"IMPROVED VOLTAGE AND CURRENT CONTROL METHOD APPLIED TO A CONTROL APPARATUS FOR A DEVICE FOR THE PRODUCTION OF ELECTRICAL ENERGY". DESCRIPTION

The present invention concerns an improved method for voltage and current control for an inverter device, particularly suitable for use in a device for the production of electrical energy.

Moreover, the invention relates to a control apparatus to which the improved control method is applied, and to a device for the production of electrical energy in which said control apparatus is installed.

It is common knowledge that in order to use various types of electrically- operated equipment in situations where a connection to the national electricity grid is unavailable, particular means like generator sets or systems for the production of alternative energy, for example wind energy, are used, which enable the independent generation of the electrical energy required. For example, generator sets A of the known art, as shown in Figure 1 , comprise drive means B, consisting of an internal combustion engine or the like, which enable the transmission of a rotational movement to one or more electrical energy generators C, connected downstream. In order to properly use the electrical energy generated, it naturally becomes necessary to transform it and stabilise its characteristics. That is why one or more inverter devices D, and their control means E, usually of the feedback type, are installed downstream of the electrical energy generators C.

Finally, the output from the above-mentioned inverter devices D is delivered to the user devices F1 , which may have different internal and behavioural characteristics, making it necessary to design the control apparatus located downstream so as to keep the whole configuration as stable as possible in the various operating conditions.

It is also common knowledge that these generator sets A for generating electrical energy can be used for civil and domestic applications only if compliance is assured with the ISO standard 8528-3, which establishes that these devices must meet accurate specifications in terms of stability, prompt response and signal purity. In the case of the inverter devices D, in particular, the rated characteristics

include a rated output voltage coming between 100 and 120 V or 220 and 240 V, with a frequency that can be 50 or 60 Hz.

In addition, the required value of the total harmonic distortion, or THD, must be less than 2% of the fundamental, while the rated power of the device is comprised between 5 and 10 kW.

It is likewise well known that the control apparatus E for an inverter device D carried out according to the known art, as shown in Figure 2, comprises a voltage regulator assembly E1 and a current regulator assembly E2, as well as an inverter device D, of course, and a user device F1 , represented by the load F, and a filter G of the LC type, the main function of which is to reduce the voltage fluctuations on the load.

As shown in the flow chart, a cascade structure is clearly adopted, consisting of an internal current ring and an external voltage ring, in order to exploit the different dynamics of the apparatus, enabling both improved performance and control of the load voltage and current by means of the corresponding references.

In particular, the voltage and current are digitally controlled in a microcontroller that processes the input signals and generates the corresponding output signals in order to obtain the previously-specified performance. More in detail, as shown in Figure 2 illustrating the known art, the configuration of the control apparatus E comprises a first voltage regulator E1 that controls and stabilises the input voltage, taking for reference the output voltage from the load, which is fed back through the connection H1. This is followed by the regulation of the input current, taking for reference the current measured upstream of the load, which is fed back to the regulator assembly along the path H2.

Then comes the actual inverter device D, which transforms the voltage and current characteristics by means of one of the known modulating techniques, such as pulse width modulation, or PWM. Upstream of the load F there is the LC filter G that enables the harmonics to be attenuated at the modulation frequency and its multiples in the output voltage from the inverter device.

As for the load F, which in practice consists of the user F1 , this may be of various types and behave differently, depending on its resistive or inductive characteristics, or nonlinearity.

This type of configuration, and the other equivalent configurations of the known art, pose a series of drawbacks, however, as listed below.

As mentioned earlier, the great variety of possible characteristics of the load F may give rise to a first, particularly evident drawback of this type of configuration, which consists in the considerable difficulty in sizing the current regulator E2 with adequate values to ensure stability.

In fact, this variability makes it difficult to simultaneously guarantee both stability and a prompt response in all operating conditions.

Thus, under different loading conditions, the same dynamic and stability requirements for the current ring are met with different values of the regulator parameters.

A known solution for containing this drawback consists in placing a resistance with a given value, called a "bleeder resistance" in technical jargon, in parallel with the load F. In addition to increasing the stability of the system over a wider range of conditions than in the previously-described cases, this solution also introduces several major drawbacks, however.

A first drawback concerns the fact that the inclusion of an additional component, i.e. the resistance, increases the manufacturing costs of the control apparatus as a whole.

A second drawback lies in that the physical dimensions of the PCB needed for the control have to be increased in order to insert the resistance in parallel with the load, consequently increasing the space occupied by the control apparatus as a whole. Another, but not necessarily the least drawback lies in that the presence of the bleeder resistance in parallel with the load means there is a current absorption even when the equivalent resistance of the load is infinite.

The choice of the value for the bleeder resistance is thus based on a compromise between the improvement in the control's performance and the power absorption in no-load conditions, which affects the efficiency of the system.

This means that, although the introduction of the resistance contributes to improving the stability of the system in various conditions, the range in which the stability of the system is guaranteed under different types of load is still somewhat limited.

Concerning the combination of the voltage regulator with the LC filter, as mentioned previously, this guarantees the stability of the system and the attenuation of the harmonics in the presence of linear loads. This also entails a disadvantage, however, in that - in the presence of a non- linear load - the inverter device's modulating action, in combination with the LC filter, is not enough to effectively reduce the presence of multiple harmonics of the fundamental on the output voltage.

A further drawback consequently consists in the fact that the system does not enable an adequate, stable operation in all load conditions. The present invention aims to overcome the above-listed drawbacks.

The first object of the invention is to achieve an improved voltage and current control method in an inverter device that guarantees the stability of the configuration over a wider range of load variability than in the systems of the known art. Another object of the invention is to achieve a control method capable of attenuating the harmonics in the output voltage, both during transients due to sudden changes in the load, and by taking effect more slowly, compensating for the harmonics and ensuring minimal steady-state errors and THD. Another object of the invention consists in achieving a control method with better performance than those of the known art, without affecting the industrialisation costs.

Another, but not necessarily the least object of the invention is to achieve an improved control method that enables the dimensions of the control apparatus as a whole to be limited. The above objects are achieved by an improved method for controlling the voltage and current of an inverter device with characteristics according to the main claim.

The dependent claims describe further characteristics of the control method, of the control apparatus in which said method is implemented, and finally of the device for the production of electrical energy in which the control apparatus is installed, in order to achieve the set objects.

The control method that is the subject of the invention advantageously makes the generator more sturdy in relation to DC busbar voltages disturbed by harmonics relating to the imperfect nature of the sine waves of the electromotive forces (emf) produced by the permanent-magnet generator.

Another advantage of the control method lies in that it enables an improved control and regulation of the internal combustion engine, or other such means for making the electrical energy generator rotate.

The above-mentioned objects and advantages are illustrated in greater detail in the description of a preferred embodiment of the invention that is given below as a nonlimiting example with reference to the attached drawings, wherein:

- Figure 1 schematically illustrates the known art, with a device for the production of electrical energy; - Figure 2 illustrates the known art, with the equivalent circuit diagram of the control apparatus installed in the device for the production of electrical energy of Figure 1 ;

- Figure 3 schematically shows the device for the production of electrical energy; - Figure 4 shows the equivalent circuit diagram of the control apparatus contained in the device for the production of electrical energy of Figure 3, with the control elements of the invention;

- Figure 5 shows the equivalent circuit diagram of the internal bridge inverter device and the three-level modulation signal with a triangular carrier; - Figure 6 schematically shows the control apparatus in which the method of the invention is implemented;

- Figure 7 shows a flow chart of the control method that is the subject of the invention;

- Figure 8 shows the equivalent circuit diagram of the control apparatus in which the contribution of the virtual bleeder resistance can be observed;

- Figure 9 shows the equivalent circuit diagram of the introduction of the repetitive control in parallel with the voltage regulator;

- Figure 10 shows a digital representation of the repetitive control introduced in the method of the invention; - Figure 11 is a table showing the results of simulations with different R and C values of the non-linear load;

- Figure 12 is a table showing the effects on the control apparatus of changes to the components of the attenuator filter.

The method of the invention, indicated as a whole by 1 , is particularly and advantageously suitable for use in a device for the production of electrical

energy which, in the embodiment described herein, is a generator set 2, as shown in Figure 3.

In other embodiments of the invention, the device for the production of electrical energy may be a system for the production of alternative energy, for example wind energy or a different type of energy, provided that it belongs to the known art.

As shown in Figure 3, the generator set 2 is designed to connect drive means

3 with a device for generating electrical energy 4, the combination of which enables the kinetic energy generated to be transformed into electrical energy, In fact, the rotational movement generated by the drive means 3 is transmitted to the generator device 4, and to its rotor element 41 , in particular, which thus facilitates the induction of electrical energy in the windings of the stator element 42 inside the generator device.

In the particular embodiment described herein, the drive means 3 comprise an internal combustion engine.

In different embodiments, not described herein, the drive means 3 may comprise any other type of motor or means belonging to the known art that enables the generation of kinetic energy, and of a rotational motion in particular. As concerns the generator device 4, this is of the three-phase type and the rotor 41 consists of permanent magnets.

In other embodiments, the generator device 4 may naturally have different characteristics, provided that they enable the generation of electrical energy and provided that they belong to the known art. It is likewise evident and well known that the electrical energy generated by the generator device 4 does not have characteristics suitable for its use directly as the input to a load 6, as shown in the layout of Figure 3.

So, downstream of the generator device 4, the generator set 2 comprises a control apparatus 5 that enables the voltage and current delivered to the input of the load 6 to be regulated.

At this point, it is important to emphasise that the method of the invention 1 applies to this particular control apparatus 5.

In particular, this control apparatus 5, as shown in Figure 4, comprises an inverter device 7 configured in a feedback arrangement with a closed double ring, by means of which one of the operations specified in the method 1 ,

illustrated in Figure 7, is carried out.

This operation, indicated by a, consists in converting the characteristics of the voltage Ti and the current Ci delivered as input to the control apparatus 5, so as to provide the load 6, represented in practice by a user device 61 , with a voltage Tu and a current Cu having suitable characteristics.

The inverter device 7 is preferably, but not necessarily, a single-phase internal bridge inverter, as shown in the diagram of Figure 5.

Moreover, in the particular embodiment described herein, a three-level sine wave modulation with a triangular carrier 71 has been applied to the inverter device 7.

This advantageously enables a reduction of the harmonic content by comparison with any other modulation, on two levels for instance. In different embodiments of the invention it may also be possible to use an inverter device 7 with different characteristics and another type of modulation, provided that both the elements belong to the known art.

In the particular configuration described herein, moreover, the control apparatus 5 comprises a voltage regulator 8 that enables another operation b of the method 1 to be implemented, involving the regulation of the input voltage Ti by means of a first feedback control, taking for reference the output voltage from the load 6 fed back along the pathway 9.

According to the embodiment described herein, the voltage regulator 8 is of the highly dynamic resonant type that consequently takes effect during the transients due to a sudden change in the load 6. As for the third operation involved in the method 1 , indicated by the letter c, this consists in the regulation of the input current Ci delivered to the control system 5 by means of a second feedback control that collects the current signal upstream of the load 6 and uses this signal, via the path 10, as an input reference for a current regulator 11 , as shown in Figure 4. Clearly, in other embodiments, the current regulator 11 may be of a different type, provided that it belongs to the known art.

For the current regulator 11 , as for the voltage regulator 8, the resonant type was designed and implemented in the particular embodiment described herein. In another embodiment, not described herein, the current regulator 11 may be of the proportional integral type (Pl), or of any other type suited to the specifications and design needs.

Obviously, all the variants of the type of current regulator 11 must belong to the known art.

The current regulator 11 , the voltage regulator 8 and consequently also the corresponding feedback controls are implemented by means of a mathematical algorithm in a microcontroller 12 connected to the inverter device 7, as shown in Figure 6, enabling the exchange of the input and output signals between the two components.

Moreover, the method 1 of the invention involves an additional operation, indicated by d in Figure 7, which enables an attenuation of the harmonics at the modulation frequency and its multiples occurring in the output voltage Tu, by means of the introduction of an attenuator filter 13, located immediately upstream of the load 6, as shown in Figure 4.

The attenuator filter 13 is preferably, but not necessarily, of the LC type.

Finally, the pre-charactehsing operations of the method 1 include the operation e, involving the use of the output voltage Tu and current Cu by the above-mentioned load 6.

Naturally, the load 6, as mentioned previously, represents various types of user device 61 and may consequently have different structural and behavioural features that imply a different response from the control system 5, located upstream.

In the particular embodiment described herein, the load 6 is of the resistive type, but this does not rule out the possibility, in other embodiments, of the load 6 being of inductive and/or non-linear type.

According to the invention, the method 1 involves two further operations in addition to those described previously.

In particular, the first operation f consists in stabilising the characteristics of the output voltage Tu and current Cu to suit a wider variety of types of load 6 by comparison with the stability guaranteed by the configurations of the known art. Conceptually, the operation f takes place simultaneously with the operations a and b for regulating the voltage and the current, respectively, and involves the insertion of a virtual resistance 14, called a virtual "bleeder" resistance, in parallel with the load 6.

Unlike the introduction of a physical bleeder resistance which in the known art is placed in parallel with the load 6, its virtualisation by means of suitable

transfer functions advantageously enables the elimination, as already mentioned in the objects of the invention, both of the costs of the physical component and of the physical space needed to insert the resistance in the control apparatus 5. Another very important positive feature deriving from the introduction of the virtual resistance 14 lies in that it is no longer necessary to select its value on the basis of a compromise between the improvement in the performance of the control apparatus 5 and the power absorption under no-load conditions, which has a negative fallout on the system's performance. With the virtual resistance 14, in fact, the power absorption occurring with the physical bleeder resistances of the known art no longer occurs, and therefore it is possible to reduce the value of the resistance itself to increase stability without increasing resistive losses. As shown in detail in the equivalent circuit diagram of the control apparatus 5 in Figure 8, this virtual resistance 14 is included in the apparatus by adding one or more transfer functions that contain its contribution in the closed double ring feedback configuration.

In particular, it should be noted that the control apparatus 5 comprises the contribution of the virtual resistance value 14, indicated as Rb, both for the numerator and for the denominator of the transfer function of the assembly 15, and for the denominator of the transfer function of the feedback assembly 16. As concerns the second operation g, introduced by the method 1 of the invention, this consists in achieving a further attenuation of the harmonics in parallel with the voltage regulating operation a, as shown in Figure 9. This further attenuation operation involves the implementation of a repetitive control 23.

In particular, in the embodiment described herein and illustrated schematically in Figure 10, the operation g involves returning the output voltage, with the corresponding periodic disturbances, as indicated in the figure with the assembly 17, to the input of the repetitive control along the feedback path 18 after a number M of sampling periods.

As shown in Figure 10, the repetitive control 23 also comprises an assembly 19 with a gain coefficient K that is delivered to the output voltage Tu. In conclusion, this second operation g takes effect more slowly than the operation a completed by the voltage regulator 8, as described previously,

thereby compensating for the harmonics on the output voltage and ensuring minimal steady-state errors and THD.

In addition, this introduction enables a better attenuation of the harmonics in the output voltage Tu by comparison with the configurations of the known art, even in the case in which the load 6 is non-linear.

Finally, both the operation f for stabilising the control apparatus 5 with the virtual resistance 14, and the further operation g for attenuating the harmonics, done by the repetitive control 23, are implemented with a mathematical algorithm in the microcontroller 12, already contained in the control apparatus. The results of experiments conducted have demonstrated, as shown in the right-hand column 20 of the table 21 in Figure 1 1 , that the introduction of the attenuation operation g by means of the repetitive control 23 achieves a considerable reduction in the percentage of THD for different characteristics of resistance R and capacitance C of a load of non-linear type. For instance, for a value of R = 5.8 Ohm and C = 230 μF, the value of the THD changes from 5.47% without the repetitive control enabled to 0.84% with the control enabled.

A further factor to consider for the structural design of the control apparatus 5 lies in that each of the passive components used to create the attenuator filter 13 has a certain variability with respect to the rated value declared by the manufacturer, and this means consequently worse performance of the whole control apparatus 5.

That is why a valid control apparatus 5 must also have suitable characteristics to guarantee sufficient insensitivity to the above-mentioned parametric variations.

On this issue, based on experiments conducted with the repetitive control system 23, as shown in the table 22 in Figure 12, in conditions where the load resistance is fixed and the values of inductance and of the capacitor vary by 20% with respect to the rated values, the percentage of the THD does not vary in an appreciable manner.

Based on the above considerations, it is therefore clear that the method of the invention, applied to the control apparatus installed in the device for the production of electrical energy, achieves all the previously stated objects. In particular, the invention fulfils the object of achieving an improved method for controlling the voltage and current of an inverter device that guarantees the

maintenance of a stable configuration over a wider range of load variability than can be achieved with the systems of the known art.

Another object fulfilled is the implementation of a control method that attenuates the harmonics in the output voltage both during transients due to sudden changes in the load and by taking action more slowly, compensating for the harmonics and ensuring minimal steady-state error and THD.

Another object fulfilled consists in the implementation of a control method ensuring better performance than those of the known art, while preserving the industrialisation costs unchanged, Moreover, the object to achieve an improved control method that enables the dimensions of the entire device to be limited is also fulfilled.

In the executive phase, the method, the control apparatus and the device for the production of electrical energy may undergo modifications and variants, not described or illustrated herein, implemented for the purpose of improving their functionality and making their manufacture more economical.

Any such executive variants not described herein, and any others not mentioned, shall be deemed to be protected by the present patent, provided that they come within the scope of the claims that follow.

Where technical features mentioned in any claim are followed by reference signs, those reference sings have been included for the sole purpose of increasing the intelligibility of the claims and accordingly such reference signs do not have any limiting effect on the interpretation of each element identified by way of example by such reference signs.