The present invention relates to a method and a device for controlling an electrical generator whose power varies in time, in particular a generator based on a renewable power source, such as a photovoltaic or wind-powered generator, or a power carrier for which the time profile of the generated power is not known in advance, connected to an AC or DC load, or connected to the electrical mains.

More specifically, the invention relates to a control method acting on a power conversion system, comprising the steps of

coupling a DC-DC converter to the generator through a first, parallel-connected capacitor, the converter comprising at least one electronic switch;

driving the switch by means of a driving signal having a duty cycle which is variable in predetermined ways, using a reference signal generated on the basis of a predetermined MPPT algorithm.

Prior art

Figure 1 of the attached drawings shows, in the form of a diagram which is partially a block diagram, a possible configuration of the system to which the method according to the invention can be applied. The diagram relates to a photovoltaic application connected to the single-phase electrical mains, and is provided purely by way of example and solely for the purpose of describing the method proposed by the invention, being non-limiting in respect of the type of generator used and the conversion circuit used, in terms of its type and its connection to the single-phase or three-phase AC electrical mains or to an isolated load of the AC or DC type.

In Figure 1 of the attached drawings, the letters PV indicate a photovoltaic generator, formed by an array of panels, and the number 1 indicates the associated conversion system. This conversion system 1 comprises a DC-DC converter 2 and a DC-AC converter 3. A parallel-connected capacitor Cj _n is interposed between the input of the DC-DC converter 2 and the photovoltaic generator PV. A bulk capacitor C _b is connected in parallel on the DC link 4 which interconnects the converters 2 and 3. .The DC-DC converter 2 is associated with a transferred power control module, indicated as a whole by 5. This module 5 receives at its input, in a known way, a signal indicating the voltage at the input of the converter 2, and a signal indicating the current flowing from or returning towards the photovoltaic generator PV. The module 5 supplies a control signal to the converter 2, the characteristics of this signal modifying the driving of one or more electronic switches included in the converter 2.

A corresponding control module 6 is associated with the converter or inverter 3. This module drives the inverter 3 as a function of the voltage V _b detected across the capacitor Cb, and of the voltage V _g and the current I _g at the output of the converter 3.

Problems of the prior art

In the operation of a conventional converter of the type shown in Figure 1 , the inverter 3 causes an oscillation of the voltage V _b across the capacitor C _b at a frequency (of 100 Hz for example) which is equal to twice the frequency (50 Hz for example) of the network ACN. This oscillation is propagated through the converter 2 and the control module 5, and disturbs the voltage Vpv of the generator PV. This causes a decrease in the power produced by the photovoltaic generator and can lead to a malfunction of the MPPT control module 5.

In order to overcome this problem in the prior art, a capacitor C _b having a rather high capacitance is chosen. However, this makes it necessary to use electrolytic capacitors.

Objects of the invention

One object of the present invention is to overcome the aforementioned disadvantages of prior art conversion systems.

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A second object is to extend the possibilities of "immunizing" the generator from all disturbances originating from the output of the DC-DC converter (2), either in applications connected to the mains or in applications operating in "island" mode (in DC or in AC).

A third object is to increase the efficiency of the maximum power point tracking of the generator provided by the controller (5).

These and other objects are achieved according to the invention with a method whose principal characteristics are defined in the attached Claim 1.

As described below with reference to Figure 2, the aforesaid objects are achieved in practice by the provision of a double control loop in the MPPT unit 5: the first loop is dedicated to tracking the current variations caused by "exogenous" variables (such as the incident solar radiation in the case of a photovoltaic generator), while the second loop is dedicated to maximizing the power produced by the system.

In the first loop, a conversion method of the type described initially is implemented according to the invention, and is characterized in that it detects the magnitude of the current flowing in the first capacitor and determines the duty cycle of the aforesaid driving signal in predetermined ways as a function of the detected magnitude of the current.

In the second loop, the input receives, in a known way, a signal indicating the voltage at the input of the converter 2, and a signal indicating the current flowing from or returning towards the photovoltaic generator PV. The second control loop provides the first control loop with a reference signal whose characteristics modify the driving of one or more electronic switches included in the converter 2.

The aforesaid objects are also achieved according to the invention with a conversion system whose principal characteristics are defined in Claim 4.

In a conversion system of this type, for a photovoltaic generator (for example), the current flowing during operation in the parallel-connected input capacitor "reflects" very rapidly the variations in current and voltage of the photovoltaic generator due to the variations of the solar radiation conditions. This enables a fast response time to be achieved.

The conversion system according to the invention can provide good rejection not only of the oscillations at the frequency which is double the mains frequency, but also of all other disturbances, owing to the fact that, in the configuration of the system proposed as an example of application to photovoltaic systems, all the disturbances originating either from the output of the converter 2 or from the generator are reflected in the current flowing in the parallel-connected input capacitor.

The application of the present invention requires the identification of any electrical variable which is instantaneously modified by the variations of the disturbances originating from the generator or from the converter 2, and which can be used in place of the input capacitor current.

Description of the drawings

Other characteristics and advantages of the invention will become clear from the following detailed description which is given purely by way of non-limiting example with reference to the attached drawings, in which:

Figure 1, described above, shows the architecture of a prior art conversion system, and

Figure 2 is a diagram showing an embodiment of a conversion system according to the present invention.

Detailed description of embodiments

Before a detailed description of a non-limiting example of a conversion system according to the invention is given, the principal features of the conversion method according to the invention will be indicated.

In this method, a DC-DC converter is coupled, through a parallel-connected capacitor, to a direct current electrical generator whose power is variable in time, particularly a generator based on a renewable power source. The DC-DC converter is, for example, a boost converter or a buck converter, and comprises at least one controlled electronic switch, driven by a signal having a variable duty cycle.

The conversion method according to the invention is primarily characterized by the fact that a double loop control system is implemented in the MPPT control unit 5: the first loop (or inner loop) is dedicated to tracking the current variations caused by "exogenous" variables (such as the incident solar radiation in the case of a photovoltaic generator), while the second loop is dedicated to maximizing the power produced by the system.

The method according to the invention requires the identification of an electrical variable which is modified virtually instantaneously by the variations or disturbances originating from the generator or from the input converter.

In the exemplary case, relating to a photovoltaic application, the magnitude of the current flowing in the capacitor is detected during operation, and the duty cycle of the driving signal applied to the electronic switch of the DC-DC converter is determined as a function of the detected magnitude of this current.

A specific example of embodiment will now be given with reference to the diagram in Figure 2.

In Figure 2, components described previously have been given the same alphanumerical references as those used before in relation to Figure 1.

In the embodiment shown by way of example in Figure 2, the DC-DC converter 2 is of the boost type, and comprises a series inductor L, a controlled electronic switch M such as a MOSFET, and a diode D. The invention can also be applied equally well when using DC- DC converters of other types, such as buck converters.

With reference to Figure 2, the inductor L is connected between the photovoltaic generator PV and the anode of a diode D, the cathode of which is connected to the DC-AC converter or inverter 3 through one of the two conductors of the DC link 4. The drain of the transistor M is connected to the anode of the diode D, while its source is connected to the earth GND (to which the other conductor of the DC link 4 is connected), and its gate is connected to the output of a control circuit CC.

In operation, the control circuit CC drives the transistor M with a square wave signal whose duty cycle is variable in predetermined ways as a function of two control signals applied to its two inputs.

A first input signal of the control circuit CC, indicated by Ια _η , is a signal supplied by a first current detector 7 associated with the parallel-connected input capacitor Cj _n . A second input signal, indicated by I _V R, is supplied to the control circuit CC by the control module 5.

The control module 5 receives at its input a signal Ipv from a current detector 8 associated with a conductor which couples the photovoltaic generator PV to the DC-DC converter 2, particularly the conductor connected to the inductor L of the converter.

Another signal V _PV is sent to the control module 5 through a connection 9 to the conductor with which the current detector 8 is associated.

In operation, the signals V _PV and Ipv delivered to the control module 5 represent the voltage and current at the output of the photovoltaic generator PV.

In the embodiment shown schematically in Figure 2, the control module 5 comprises a multiplier 10 which receives the voltage signal Vpv and the current signal Ipv- This multiplier supplies at its output a signal Ppv representing the power at the output of the photovoltaic generator PV, in other words at the input of the DC-DC converter 2.

In an alternative embodiment (not shown), the signals Ipv and Vpv which are delivered to the multiplier 10 could be branched from the upper conductor of the DC link 4, in such a way that the multiplier 10 would supply the power at the output of the converter 2. The output of the multiplier 10 is coupled to the input of an MPPT device 1 1 , of a known type, which supplies at its output a reference voltage VREF sent to an input of a subtraction device 12.

The signal Vpy is applied to a further input of the subtraction device 12. This subtraction device 12 supplies at its output an error signal V _e proportional to the difference between the voltage Vp _V and the reference voltage VREF-

The error signal V _e is delivered to the input of a correction network 13, which supplies from its output the signal I _V R to the control circuit CC of the DC-DC converter 2.

The architecture of the control module 5 described above is substantially conventional.

A distinctive feature of the conversion system 1 according to the present invention is the fact that the operation of the DC-DC converter 2 is driven as a function of the detected magnitude Ici _n of the current flowing in the parallel-connected input capacitor Cj _n .

As mentioned above, this current, in the photovoltaic application, can "reflect" the variations of solar radiation in a very rapid way, unlike the current in the inductor L. In the conventional system of Figure 1 , the photovoltaic generator PV is immediately modified by the variations in solar radiation, but the known methods of implementing the MPPT function in the unit 5 cannot react instantaneously to these variations. It is the object of the invention to overcome this limitation.

The conversion system 1 of Figure 2 provides efficient rejection of disturbances at the frequency which is twice that of the network ACN, and of any other disturbances, because these have an immediate effect on the current in the parallel -connected input capacitor.

The disturbance rejection band depends on the design characteristics of the control circuit CC associated with the DC-DC converter 2, which must have a large bandwidth and can be based, for example, on non-linear control methods such as "sliding mode" or "one cycle" control. Naturally, the principle of the invention remaining the same, the forms of embodiment and the details of construction may be varied widely with respect to those described and illustrated, which have been given purely by way of non-limiting example, without thereby departing from the scope of the invention as defined in the attached claims.