characteristic of the PV panel 2 may have evolved during the voltage variations, for example due to a variation of irradiance of the PV panel during the at least one voltage variation V. Hence, the latest voltage variation corresponds to the latest conditions of irradiance of the PV panel, which are most likely to be the most accurate. The identified voltage V.sub.Pmax having maximized the power supplied by the PV panel during the latest voltage variation is illustrated in FIGS. 6a and 6b. In FIG. 6a, it corresponds to the voltage V.sub.Pmax associated to the GMPP but we understand that depending on the voltage variation V, the voltage can correspond to an identified voltage V.sub.Pmax associated with another maximum of power. The identified voltage V.sub.Pmax having maximized the power supplied by the PV panel 2 during the latest voltage may for example corresponds to a boundary of the latest voltage variation V, as illustrated in FIG. 6b where the voltage V.sub.Pmax corresponds to the voltage V.sub.a of the voltage variation V.

[0060] As illustrated in block 140, the method 100 comprises defining a target voltage V.sub.T such that applying a MPPT technique from the target voltage V.sub.T controls the output voltage of the PV panel to reach the identified voltage V.sub.Pmax.

[0061] In some examples, the target voltage V.sub.T belongs to a voltage range V.sub.T of the output voltage of the PV panel. The voltage range V.sub.T is defined below such that any target voltage V.sub.T chosen in this voltage range V.sub.T is a voltage starting point which leads a MPPT technique activated from this voltage starting point to control the output voltage of the PV panel to reach the identified voltage V.sub.Pmax.

[0062] The voltage range V.sub.T is defined as comprising a single maximum of power of the latest voltage variation V and as being free of any minimum of power of the latest voltage variation V. The single maximum of power of the latest voltage variation corresponds to the maximum of power associated to the identified voltage V.sub.Pmax. A voltage range V.sub.T which comprises a single maximum of power of the latest voltage variation V, and which is free of any minimum of power of the latest voltage variation V should be understood as an interval of voltages of the latest voltage variation V, wherein the changes of power in this interval, monitored during block 120, comprises a unique maximum of voltage and does not comprise any minimum of voltage (no local minimum of power or no minimum of power associated to a boundary of the latest voltage variation V). As illustrated in FIG. 6b, the maximum of power can be associated with a boundary of voltage (V.sub.a, V.sub.b, V.sub.c, V.sub.d) of the latest voltage variation V. Different possibilities of voltage range V.sub.T corresponding to the definition above are illustrated in the figures.

[0063] As illustrated in block 150, the method 100 comprises controlling the electric motor to reach the target voltage at the output voltage of the PV panel 2. As explained in block 110, the method 100 cleverly uses the control of the electric motor 4 to control the output voltage of the PV panel 2.

[0064] As illustrated in block 160, the method 100 comprises starting the MPPT technique from the target voltage. This block would allow the output voltage of the PV panel 2 to reach the identified voltage V.sub.Pmax from the target voltage V.sub.T, which therefore allows the PV panel to deliver the maximum of power identified during the latest voltage variation V, in view of its latest conditions of irradiance.

[0065] The method 100 therefore allows using an energy supplied by a PV panel 2 to drive an electric motor in a system wherein there is no DC/DC converter for modifying the output voltage of the PV panel, while controlling the output voltage of the PV panel to benefit from a maximum power (not necessarily a global maximum of power) in view of the conditions of irradiance of the PV panel. The method 100 ingeniously uses the control of the electric motor to control the output voltage of the PV panel 2, and therefore identifies and reaches a voltage associated with a maximum of power of the latest voltage variation of the output voltage of a PV panel 2.

[0066] In some examples, the block 110 of controlling the electric motor to produce at least one voltage variation may comprise controlling the electric motor to produce a first voltage variation V.sub.1. The first voltage variation V.sub.1 corresponds to an evolution of the output voltage of the PV panel from a first voltage V.sub.1 to a second voltage V.sub.2. These examples are illustrated in FIG. 7 by the block 111.

[0067] In some examples of method 100 comprising the block 111, the first voltage V.sub.1 may be determined based on: [0068] a current V.sub.cur voltage at an output voltage of the PV panel; or [0069] an output voltage of the PV panel when a speed of an electric motor 4 is at a minimum point.
The first voltage V.sub.1 may especially correspond to the current voltage at the output voltage of the PV panel or may correspond to the output voltage of the PV panel when a speed of an electric motor 4 is at a minimum point.
The minimum point of speed of the electric motor may be defined depending on a current application using the electric motor of the system 1. If the current application allows the electric motor to be completely stopped without any injuries, a speed of the electric motor at a minimum point may correspond to 0. However, some applications may present another minimum point of speed for the electric motor to avoid injuries. For example, when the electric motor drives a pump, the characteristics of some pumps may not allow the pump to be completely stopped in any conditions unless being submitted to damages (for example due to a lack of lubrication at a lower speed), such that an operator of the system 1 in its current application may define a minimum point of speed of the electric motor different from 0.

[0070] Having a first voltage V.sub.1 determined based on the current voltage V.sub.cur at the output voltage of the PV panel allows starting the method 100 from the current operating point of the system 1. That is, no initialization of the output voltage of the PV panel 2 is performed to produce the first voltage variation V.sub.1 of the method 100.

[0071] Having a first voltage V.sub.1 determined based on an output voltage of the PV panel when a speed of an electric motor is at a minimum point allows initializing the method 100 at a maximum voltage of the Power/Voltage characteristic of the PV panel for the current application of the system 1. Indeed, although the inventors have determined that a relationship between a speed of the electric motor and an output voltage of the PV panel 2 is not linear, they have cleverly noticed that a maximum voltage of the Power/Voltage characteristic of the PV panel for a given application is reached when a speed of the electric motor is controlled at a minimum point which depends on the given application. This point is particularly important since the speed of the electric motor is already the reference used in the control law of the electric motor 4. Hence, the control of the electric motor 4 to reach the minimum point of speed (and therefore the maximum voltage of the Power/Voltage characteristic) does not require to amend an existing control law of the VSD and simply requires, for a user of the method 100, to provide a reference speed corresponding to the minimum point of speed allowed by the application.

[0072] In some examples of method 100 comprising the block 111, the second voltage V.sub.2 may be determined based on a minimum operating voltage allowing the VSD to control the electric motor by producing an alternating voltage supplying the electric motor. The second voltage V.sub.2 may especially correspond to the minimum operating voltage allowing the VSD to control the electric motor. That is, the second voltage V.sub.2 is determined such that the output voltage of the PV panel 2 is sufficient (i.e. high enough) to allow the VSD to control the electric motor. Hence, the minimum operating voltage may be determined based on the intrinsic parameters of the VSD 3. The minimum operating voltage may especially be determined based on a technical specification of the VSD 3.

[0073] We therefore understand that in a first option wherein the first voltage V.sub.1 corresponds to the current output voltage V.sub.cur of the PV panel, the first voltage variation V.sub.1 may evolve from the current output voltage V.sub.cur of the PV panel to the minimum voltage (V.sub.2) allowing the VSD to control the electric motor. The FIGS. 9a and 9c illustrate this first option of first voltage variation V.sub.1. In these figures, the minimum voltage V.sub.2 allowing controlling the VSD is different to show two different situations of second voltage variation V.sub.2.

[0074] We also understand than in a second option wherein the first voltage V.sub.1 corresponds to the output voltage of the PV panel when the speed of an electric motor is at the minimum point, the first voltage variation V.sub.1 evolves in all the voltage range allowed by the current application of the system 1. Indeed, the maximum voltage of the possible voltage range allowed by the current application of the system 1 is fixed by the allowed minimum speed of the electric motor in view of the current application of the system 1, while the minimum voltage of the possible voltage range is defined as the minimum voltage allowing the VSD to control the electric motor. Hence, in this second option, the first voltage variation V.sub.1 allows determining, by the monitoring of the power supplied by the PV panel 2 implemented in block 120, the whole possible Power/Voltage characteristic of the PV panel associated to the current application of the system 1. The FIG. 9b illustrates this second option of first voltage variation V.sub.1.

[0075] In some examples of method 100 comprising the block 111, controlling the electric motor to produce at least one voltage variation at the output voltage of the PV panel may comprise controlling the electric motor to produce a second voltage variation V.sub.2. These examples are illustrated in the FIG. 8 by the block 112. The second voltage variation is successive to the first voltage variation V.sub.2. The second voltage variation V.sub.2 corresponds to an evolution of the output voltage of the PV panel from the second voltage V.sub.2 to a third voltage V.sub.3. A direction of a voltage variation V of the second voltage variation V.sub.2 may be opposite to a direction of a voltage variation V of the first voltage variation V.sub.1. In other words, if the first voltage variation V.sub.1 has reduced (respectively raised) the output voltage of the PV panel 2, the second voltage variation V.sub.2 will raise (respectively reduce) the output voltage of the PV panel 2. These examples allow determining a second time the Power/Voltage characteristic of the PV panel 2 and using the latest voltage variation V to identify the voltage V.sub.Pmax associated with the maximum of power.

[0076] In some examples of method 100 comprising the block 112, the third voltage V.sub.3 is determined based on the output voltage of the PV panel when the speed of the electric motor is at the minimum point. The third voltage V.sub.3 may especially correspond to the output voltage of the PV panel when the speed of the electric motor is at the minimum point. In these examples, the second voltage variation V.sub.2, since this voltage variation produces an evolution of the voltage from the second voltage V.sub.2 (determined based on the minimum voltage allowing the VSD to control the electric motor) to the third voltage V.sub.3 (determined based on the minimum point of speed of the motor), also allows determining the whole possible Power/Voltage characteristic of the PV panel 2 with respect to the current application of the system 1.

[0077] That is, at the end of the block 112, the whole possible Power/Voltage characteristic of the PV panel 2, in view of the current application, is determined. Hence, when the voltage V.sub.Pmax having maximized the power supplied by the PV panel during the latest voltage variation of the at least one voltage variation is identified, it corresponds to the voltage associated to the maximum available power supplied by the PV 2 panel in view of the current application of the system 1. This phenomenon is well illustrated in FIG. 9c where the first voltage variation V.sub.1 only allows determining a little portion of the Power/Voltage characteristic of the PV panel 2 in view of the current application. At the end of the first voltage variation V.sub.1, the voltage maximizing the power supplied by the PV panel corresponds to the first voltage V.sub.1. Hence, if the method 100 was only based on the first voltage variation V.sub.1 in this case, the method 100 would start the MPPT for reaching the first voltage V.sub.1. However, when the electric motor is also controlled to produce the second voltage variation V.sub.2, the voltage maximizing the power supplied by the PV panel of the latest voltage variation V becomes the voltage V.sub.Pmax associated to the GMPP such that the method 100 would start the MPPT to reach the voltage V.sub.Pmax.

[0078] In some examples of method 100 comprising the block 111, controlling the electric motor to produce the first voltage variation V.sub.1 may comprise a block 1111 of determining a first voltage reference V.sub.ref1 in a control law of the electric motor. The first voltage reference V.sub.ref1 is associated with the voltage of the DC-link of the VSD (or with the output voltage of the PV panel, which corresponds to the same voltage). The first voltage reference V.sub.ref1 corresponds to the second voltage V.sub.2 of the first voltage variation V.sub.1. In these examples, the first voltage reference V.sub.ref1 is used in the control law of the electric motor to determine a first speed reference .sub.ref1 for the electric motor. That is, when the VSD controls the electric motor using the first speed reference .sub.ref1, the control of the electric motor will lead the voltage in the DC-link to reach the second voltage V.sub.2. Since the voltage of the DC-link corresponds to the output voltage of the PV panel 2, these examples allow controlling the output voltage of the PV panel using the control law of the electric motor applied by the VSD. These examples are schematically illustrated in FIG. 10.

[0079] The FIG. 11 illustrates an example of a functioning diagram of a control law of the electric motor allowing controlling the electric motor to produce a voltage variation V in the output voltage of the PV panel. As previously explained, the control law of the electric motor 4 may be implemented by the VSD, for example by the control circuit 311 of the VSD. The control law may especially be performed by the processor PROC of the control circuit 311 of the VSD.

[0080] In the example of functioning diagram of FIG. 11, the control law of the electric motor 4 comprises a function Vf, a function of and a function PWMf.

[0081] The function Vf is configured to determine a voltage reference V.sub.ref based on an input voltage V.sub.i. The voltage reference V.sub.ref may especially correspond to the input voltage V.sub.i. The voltage V.sub.i may therefore correspond to the second voltage V.sub.2 when the control law performs the first voltage variation V.sub.1, to the third voltage V.sub.3 when the control law performs the second voltage variation V.sub.1, or to the target voltage V.sub.T when the control law controls the electric motor to reach said target voltage V.sub.T in block 150. The input V.sub.i voltage may also correspond to the current output voltage of the PV panel 2 (which corresponds to the voltage of the DC-link) with an increment or a decrement of voltage when the control law controls the electric motor to perform the MPPT technique. In some examples of method 100 comprising the block 1111, the function Vf may therefore be configured to determine the first voltage reference V.sub.ref1 based on the second voltage V.sub.2.

[0082] The function of is configured to determine a speed reference .sub.ref based on the voltage reference V.sub.ref determined by the function Vf and on the current voltage V.sub.DC in the DC-link (which corresponds to the output voltage of the PV panel). In some examples of method 100 comprising the block 1111, the function of may therefore be configured to determine the first speed reference .sub.ref1 based on the first voltage reference V.sub.ref1.

[0083] The function PWMf is configured to apply a determined voltage U.sub.abc to the electric motor based on the speed reference .sub.ref. The function PWMf may especially determine PWM signals allowing controlling the IGBT of the inverter 33 to apply a determined voltage U.sub.abc in each winding of the electric motor 4 to control the electric motor 4 according to the speed reference .sub.ref.

[0084] It should be noted that the example of functioning diagram of a control law of the electric motor illustrated in FIG. 11 may be implemented to control the electric motor for at least one of the following operations: [0085] producing the first voltage variation V.sub.1; [0086] producing the second voltage variation V.sub.2; [0087] reaching the target voltage V.sub.T; or [0088] producing an increment or a decrement of voltage in the MPPT technique.

[0089] In some first examples of method 100 comprising the block 112, controlling the electric motor to produce the second voltage variation V.sub.b may comprise a block 1121a of determining a second voltage reference V.sub.ref2 in a control law of the electric motor. The second voltage reference V.sub.ref2 is associated with the voltage of the DC-link of the VSD. The second voltage reference V.sub.ref2 may correspond to the third voltage V.sub.3 of the second voltage variation V.sub.2. In these examples, the second voltage reference V.sub.ref2 may be used in the control law of the electric motor to determine a second speed reference .sub.ref2 for the electric motor. That is, when the VSD controls the motor using the second speed reference .sub.ref2, the control of the electric motor will lead the voltage in the DC-link to reach the third voltage V.sub.3. These examples are schematically illustrated in FIG. 12a. We understand here that, in the functioning diagram of FIG. 11, the function Vf may be configured to determine the second voltage reference V.sub.ref2 based on the third voltage V.sub.3, and that the function of may be configured to determine the second speed reference .sub.ref2 based on the second voltage reference V.sub.ref2.

[0090] In some second examples of method 100 comprising the block 112, controlling the electric motor to produce the second voltage variation V.sub.2 (from the second voltage V.sub.2 to the third voltage V.sub.3) may comprise a block 1121b of determining a minimum speed reference .sub.min for the electric motor. The minimum speed reference .sub.min may be determined in a control law of the electric motor. As explained above, VSDs already control the electric motors based on a speed reference .sub.ref such that the control law of the electric motor implemented by VSDs may not be amended to produce the second voltage variation V.sub.2. These examples are schematically illustrated in FIG. 12b.

[0091] The FIG. 13 illustrates another example of functioning diagram of a control law of the VSD according to the examples of FIG. 12b. In this diagram, a minimum speed reference .sub.min may be directly inputted into the function PWMf allowing applying a determined voltage U.sub.abc to the electric motor based on said speed reference. In other words, in this example of functioning diagram, there is no need to convert a voltage reference V.sub.ref to a speed reference .sub.ref, the minimum speed reference .sub.min is directly applied as input parameter of the control law of the electric motor. Hence, when the VSD controls the motor using the minimum speed reference .sub.min, the control of the electric motor will lead the voltage in the DC-link to reach the third voltage V.sub.3.

[0092] In some examples, controlling the electric motor to reach the target voltage V.sub.T at the output voltage of the PV panel may comprise a block 151 of determining a third voltage reference V.sub.ref3 in a control law of the electric motor. The third voltage reference V.sub.ref3 is associated to the voltage of the DC-link of the VSD. The third voltage reference V.sub.ref3 may be used in the control law of the electric motor to determine a third speed reference .sub.ref3 for the electric motor. That is, when the VSD controls the electric motor using the third speed reference .sub.ref3, the control of the electric motor will lead the voltage in the DC-link to reach the target voltage V.sub.T. These examples are schematically illustrated in FIG. 14. We understand here that, in the functioning diagram of FIG. 11, the function Vf may be configured to determine the third voltage reference V.sub.ref3 based on the target voltage V.sub.T, and that the function of may be configured to determine the third speed reference .sub.ref3 based on the third voltage reference V.sub.ref3.

[0093] In some examples, the target voltage V.sub.T is defined as a voltage greater than the identified voltage V.sub.Pmax. The inventors have found out that controlling the electric motor to reduce the output voltage of the PV panel 2 is faster than controlling the electric motor to increase the output voltage of the PV panel, due to the non-linearity of the relationship between speed of the electric motor and output voltage of the PV panel 2. Moreover, in the examples of method 100 comprising the block 112, since the second voltage variation V.sub.2 may lead the output voltage of the PV panel to its maximum, having a target voltage V.sub.T lower than the identified voltage means that the identified voltage V.sub.Pmax will be reached before reaching the target voltage V.sub.T. It is therefore faster to determine the target voltage V.sub.T at a voltage greater than the identified voltage V.sub.Pmax and starts the MPPT technique from the target voltage V.sub.T to converge to the identified voltage V.sub.Pmax.

[0094] In some examples, the method 100 may further comprise two other blocks 152 and 153, as illustrated in FIG. 15. In block 152, the method 100 monitors the power supplied by the PV panel 2 during the control of the electric motor to reach the target voltage V.sub.T. That is, when the method 100 implements the block 150 of reaching the target voltage V.sub.T, the method 100 also monitors the power supplied by the PV panel 2.

[0095] Then, the block 153 is implemented if the power supplied by the PV panel 2 during the control of the electric motor 4 to reach the target voltage V.sub.T becomes greater than the power associated to the identified voltage V.sub.Pmax. Indeed, as explained above, the irradiance conditions of the PV panel 2 may evolve and may especially evolve when the method 100 implements the block 150 of controlling the electric motor to reach the target voltage V.sub.T. Hence, if during the control of the electric motor to reach the target voltage V.sub.T, a power supplied by the PV 2 becomes greater than a power associated to the identified voltage V.sub.Pmax (corresponding to the maximum of power of the latest voltage variation V), this means that the irradiance conditions of the PV panel 2 have evolved and that the power associated to the identified voltage V.sub.Pmax is no more an available maximum of power to be supplied by the PV panel 2.

[0096] The block 153 corresponds to redefine the target voltage V.sub.T as the current output voltage of the PV panel 2. Since the block 160 of starting the MPPT technique is implemented when the output voltage of the PV panel reaches the target voltage V.sub.T, this redefinition of the target voltage V.sub.T allows therefore implementing the MPPT technique of the block 160 if the power of the PV panel becomes greater than the power associated to the identified voltage V.sub.Pmax. Hence, a new maximum of power will be reached by the MPPT technique starting from the redefined target voltage V.sub.T (corresponding to the current output voltage of the PV panel 2).

[0097] In some examples, the block 160 of starting a MPPT technique from the target voltage V.sub.T may comprise a block 161 of determining successive voltage references V.sub.ref in a control law of the electric motor. The successive voltage references V.sub.ref are associated to the voltage of the DC-link of the VSD. These examples are illustrated in FIG. 16. Each voltage reference V.sub.ref of the successive voltage references may be inputted in the function of to determine a respective speed reference .sub.ref.

[0098] In some examples, a new iteration of the method 100 may be started when a timer started at a latest iteration of the method 100 becomes greater than a first threshold. This allows restarting the method 100 to eventually find and reach a new maximum of power available if the irradiance conditions of the PV panel 2 have evolved. The first threshold may for example be lower than 5 minutes, and preferably be lower than 1 minute.

[0099] In some examples, a new iteration of the method 100 may be started when a gap between a maximum voltage and a minimum voltage of the output voltage of the PV panel in a first predetermined time window is greater than a second threshold. This allows restarting the method 100 when a brutal voltage variation of the output voltage of the PV panel is observed. This brutal voltage variation indicates that the irradiance conditions of the PV panel 2 have been changed and that finding and reaching a new maximum of power may be relevant. In some examples, the second threshold may be greater than 3V, preferably be greater than 10V.

[0100] In some examples, a new iteration of the method 100 may be started when a gap between a maximum current and a minimum current of the output current of the PV panel in a second predetermined time window is greater than a third threshold. This allows restarting the method 100 when a brutal current variation of the output current of the PV panel is observed. This brutal current variation indicates that the irradiance conditions of the PV panel 2 have been changed and that finding and reaching a new maximum of power may be relevant. In some examples, the third threshold may be greater than 10 A, preferably be greater than 50 A.

[0101] The first and the second predetermined time windows may for example be lower than 10 seconds and preferably be lower than 1 seconds.

[0102] The present disclosure also presents a computer-readable storage medium comprising instructions which, when executed by a controller, cause the controller to carry out any one of the methods presented hereby.

[0103] The present disclosure also describes a computer program product comprising instructions which, when the program is executed by a computer, cause the computer to carry out any one of the methods hereby described.

[0104] The present disclosure therefore allows using an energy supplied by a PV panel 2 to drive an electric motor in a system wherein there is no DC/DC converter for modifying the output voltage of the PV panel, while controlling the output voltage of the PV panel to benefit from a maximum power in view of the conditions of irradiance of the PV panel. The solution presented in the present disclosure ingeniously uses the control of the electric motor to control the output voltage of the PV panel 2, and therefore identifies and reaches a voltage associated with a maximum of power of the latest voltage variation of the output voltage of a PV panel 2.