Field of the disclosure The disclosure relates generally to control of electric energy. More particularly, the disclosure relates to a power converter for an electric power plant. Furthermore, the disclosure relates to an electric power plant, to a method for controlling an electric power plant, and to a computer program for controlling an electric power plant.

Background In many cases, an electric power plant for producing electric energy comprises one or more direct voltage energy sources and one or more direct voltage energy storages for storing energy and for responding to momentary power needs which cannot be satisfied by the direct voltage energy sources. Each direct voltage energy source can be for example a photovoltaic panel, a fuel cell, or another suitable direct voltage energy source. Each direct voltage energy storage may comprise for example a battery system and/or a capacitor bank. Furthermore, the electric power plant comprises one or more power converters for transferring energy from the direct voltage energy sources and from the direct voltage energy storages to an alternating voltage power grid. Each power converter comprises typically an inverter-bridge and a network filter for connecting the inverter-bridge to a supply transformer of the alternating voltage power grid. A network filter may comprise for example serial inductors or an inductor-capacitor-inductor "LCL" filter.

The electric power plant may comprise a separate power converter of the kind described above for each direct voltage energy source and for each direct voltage energy storage. Each power converter connected to a direct voltage energy storage is capable of transferring energy in both directions between the direct voltage energy storage and a supply transformer of an alternating voltage power grid. In this exemplifying case, energy flows via two power converters and via the supply transformer from a direct voltage energy source to a direct voltage energy storage when the direct voltage energy source is charging the direct voltage energy storage. In this situation, the energy transfer path from the direct voltage energy source to the direct voltage energy storage is rather long and comprises many elements. Thus, there may be significant power loss in the above-mentioned energy transfer path. Furthermore, when one or more direct voltage energy storages are charged by direct voltage energy sources, the total power transferred by power converters connected to the direct voltage energy sources is higher than output power of the electric power plant because a part of energy transferred by these power converters is directed to the one or more direct voltage energy storages. Thus, there is a need to design the power converters connected to the direct voltage energy sources for a power level higher than the output power of the electric power plant.

In another example, an electric power plant comprises a single power converter of the kind described above and direct voltage converters for connecting each direct voltage energy source and each direct voltage energy storage to the direct voltage side of the power converter. Each direct voltage converter connected to a direct voltage energy storage is capable of transferring energy in both directions between the direct voltage energy storage and the direct voltage side of the power converter. In this exemplifying case, energy flows via two direct voltage converters from a direct voltage energy source to a direct voltage energy storage when the direct voltage energy source is charging the direct voltage energy storage. When one or more direct voltage energy storages are charged by direct voltage energy sources, the total power transferred by direct voltage converters connected to the direct voltage energy sources is higher than output power of the electric power plant because a part of energy transferred by these direct voltage converters is directed to one or more direct voltage energy storages. Thus, there is a need to design the direct voltage converters connected to the direct voltage energy sources for a power level higher than the output power of the electric power plant.

Summary

The following presents a simplified summary in order to provide basic understanding of some aspects of various invention embodiments. The summary is not an extensive overview of the invention. It is neither intended to identify key or critical elements of the invention nor to delineate the scope of the invention. The following summary merely presents some concepts of the invention in a simplified form as a prelude to a more detailed description of exemplifying embodiments of the invention.

In accordance with the invention, there is provided a new power converter for a direct voltage energy storage of an electric power plant. A power converter according to the invention comprises:

- an inverter-bridge comprising alternating voltage poles and first direct voltage poles for connecting to a direct voltage energy storage,

- a network filter for connecting the alternating voltage poles to an alternating voltage power grid, - a controller for controlling, in a first operating mode of the power converter, the inverter-bridge to transfer energy from the first direct voltage poles to the alternating voltage poles,

- second direct voltage poles for connecting to a direct voltage energy source, and a contactor for arranging, in a second operating mode of the power converter, electric current paths from a first one of the second direct voltage poles to one or more of the alternating voltage poles and from one of the first direct voltage poles to a second one of the second direct voltage poles so that at least one of the electric current paths comprises at least one inductive component.

The above-mentioned controller is configured to control, in the second operating mode, each of one or more controllable switches between each of the one or more alternating voltage poles and the one of the first direct voltage poles to be alternately conductive and non-conductive. Thus, the above-mentioned at least one inductive component is alternately charged via the second direct voltage poles and discharged via the first direct voltage poles. The at least one inductive component may comprise for example one or more inductive components of the network filter. The above-mentioned inverter-bridge is usable as an inverter for supplying energy to the alternating voltage power grid, as well as a voltage-increasing direct voltage converter, i.e. as a boost-converter, for charging a direct voltage energy storage connected to the first direct voltage poles from a direct voltage energy source connected to the second direct voltage poles. Thus, the energy transfer path from the direct voltage energy source to the direct voltage energy storage can be short when the voltage of the direct voltage energy source is lower than the voltage of the direct voltage energy storage. This reduces power losses related to charging the direct voltage energy storage. Furthermore, there is typically no need to design power converters or direct voltage converters connected to direct voltage energy sources for power level higher than output power of an electric power plant because charging energy of direct voltage energy storages does not flow through the power converters or direct voltage converters connected to the direct voltage energy sources. In accordance with the invention, there is provided also a new electric power plant that comprises:

- at least one direct voltage energy source, e.g. a photovoltaic panel,

- at least one direct voltage energy storage, e.g. a battery system,

- a first power converter for transferring electric energy from the direct voltage energy source to an alternating voltage power grid, and

- a second power converter according to the invention for transferring, in a first operational mode of the electric power plant, electric energy from the direct voltage energy storage to the alternating voltage power grid and for transferring, in a second operational mode of the electric power plant, electric energy from the direct voltage energy source to the direct voltage energy storage.

In accordance with the invention, there is provided also a new method for controlling an electric power plant of the kind described above. A method according to the invention comprises: - controlling, in the first operational mode, the second power converter to transfer electric energy from the direct voltage energy storage to the alternating voltage power grid,

- controlling, in the second operational mode, a contactor of the second power converter to arrange electric current paths from a first direct voltage pole of the direct voltage energy source to one or more alternating voltage poles of the second power converter and from a direct voltage pole of the second power converter to a second direct voltage pole of the direct voltage energy source so that at least one of the electric current paths comprises at least one inductive component, and

- controlling, in the second operational mode, each of one or more controllable switches between each of the one or more alternating voltage poles of the second power converter and the direct voltage pole of the second power converter to be alternately conductive and non-conductive so as to alternately charge energy to the at least one inductive component from the direct voltage energy source and discharge the energy from the at least one inductive component to the direct voltage energy storage.

In accordance with the invention, there is provided also a new computer program for controlling an electric power plant of the kind described above. A computer program according to the invention comprises computer executable instructions for controlling a programmable processing system to:

- control, in the first operational mode, the second power converter to transfer electric energy from the direct voltage energy storage to the alternating voltage power grid,

- control, in the second operational mode, a contactor of the second power converter to arrange electric current paths from a first direct voltage pole of the direct voltage energy source to one or more alternating voltage poles of the second power converter and from a direct voltage pole of the second power converter to a second direct voltage pole of the direct voltage energy source so that at least one of the electric current paths comprises at least one inductive component, and

- control, in the second operational mode, each of one or more controllable switches between each of the one or more alternating voltage poles of the second power converter and the direct voltage pole of the second power converter to be alternately conductive and non-conductive so as to alternately charge energy to the at least one inductive component from the direct voltage energy source and discharge the energy from the at least one inductive component to the direct voltage energy storage. In accordance with the invention, there is provided also a new computer program product. The computer program product comprises a non-volatile computer readable medium, e.g. a compact disc "CD", encoded with a computer program according to the invention.

Various exemplifying and non-limiting embodiments of the invention are described in accompanied dependent claims.

Various exemplifying and non-limiting embodiments of the invention both as to constructions and to methods of operation, together with additional objects and advantages thereof, will be best understood from the following description of specific exemplifying and non-limiting embodiments when read in conjunction with the accompanying drawings.

The verbs "to comprise" and "to include" are used in this document as open limitations that neither exclude nor require the existence of un-recited features. The features recited in dependent claims are mutually freely combinable unless otherwise explicitly stated. Furthermore, it is to be understood that the use of "a" or "an", i.e. a singular form, throughout this document does not exclude a plurality.

Brief description of the figures

Exemplifying and non-limiting embodiments of the invention and their advantages are explained in greater detail below in the sense of examples and with reference to the accompanying drawings, in which: figure 1 illustrates a power converter according to an exemplifying and non-limiting embodiment of the invention, figure 2 illustrates a power converter according to another exemplifying and non- limiting embodiment of the invention, figures 3a, 3b, and 3c illustrate electric power plants according to exemplifying and non-limiting embodiments of the invention, and figure 4 is a flowchart of a method according to an exemplifying and non-limiting embodiment of the invention for controlling an electric power plant.

Description of exemplifying and non-limiting embodiments The specific examples provided in the description below should not be construed as limiting the scope and/or the applicability of the accompanied claims. Lists and groups of examples provided in the description are not exhaustive unless otherwise explicitly stated.

Figure 1 illustrates a power converter 101 according to an exemplifying and non- limiting embodiment of the invention. The power converter 101 is capable of operating in a first operational mode where the power converter 101 transfers energy from a direct voltage energy storage 1 13 to an alternating voltage power grid 1 15, and in a second operational mode where the power converter 101 transfers energy from a direct voltage energy source 1 14 to the direct voltage energy storage 1 13. In the exemplifying situation shown in figure 1 , the direct voltage energy storage 1 13 is a battery system and the direct voltage energy source 1 14 is a photovoltaic panel. The power converter 101 comprises an inverter-bridge 102 that comprises first direct voltage poles 103a and 103b and alternating voltage poles 104a, 104b, and 104c. In this exemplifying case, the inverter-bridge 102 is a three- phase inverter-bridge that comprises a controllable switch 1 1 1 a between the alternating voltage pole 104a and the direct voltage pole 103b, a controllable switch 1 1 1 b between the alternating voltage pole 104b and the direct voltage pole 103b, a controllable switch 1 1 1 c between the alternating voltage pole 104c and the direct voltage pole 103b, a controllable switch 1 12a between the alternating voltage pole 104a and the direct voltage pole 103a, a controllable switch 1 12b between the alternating voltage pole 104b and the direct voltage pole 103a, and a controllable switch 1 12c between the alternating voltage pole 104c and the direct voltage pole 103a. It is, however, also possible that an inverter-bridge of a power converter according to another embodiment of the invention has less than three phases or more than three phases. The controllable switches 1 1 1 a-1 1 1 c and 1 12a-1 12c can be for example insulated gate bipolar transistors "IGBT", gate turn-off thyristors "GTO", metal oxide semiconductor field-effect transistors "MOSFET", or bipolar transistors.

The power converter 101 comprises a network filter 105 for connecting the alternating voltage poles 104a, 104b, and 104c to the alternating voltage power grid 1 15. In this exemplifying case, the network filter 105 is an inductor-capacitor- inductor "LCL" filter for suppressing harmonics of alternating electric currents supplied to the alternating voltage power grid 1 15. It is, however, also possible that a network filter of a power converter according to another embodiment of the invention comprises only a serial inductor in each phase. The power converter 101 comprises a controller 106 for controlling, in the first operating mode of the power converter, the inverter-bridge 102 to transfer energy from the first direct voltage poles 103a and 103b to the alternating voltage poles 104a, 104b, and 104c so as to transfer energy from the direct voltage energy storage 1 13 to the alternating voltage power grid 1 15.

The power converter 101 further comprises second direct voltage poles 107a and 107b. In the exemplifying situation shown in figure 1 , the direct voltage poles 107a and 107b are connected to the direct voltage energy source 1 14. The power converter 101 comprises a contactor 108 for arranging, in the second operating mode of the power converter, electric current paths from the direct voltage pole 107a to each of the alternating voltage poles 104a, 104b, and 104c, and from the direct voltage pole 103b to the direct voltage pole 107b. In this exemplifying case, the electric current paths from the direct voltage pole 107a to each of the alternating voltage poles 104a, 104b, and 104c comprise inductive components 109a, 109b, 109c and 1 10. The inductive components 109a, 109b, and 109c are components of the network filter 105. The controller 106 is configured to control, in the second operating mode, the controllable switches 1 1 1 a, 1 1 1 b, and 1 1 1 c to be alternately conductive and non- conductive so as to alternately charge energy to the inductive components 109a, 109b, 109c, and 1 10 from the direct voltage energy source 1 14 and discharge the energy from these inductive components to the direct voltage energy storage 1 13 via diodes of the controllable switches 1 12a, 1 12b, and 1 12c. Therefore, the inverter-bridge 102 is used, in the second operating mode, as a voltage-increasing direct voltage converter, i.e. as a boost converter where VDCI > VDC2, for charging the direct voltage energy storage 1 13 from the direct voltage energy source 1 14. In a power converter according to an exemplifying and non-limiting embodiment of the invention, the controller 106 is configured to operate the controllable switches 1 1 1 a, 1 1 1 b, and 1 1 1 c in a phase-shifted way so as to reduce ripple of the direct current supplied to the direct voltage energy storage 1 13. In the exemplifying case illustrated in figure 1 , the direct voltage pole 107a having a positive polarity is connected to the alternating voltage poles 104a, 104b, and 104c and the direct voltage poles 103b and 107b having a negative polarity are connected to each other, and the controllable switches 1 1 1 a-1 1 1 c are controlled to be alternately conductive and non-conductive. It is, however, also possible that the direct voltage pole 107b having a negative polarity is connected to the alternating voltage poles 104a, 104b, and 104c and the direct voltage poles 103a and 107a having a positive polarity are connected to each other, and the controllable switches 1 12a-1 12c are controlled to be alternately conductive and non-conductive. The inductive component 1 10 is not necessary in cases where the inductive components 109a, 109b, 109c have sufficient inductances. The required inductance depends on switching frequency of the controllable switches.

Figure 2 illustrates a power converter 201 according to an exemplifying and non- limiting embodiment of the invention. The power converter 201 is capable of operating in a first operational mode where the power converter 201 transfers energy from a direct voltage energy storage 213 to an alternating voltage power grid 215, and in a second operational mode where the power converter 201 transfers energy from a direct voltage energy source 214 to the direct voltage energy storage 213. The power converter 201 comprises an inverter-bridge 202 that comprises first direct voltage poles 203a and 203b, alternating voltage poles 204a, 204b, and 204c, and controllable switches 21 1 a, 21 1 b, 21 1 c, 212a, 212b, 212c.

The power converter 201 comprises a network filter 205 for connecting the alternating voltage poles 204a, 204b, and 204c to the alternating voltage power grid 215. The power converter 201 comprises a controller 206 for controlling, in the first operating mode of the power converter, the inverter-bridge 202 to transfer energy from the first direct voltage poles 203a and 203b to the alternating voltage poles 204a, 204b, and 204c so as to transfer energy from the direct voltage energy storage 213 to the alternating voltage power grid 215. The power converter 201 further comprises second direct voltage poles 207a and 207b. The power converter 201 comprises a contactor 208 for arranging, in the second operating mode of the power converter, electric current paths from the direct voltage pole 207a to the alternating voltage pole 204c, and from the direct voltage pole 203b to the direct voltage pole 207b. In this exemplifying case, the electric current path from the direct voltage pole 207a to the alternating voltage pole 204c comprises an inductive component 209c. The inductive component 209c is one of inductive components of the network filter 205.

The controller 206 is configured to control, in the second operating mode, the controllable switch 21 1 c to be alternately conductive and non-conductive so as to alternately charge energy to the inductive component 209c from the direct voltage energy source 214 and discharge the energy from this inductive component to the direct voltage energy storage 213 via a diode of the controllable switch 212c. Therefore, one branch of the inverter-bridge 202 is used, in the second operating mode, as a voltage-increasing direct voltage converter, i.e. as a boost converter where VDCI > VDC2, for charging the direct voltage energy storage 213 from the direct voltage energy source 214.

The implementation of the controller 106 shown in figure 1 , as well as the implementation of the controller 206 shown in figure 2, can be based on one or more processor circuits, each of which can be a programmable processor circuit provided with appropriate software, a dedicated hardware processor such as for example an application specific integrated circuit "ASIC", or a configurable hardware processor such as for example a field programmable gate array "FPGA". Furthermore, the controller 106 as well as the controller 206 may comprise one or more memory devices such as e.g. random-access memory "RAM" circuits.

It is worth noting that the above-described principle to use the power converters 101 and 201 shown in figures 1 and 2 as voltage-increasing direct voltage converters, i.e. as boost-converters, is applicable also in cases where an inverter-bridge has a three-level topology or a M-level topology, M being greater than three. In these cases, there are two or more controllable switches between each alternating voltage pole and each direct voltage pole of the inverter-bridge. Also in conjunction with a three or more -level topology, the controllable switches can be operated in the above-described way so as to alternately charge energy to at least one inductive component and discharge the energy from the at least one inductive component so that a direct voltage energy storage is charged from a direct voltage energy source.

Figure 3a illustrates an electric power plant according to an exemplifying and non- limiting embodiment of the invention. The electric power plant comprises direct voltage energy sources 314a and 314b and a direct voltage energy storage 313. In this exemplifying case, the direct voltage energy sources 314a and 314b comprise photovoltaic panels and the direct voltage energy storage 313 comprise a battery system. The electric power plant comprises first power converters 316a and 316b for transferring electric energy from the direct voltage energy sources 314a and 314b to an alternating voltage power grid 315. In this exemplifying case, each of the power converters 316a and 316b comprises a serial inductance which is connected to a common capacitor-inductor "CL" element 317 so as to constitute a LCL-filter for each of the power converters 316a and 316b. The electric power plant comprises a second power converter 301 a according to an exemplifying and non-limiting embodiment of the invention. The power converter 301 a is capable of operating in a first operational mode where the power converter 301 a transfers energy from the direct voltage energy storage 313 to the alternating voltage power grid 315, and in a second operational mode where the power converter 301 a transfers energy from the direct voltage energy source 314b to the direct voltage energy storage 313. As illustrated in figure 3a, first direct voltage poles 303 of the power converter 301 a are connected to the direct voltage energy storage 313 and second direct voltage poles 307 of the second power converter are connected to the direct voltage energy source 314b.

Referring to figures 1 and 2, one can understand that the second power converter 301 a can operate as a voltage increasing direct "DC" voltage converter, i.e. as a boost DC-DC converter, for charging the direct voltage energy storage 313 from the direct voltage energy source 314b and/or 314a. Thus, the direct voltage energy storage 313 can be charged using the DC-DC conversion only when the DC-voltage of the direct voltage energy source 314b and/or 314a is lower than the DC-voltage of the direct voltage energy storage 313. In a case where the DC-voltage of the direct voltage energy source 314b and/or 314a is higher than the DC-voltage of the direct voltage energy storage 313, the DC-DC conversion is difficult to use because there would be an uncontrolled power flow through the diodes 1 12a-1 12c shown in figure 1 or through the diode 212c shown in figure 2. In these cases, the direct voltage energy storage 313 can be charged so that the DC-voltage of the direct voltage energy source 314b and/or 314a is first converted into alternating "AC" voltage with the power converter 316b and/or 316a and then the AC-voltage is converted to DC-voltage suitable for the direct voltage energy storage 313 with the power converter 301 a, i.e. there is a DC-AC-DC conversion. A situation where the DC-voltage of the direct voltage energy source 314b and/or 314a is higher than the DC-voltage of the direct voltage energy storage 313 may take place for example when output power of the direct voltage energy source 314b and/or 314a, e.g. photovoltaic panels, is limited by increasing their voltage.

Figure 3b illustrates an electric power plant according to an exemplifying and non- limiting embodiment of the invention. The electric power plant comprises first power converters 326a and 326b for transferring electric energy from the direct voltage energy sources 314a and 314b to the alternating voltage power grid 315. The electric power plant comprises a second power converter 301 b according to an exemplifying and non-limiting embodiment of the invention. In this exemplifying case, each of the power converters 326b, 326b and 301 b comprises a serial inductance which is connected to a common capacitor-inductor "CL" element 327 so as to constitute a LCL-filter for each of the power converters 326b, 326b and 301 b. The power converter 301 b is capable of operating in a first operational mode where the power converter 301 b transfers energy from the direct voltage energy storage 313 to the alternating voltage power grid 315, and in a second operational mode where the power converter 301 b transfers energy from the direct voltage energy source 314b to the direct voltage energy storage 313. First direct voltage poles 303 of the power converter 301 a are connected to the direct voltage energy storage 313, and second direct voltage poles 307 of the second power converter are connected to the direct voltage energy source 314b. The electric power plant further comprises a contactor 318 which the aid of which the direct voltage energy source 314b can be directly connected to the direct voltage energy storage 313. Figure 3c illustrates an electric power plant according to an exemplifying and non- limiting embodiment of the invention. The electric power plant comprises first power converters 336a and 336b for transferring electric energy from the direct voltage energy sources 314a and 314b to the alternating voltage power grid 315. The electric power plant comprises a second power converter 301 c according to an exemplifying and non-limiting embodiment of the invention. In this exemplifying case, each of the power converters 336b, 336b and 301 b comprises an own inductor-capacitor-inductor "LCL" filter for connecting to the alternating voltage power grid 315 via a transformer. The power converter 301 c is capable of operating in a first operational mode where the power converter 301 c transfers energy from the direct voltage energy storage 313 to the alternating voltage power grid 315, and in a second operational mode where the power converter 301 c transfers energy from the direct voltage energy source 314b to the direct voltage energy storage 313. First direct voltage poles 303 of the power converter 301 a are connected to the direct voltage energy storage 313, and second direct voltage poles 307 of the second power converter are connected to the direct voltage energy source 314b. The electric power plant further comprises a contactor 318 which the aid of which the direct voltage energy source 314b can be directly connected to the direct voltage energy storage 313.

The direct voltage energy storage 313 can be discharged with the aid of the power converter 301 c capable of transferring energy from the direct voltage energy storage 313 to the alternating voltage power grid 315. When the power of the direct voltage energy source 314b and/or 314a is zero or limited, the discharging of the direct voltage energy storage 313 can be done together with the power converter 336b and/or 336a by closing the contactor 318. Adding one or both of the power converters 336b and 336a for the discharging of the direct voltage energy storage 313 will increase the discharging power capacity. The contactor 318 also enables the power converter 301 c to be used for transferring energy from the the direct voltage energy source 314b and/or 314a to the alternating voltage power grid 315 when a contactor 308 is open and contactors 305 and 355 are closed. This will increase system versatility and redundancy.

Figure 4 is a flowchart of a method according to an exemplifying and non-limiting embodiment of the invention for controlling an electric power plant that comprises:

- at least one direct voltage energy source,

- at least one direct voltage energy storage,

- a first power converter between the direct voltage energy source and an alternating voltage power grid, and - a second power converter between the direct voltage energy storage and the alternating voltage power grid.

The method comprises, in a first operational mode of the electric power plant:

- action 401 : controlling the second power converter to transfer electric energy from the direct voltage energy storage to the alternating voltage power grid. The method comprises in a second operating mode of the electric power plant:

- action 402: controlling a contactor to arrange electric current paths from a first direct voltage pole of the direct voltage energy source to one or more alternating voltage poles of the second power converter and from a direct voltage pole of the second power converter to a second direct voltage pole of the direct voltage energy source so that at least one of the electric current paths comprises at least one inductive component, and - action 403: controlling each of one or more controllable switches between each of the one or more alternating voltage poles of the second power converter and the direct voltage pole of the second power converter to be alternately conductive and non-conductive so as to alternately charge energy to the at least one inductive component from the direct voltage energy source and discharge the energy from the at least one inductive component to the direct voltage energy storage.

The above-mentioned second operating mode is applicable in cases where the voltage of the direct voltage energy storage is higher than the voltage of the direct voltage energy source.

In a method according to an exemplifying and non-limiting embodiment of the invention, the above-mentioned electric current path from the first direct voltage pole of the direct voltage energy source is arranged to extend to two or more of the alternating voltage poles of the second power converter. In a method according to an exemplifying and non-limiting embodiment of the invention, the controllable switches are controlled in a phase-shifted way so as to reduce ripple of direct current supplied to the direct voltage energy storage in the second operating mode.

A computer program according to an exemplifying and non-limiting embodiment of the invention comprises computer executable instructions for controlling a programmable processing system to carry out actions related to a method according to any of the above-described exemplifying and non-limiting embodiments of the invention.

A computer program according to an exemplifying and non-limiting embodiment of the invention comprises software modules for controlling an electric power plant of the kind described above. The software modules comprise computer executable instructions for controlling a programmable processing system to:

- control, in a first operating mode of the electric power plant, a power converter to transfer electric energy from a direct voltage energy storage to an alternating voltage power grid, - control, in a second operating mode of the electric power plant, a contactor to arrange electric current paths from a first direct voltage pole of a direct voltage energy source to one or more alternating voltage poles of the power converter and from a direct voltage pole of the power converter to a second direct voltage pole of the direct voltage energy source so that at least one of the electric current paths comprises at least one inductive component, and

- control, in the second operating mode, each of one or more controllable switches between each of the one or more alternating voltage poles of the power converter and the direct voltage pole of the power converter to be alternately conductive and non-conductive so as to alternately charge energy to the at least one inductive component from the direct voltage energy source and discharge the energy from the at least one inductive component to the direct voltage energy storage.

The software modules can be for example subroutines or functions implemented with programming tools suitable for the programmable processing system.

A computer program product according to an exemplifying and non-limiting embodiment of the invention comprises a computer readable medium, e.g. a compact disc "CD", encoded with a computer program according to an exemplifying embodiment of invention. A signal according to an exemplifying and non-limiting embodiment of the invention is encoded to carry information defining a computer program according to an exemplifying embodiment of invention.

The specific examples provided in the description given above should not be construed as limiting the scope and/or the applicability of the appended claims. Lists and groups of examples provided in the description given above are not exhaustive unless otherwise explicitly stated.