METHOD, CONTROLLER AND COMPUTER PROGRAM FOR OPERATING AN INVERTER, AND INVERTER SYSTEM

Abstract

A method for operating an inverter configured to drive a generator is provided. The inverter comprises a direct current link (DC-link) having a DC-link capacitor, and the inverter is configured to convert a three-phase alternating current (AC) voltage generated by the generator into a DC-link voltage to be applied to the DC-link capacitor. The method includes receiving a DC-link voltage reference to be applied to the DC-link, receiving an actual DC-link voltage currently present over the DC-link, determining a stator parameter reference based on the received DC-link voltage reference and the received actual DC-link voltage, modifying the determined stator parameter reference by adding a dynamic stabilizing term to the stator parameter reference, generating a switching signal for the inverter based on the modified stator parameter reference, and operating the inverter by supplying the switching signal to the inverter.

Claims

1. A method for operating an inverter for driving a generator, the inverter comprising a direct current link (DC-link) having a DC-link capacitor and the inverter configured to convert a three-phase alternating current (AC) voltage generated by the generator into a DC-link voltage to be applied to the DC-link capacitor, the method comprising: receiving a DC-link voltage reference to be applied to the DC-link; receiving an actual DC-link voltage currently present over the DC-link; determining a stator parameter reference based on the received DC-link voltage reference and the received actual DC-link voltage; modifying the determined stator parameter reference by adding a dynamic stabilizing term to the stator parameter reference; generating a switching signal for the inverter based on the modified stator parameter reference; and operating the inverter by supplying the switching signal to the inverter.

2. The method according to claim 1, wherein: when determining the stator parameter reference, a d-axis stator parameter reference and a q-axis stator parameter reference are determined, and the dynamic stabilizing term is added to at least one term of the stator parameter reference.

3. The method according to claim 2, wherein: the dynamic stabilizing term is added to the d-axis stator parameter reference.

4. The method according to claim 1, wherein before adding the dynamic stabilizing term to the stator parameter reference, the method further comprises: determining the dynamic stabilizing term such that an electric energy needed to change an actual current generated by the generator and corresponding to actual stator currents or actual stator fluxes is minimized.

5. The method according to claim 4, wherein: when determining the dynamic stabilizing term such that the electric energy needed to change the actual current is minimized, the dynamic stabilizing term is determined such that a change of the electric energy stored in the actual stator currents or the actual stator fluxes corresponding to the actual current generated by the generator is minimized.

6. The method according to claim 5, wherein: the dynamic stabilizing term is determined such that $\frac{L}{2}\frac{{\mathrm{dI}}_{D,\mathrm{REF}}^2}{dt}+\frac{L}{2}\frac{{\mathrm{dI}}_{Q,\mathrm{REF}}^2}{dt}=0$ $\mathrm{and}/\mathrm{or}$ $\frac{1}{2L}\frac{d{}_{D,\mathrm{REF}}^2}{dt}+\frac{1}{2L}\frac{d{}_{Q,\mathrm{REF}}^2}{\mathrm{dt}}=0$ wherein L comprises a stator inductance of a stator of the generator.

7. The method according to claim 1, further comprising: before determining the stator parameter reference, determining a rotational speed of the generator; and determining the stator parameter reference based on the rotational speed of the generator.

8. The method according to claim 1, further comprising: receiving an actual stator current generated by the generator; determining a stator voltage reference corresponding to the three-phase AC voltage generated by the generator based on the actual stator current and the modified stator parameter reference; and generating the switching signal based on the modified stator parameter reference by generating the switching signal based on the determined stator voltage reference.

9. The method according to claim 8, after receiving the actual stator current and before determining the stator voltage reference, the method further comprises: determining an actual d-axis stator current and an actual q-axis stator current from the actual stator current; and determining the stator voltage reference based depending on the actual d-axis stator current, the actual q-axis current, and the modified stator parameter reference.

10. The method according to claim 1, wherein: the stator parameter reference comprises a stator current reference or a stator flux reference.

11. A controller configured to operate an inverter to drive a generator, the inverter comprising a direct current link (DC-link) having a DC-link capacitor, wherein the inverter is configured to convert a three-phase alternating current (AC) voltage generated by the generator into a DC-link voltage to be applied to the DC-link capacitor, the controller comprising: a memory configured to store one or more measured, determined, and/or predetermined current and/or voltage values; and at least one processor communicatively coupled to the memory, the at least one processor configured to: receive a DC-link voltage reference to be applied to the DC-link; receive an actual DC-link voltage currently present over the DC-link; determine a stator parameter reference based on the received DC-link voltage reference and the received actual DC-link voltage; modify the determined stator parameter reference by adding a dynamic stabilizing term to the stator parameter reference; generate a switching signal for the inverter based on the modified stator parameter reference; and operate the inverter by supplying the switching signal to the inverter.

12. An inverter system, comprising: the controller according to claim 11; and an inverter configured to drive a generator, the inverter including the DC-link having the DC-link capacitor, wherein the inverter is configured to convert the three-phase AC voltage generated by the generator into the DC-link voltage to be applied to the DC-link capacitor.

13. The controller of claim 11, wherein: the at least one processor is further configured to determine a d-axis stator parameter reference and a q-axis stator parameter reference when determining the stator parameter reference, and the at least one processor is further configured to add the dynamic stabilizing term to at least one of the stator parameter references.

14. The controller of claim 13, wherein the at least one processor is further configured to add the dynamic stabilizing term to the d-axis stator parameter reference.

15. The controller of claim 11, wherein the at least one processor is further configured, before adding the dynamic stabilizing term to the stator parameter reference, to: determine the dynamic stabilizing term such that an electric energy needed to change an actual current generated by the generator and corresponding to actual stator currents or actual stator fluxes is minimized.

16. The controller of claim 15, wherein the at least one processor is further configured, when determining the dynamic stabilizing term such that the electric energy needed to change the actual current is minimized to: determine the dynamic stabilization term such that a change of the electric energy stored in the actual stator currents or the actual stator fluxes corresponding to the actual current generated by the generator is minimized.

17. The controller of claim 16, wherein: the dynamic stabilizing term is determined such that $\frac{L}{2}\frac{{\mathrm{dI}}_{D,\mathrm{REF}}^2}{dt}+\frac{L}{2}\frac{{\mathrm{dI}}_{Q,\mathrm{REF}}^2}{dt}=0$ $\mathrm{and}/\mathrm{or}$ $\frac{1}{2L}\frac{d{}_{D,\mathrm{REF}}^2}{dt}+\frac{1}{2L}\frac{d{}_{Q,\mathrm{REF}}^2}{\mathrm{dt}}=0$ and L comprises a stator inductance of a stator of the generator.

18. The controller of claim 11, wherein the at least one processor is further configured, before determining the stator parameter reference, to: determine a rotational speed of the generator; and determine the stator parameter reference based on the rotational speed of the generator.

19. The controller of claim 11, wherein the at least one processor is further configured to: receive an actual stator current generated by the generator; determine a stator voltage reference corresponding to the three-phase AC voltage generated by the generator based on the actual stator current and the modified stator parameter reference; and generate the switching signal based on determined stator voltage reference.

20. A non-transitory computer-readable medium embodying programmed instructions which, when executed by at least one processor of a controller configured to operate an inverter to drive a generator, the inverter comprising a direct current link (DC-link) having a DC-link capacitor, wherein the inverter is configured to convert a three-phase alternating current (AC) voltage generated by the generator into a DC-link voltage to be applied to the DC-link capacitor, direct the at least one processor to: receive a DC-link voltage reference to be applied to the DC-link; receive an actual DC-link voltage currently present over the DC-link; determine a stator parameter reference based on the received DC-link voltage reference and the received actual DC-link voltage; modify the determined stator parameter reference by adding a dynamic stabilizing term to the stator parameter reference; generate a switching signal for the inverter based on the modified stator parameter reference; and operate the inverter by supplying the switching signal to the inverter.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0049] The subject matter of the present disclosure will be explained in more detail in the following text with reference to exemplary embodiments which are illustrated in the attached drawings.

[0050] FIG. 1 shows an example of an inverter for driving a generator.

[0051] FIG. 2 shows an exemplary embodiment of a controller for operating an inverter.

[0052] FIG. 3 shows an exemplary embodiment of a controller for operating an inverter.

[0053] The reference symbols used in the drawings, and their meanings, are listed in summary form in the list of reference symbols. In principle, identical parts are provided with the same reference symbols in the figures.

DETAILED DESCRIPTION

[0054] FIG. 1 shows an inverter 20, in particular an electrical inverter. The inverter 20 is electrically coupled to a generator 22. The inverter 20 is configured for driving the generator 22. The generator 22 is configured for generating a three-phase AC voltage.

[0055] The inverter 20 comprises a DC-link 24 having a DC-link capacitor 26, a first DC-terminal 28, a second DC-terminal 30, a set of semiconductor switches 32, e.g. six semiconductor switches 32, and an AC-terminal 34. The inverter 20 is a two-level voltage source inverter. However, in an alternative embodiment, a three-level voltage source inverter may be used, for example.

[0056] The AC-terminal 34 is coupled to the generator 22. The AC-terminal 34 is configured for receiving the three-phase AC voltage generated by the generator 22. The semiconductor switches 32 are configured for converting the three-phase AC voltage into a DC voltage, in particular a DC-link voltage, applied to the DC-link capacitor 26. To this end, the semiconductor switches 32, in particular gates of the semiconductor switches 32, are coupled to a controller 40 (see FIG. 2) for controlling the inverter 20.

[0057] The three-phase AC voltage consists of a phase u voltage U.sub.u, a phase v voltage U.sub.v, and a phase w voltage U.sub.w applied to the AC-terminal 34 by the generator 22. As a result, the generator 22 feeds a three-phase current into the AC-terminal 34. The three-phase current consists of a phase u current I.sub.u, a phase v current I.sub.v, and a phase w current I.sub.w.

[0058] The three-phase AC voltage is converted by the semiconductor switches 32 into an actual DC-link voltage U.sub.C resulting over the DC-link capacitor 26. This enables to feed a corresponding DC current I.sub.DC to the external device.

[0059] The DC-terminals 28, 30 are configured for being connected to a DC device (not shown) or to a DC grid (not shown). The first DC-terminal 28 may be coupled to a positive potential of the DC device and the second DC-terminal 30 may be coupled to a negative potential of the DC device. The DC device may be external with respect to the inverter 20. The DC device may be a rechargeable battery or a DC grid. The inverter 20 and the generator 22 may be arranged in a ship. In this case, the DC grid may be a DC grid of the ship. Alternatively, the inverter 20 and the generator 22 may be arranged in an electric vehicle, such as a train or subway. Alternatively, the generator 22 may be a wind turbine.

[0060] The inverter 20 may comprise one or more components to measure a current space vector, e.g., depending on the phase u current I.sub.u, the phase v current I.sub.v, and the phase w current I.sub.w or by using actual states of the switches 32 and the DC-link current I.sub.DC, and/or to measure the actual DC-link voltage U.sub.C. These component(s) may be one or more current and/or, respectively, voltage sensors (not shown).

[0061] FIG. 2 shows an exemplary embodiment of the controller 40 for operating an inverter, e.g., the inverter 20 of FIG. 1. The controller 40 may comprise a DC voltage controller 42, a current controller 44, a first transformation block 46, a second transformation block 48, and a modulator 50. In addition, the controller 40 comprises a memory (not shown) for storing one or more measured, determined, and/or predetermined current and/or voltage values, and a processor (not shown) communicatively coupled to the memory and being configured to carry out a method for operating the inverter 20, as described in the following.

[0062] A DC-link voltage reference U.sub.C,REF to be applied to the DC-link 24 may be received by the controller 40, in particular by the DC voltage controller 42. The DC-link voltage reference U.sub.C,REF may be predetermined and/or may be generated by the external device. The external device may be external with respect to the controller 40 and with respect to the inverter 20.

[0063] In addition, the actual DC-link voltage U.sub.C currently being present over the DC-link 26 may be received by the controller 40, in particular by the DC voltage controller 42. The actual DC-link voltage U.sub.C may be measured, e.g., by a voltage sensor (not shown) of the inverter 20.

[0064] A stator parameter reference, e.g., a stator current reference I.sub.D,REF, I.sub.Q,REF corresponding to a stator current to be generated by the generator 22 may be determined depending on the received DC-link voltage reference U.sub.C,REF and on the received actual DC-link voltage U.sub.C, e.g., by the DC voltage controller 42. When determining the stator current reference I.sub.D,REF, I.sub.Q,REF, a d-axis stator current reference I.sub.D,REF and a q-axis stator current reference I.sub.Q,REF may be determined. The DC voltage controller 42 may have a control law for regulating a DC voltage level at the DC-link 24.

[0065] Optionally, before determining the stator current reference I.sub.D,REF, I.sub.Q,REF, a rotational speed of the generator 22 may be determined. In this case, the stator current reference I.sub.D,REF, I.sub.Q,REF may be determined depending on the rotational speed of the generator 22 also. The rotational speed may be measured, e.g., by an encoder (not shown), and may be sent to the controller 40. Alternatively, the rotational speed may be estimated from an output voltage U.sub.u, U.sub.v, U.sub.w, and from an output current I.sub.u, I.sub.v, I.sub.w, of the inverter by the controller 40 or by a flux observer (not shown) known in the art and used to obtain the rotational speed .

[0066] Before sending the determined stator current reference I.sub.D,REF, I.sub.Q,REF to the current controller 44, the determined stator current reference I.sub.D,REF, I.sub.Q,REF may be modified by adding a dynamic stabilizing term to the stator current reference I.sub.D,REF, I.sub.Q,REF. The dynamic stabilizing term may be added to at least one of the stator current references I.sub.D,REF, I.sub.Q,REF. For example, the dynamic stabilizing term may be added to the d-axis stator current reference I.sub.D,REF. The current controller 44 may have a control law for regulating the inverter current to a value defined by the determined stator current reference I.sub.D,REF, I.sub.Q,REF coming from the DC voltage controller 42.

[0067] The dynamic stabilizing term may be determined such that the variation in the energy stored in the magnetic field of the generator 22 and corresponding to actual stator currents I.sub.D, I.sub.Q is minimized. In particular, in order to allow for high proportional gain at high load the stator current reference I.sub.D,REF, I.sub.Q,REF, e.g., the d-axis stator current reference I.sub.D,REF, may be chosen such that the energy needed to change actual stator current components I.sub.D, I.sub.Q is minimized:

[00020]$\begin{array}{cc}\frac{L}{2}\frac{{\mathrm{dI}}_{D,\mathrm{REF}}^2}{dt}+\frac{L}{2}\frac{{\mathrm{dI}}_{Q,\mathrm{REF}}^2}{dt}=0& \left(24\right)\end{array}$

[0068] with L being a stator inductance of a stator of the generator 22.

[0069] Assuming a slowly changing integral term condition 24 can be satisfied, for example, with

[00021]$\begin{array}{cc}{I}_{D,REF}=-\sqrt{I_0^2-\frac{K_P}{}\left(U_{C,REF}^2-U_C^2\right)\left(\frac{K_P}{}\left(U_{C,REF}^2-U_C^2\right)+2\frac{K_I}{}\left(U_{C,REF}^2-U_C^2\right)dt\right)}& \left(25\right)\end{array}$

[0070] where I.sub.0 is the d-axis stator current reference in steady state. However, any other dynamic stabilizing term fulfilling the requirement of equation 24 may be used.

[0071] For the following analysis the integral part of equation 25 is replaced by its steady state value

[00022]$\begin{array}{cc}{I}_{D,REF}=-\sqrt{I_0^2-\frac{K_P}{}\left(U_{C,REF}^2-U_C^2\right)\left(\frac{K_P}{}\left(U_{C,REF}^2-U_C^2\right)+2\frac{P_{LOAD}}{}\right)}& \left(26\right)\end{array}$

[0072] Assuming that the current controller 44 is much faster than the DC-link voltage controller 42, in other words that the current control is much faster than DC-link voltage control, it is reasonable to assume that the stator current components I.sub.D, I.sub.Q follow their corresponding references:

[00023]$\begin{array}{cc}{I}_D=I_{D,REF}& \left(27\right)\end{array}$$\begin{array}{cc}\frac{L}{2}\frac{{\mathrm{dI}}_D^2}{dt}=\frac{L}{2}\frac{d\left(I_0^2-\frac{K_P}{}\left(U_{C,REF}^2-U_C^2\right)\left(\frac{K_P}{}\left(U_{C,REF}^2-U_C^2\right)+2\frac{P_{LOAD}}{}\right)\right)}{dt}& \left(28a\right)\end{array}$$\begin{array}{cc}\frac{L}{2}\frac{{\mathrm{dI}}_D^2}{dt}=L\frac{K_P}{}\left(\frac{K_P}{}\left(U_{C,REF}^2-U_C^2\right)+\frac{P_{LOAD}}{}\right)\frac{d{U}_C^2}{dt}& \left(28b\right)\end{array}$

[0073] By inserting equation 16b and equation 28b into equation 9 and by simplifying a relation between the power of the inverter and the square of the DC-link voltage it is found that

[00024]$\begin{array}{cc}{P}_{\mathrm{INVENTER}}=-K_P\left(U_{C,REF}^2-U_C^2\right)-P_{\mathrm{LOAD}}& \left(29\right)\end{array}$

[0074] By inserting equation 29 into equation 4 and by simplifying a relation between the power of the DC-link and the square of the DC-link voltage it is found that

[00025]$\begin{array}{cc}{P}_C=K_P\left(U_{C,REF}^2-U_C^2\right)& \left(30\right)\end{array}$

[0075] By inserting equation 30 into equation 3 and by simplifying an equation describing the dynamics of the DC-link voltage control it is found that

[00026]$\begin{array}{cc}\frac{d{U}_C^2}{dt}=\frac{-2K_P}{C}\left(U_C^2-U_{C,REF}^2\right)& \left(31\right)\end{array}$

[0076] From equation 31 it is evident that stability is maintained even if the proportional gain is set to higher values than suggested by equation 23. Therefore, higher performance for the DC-link voltage control is possible. It should be noted that similar results can be achieved with alternative forms of equation 25 for the d-axis stator current reference I.sub.D,REF. Therefore, the present disclosure is not restricted to this form of the equation but to the principle of adding the dynamic stabilizing term to the corresponding current reference in order to allow for higher proportional gain.

[0077] It has to be noted that the steady state d-axis current reference I.sub.0 mentioned above may be chosen such that equation 25 gives a negative real value rather than an imaginary value. Once equation 25 goes imaginary the d-axis current reference could be set to zero or some other constant value and the proportional gain K.sub.P could be reduced to avoid instability. The steady state d-axis current reference I.sub.0 may be chosen such that the dynamic performance requirements for the corresponding application are fulfilled at every operation point. For example, at full load the steady state d-axis current reference I.sub.0 may be reduced since load cannot increase anymore.

[0078] An actual stator current I generated by the generator 22 may be received by the controller 40, in particular by the current controller 44. The actual stator current I may be measured by a current sensor (not shown) or may be estimated by the controller 40. The current sensor may be a component of the generator 22. Then, a stator voltage reference U.sub.D,REF, U.sub.Q,REF corresponding to the three-phase AC voltage U.sub.u, U.sub.v, U.sub.w to be generated by the generator 22 may be determined depending on the actual stator current I and on the modified stator current reference I.sub.D,REF, I.sub.Q,REF, e.g. by the current controller 44.

[0079] After receiving the actual stator current I and before determining the stator voltage reference U.sub.D,REF, U.sub.Q,REF an actual d-axis stator current I.sub.D and an actual q-axis current I.sub.Q may be determined from the actual stator current I. In this case, the stator voltage reference U.sub.D,REF, U.sub.Q,REF may be determined depending on the actual d-axis stator current I.sub.D, the actual q-axis current I.sub.Q, and on the modified stator current reference I.sub.D,REF, I.sub.Q,REF. The actual d-axis stator current I.sub.D and the actual q-axis stator current I.sub.Q may be determined from the actual stator current I by a space vector transformation, for example being carried out by the first transformation block 46. In general, a space vector transformation may be used to transform signals from three-phase signals into space vector rotating in synchronous coordinates by utilizing Park and Clarke transformations.

[0080] The stator voltage reference U.sub.D,REF, U.sub.Q,REF may comprise two stator voltage reference components, e.g., a d-axis voltage reference component U.sub.D,REF and a q-axis voltage reference component U.sub.Q,REF. The d-axis voltage reference component U.sub.D,REF and the q-axis voltage reference component U.sub.Q,REF in the dq-frame may be transferred into three voltage reference components in the uvw-frame, i.e., an u-phase voltage reference component U.sub.u,REF, a v-phase voltage reference component U.sub.v,REF and a w-phase voltage reference component U.sub.w,REF, e.g., by an inverse space vector transformation which may be carried out by the second transformation block 48. When transferring the stator voltage references U.sub.D,REF, U.sub.Q,REF in the dq-frame into the three-phase voltage reference components U.sub.u,REF, U.sub.v,REF, U.sub.w,REF in the uvw-frame, an actual angle of the stator may be considered, in particular within the first and/or second transformation block 46, 48. The actual angle may be measured by an appropriate sensor, e.g., a hall-sensor, or may be estimated, e.g., by the controller 40.

[0081] Then, a switching signal SWS for the inverter 40 may be generated depending on the modified stator current reference I.sub.D,REF, I.sub.Q,REF, e.g., by the modulator 50. The switching signal SWS may be determined depending on the modified stator current reference I.sub.D,REF, I.sub.Q,REF by generating the switching signal depending on the determined stator voltage reference U.sub.D,REF, U.sub.Q,REF, e.g., by the three voltage reference components U.sub.u,REF, U.sub.v,REF, U.sub.w,REF in the uvw-frame. The switching signal SWS may comprise switching commands for the semiconductor switches 32. When generating the switching signal SWS, timing limitations of the semiconductor switches 32, such as an interlocking time and/or a minimal on/off time of the semiconductor switches 32, may be considered. The switching signal SWS, in particular the switching commands, then may be translated into gate voltages for controlling the semiconductor switches 32, as it is known in the art. So, the modulator 50 translates the voltage reference components U.sub.u,REF, U.sub.v,REF, U.sub.w,REF into gate driver signals in a semiconductor bridge comprising the semiconductor switches 32, which will then realize the corresponding three-phase AC voltage on average over a switching cycle of the inverter 20.

[0082] Finally, the inverter 20 may be operated by the controller 40 by supplying the switching signal SWS to the inverter 20, in particular to the gates of the semiconductor switches 32.

[0083] FIG. 3 shows an exemplary embodiment of a controller 40 for operating an inverter, e.g. for operating the inverter 20 of FIG. 1. The controller 40 shown in FIG. 3 may widely correspond to the controller 40 described with respect to FIG. 2. Therefore, in order to provide a concise description and to avoid any unnecessary repetitions, only those features of the controller 40 of FIG. 3 are described in the following in which the controller of FIG. 3 differs from the controller of FIG. 2.

[0084] The controller 40 may comprise a flux controller 54 instead of the current controller 44. In this case, the controller 40 may comprise a flux calculator 52.

[0085] Another stator parameter reference, e.g., a stator flux reference .sub.D,REF, .sub.Q,REF corresponding to a stator flux to be generated by the generator 22 may be determined depending on the received DC-link voltage reference U.sub.C,REF and on the received actual DC-link voltage U.sub.C, e.g., by the DC voltage controller 42. When determining the stator flux reference .sub.D,REF, .sub.Q,REF, a d-axis stator flux reference .sub.D,REF and a q-axis stator flux reference .sub.Q,REF may be determined. When the rotational speed is determined, the stator flux reference .sub.D,REF, .sub.Q,REF may be determined depending on the rotational speed also.

[0086] Before sending the determined stator flux reference .sub.D,REF, .sub.Q,REF to the flux controller 54, the determined stator flux reference .sub.D,REF, .sub.Q,REF may be modified by adding a dynamic stabilizing term to the stator flux reference .sub.D,REF, .sub.Q,REF. The dynamic stabilizing term may be added to at least one of the stator flux references .sub.D,REF, .sub.Q,REF. For example, the dynamic stabilizing term may be added to the d-axis stator flux reference .sub.D,REF. The flux controller 54 may have a control law for regulating the inverter current to a value defined by the determined stator flux reference .sub.D,REF, .sub.Q,REF coming from the DC voltage controller 42.

[0087] The dynamic stabilizing term may be determined such that the variation in the energy stored in the magnetic field of the generator 22 and corresponding to actual stator currents I.sub.D, I.sub.Q or to actual stator fluxes .sub.D, .sub.Q is minimized, wherein the actual stator fluxes .sub.D, .sub.Q may be determined from the actual stator currents I.sub.D, I.sub.Q by the flux calculator 52, as it is known in the art. In particular, in order to allow for high proportional gain at high load the stator flux reference .sub.D,REF, .sub.Q,REF, e.g., the d-axis stator flux reference .sub.D,REF, may be chosen such that the energy needed to change actual stator flux components .sub.D, .sub.Q is minimized:

[00027]$\frac{1}{2L}\frac{d{I}_{D,\mathrm{REF}}^2}{\mathrm{dt}}+\frac{1}{2L}\frac{d{}_{Q,\mathrm{REF}}^2}{dt}=0$

[0088] with L being the stator inductance of the stator of the generator 22.

[0089] An inverter system comprises the inverter 20 for driving the generator 22 and the controller 40 for operating the inverter 20.

[0090] A computer program for operating the inverter 20 for driving the generator 22 may be provided. The computer program may comprise computer-readable instructions which, when being executed by the processor of the controller 40, carry out the method as described above. The computer program may be stored on a computer-readable medium. The computer-readable medium may be a floppy disk, a hard disk, an USB (Universal Serial Bus) storage device, a RAM (Random Access Memory), a ROM (Read Only Memory), an EPROM (Erasable Programmable Read Only Memory) or a FLASH memory. The computer readable medium may also be a data communication network, e.g. the Internet, which allows downloading a program code. In general, the computer-readable medium may be a non-transitory or transitory medium.

[0091] While the present disclosure has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive; the present disclosure is not limited to the disclosed embodiments. Other variations to the disclosed embodiments can be understood and effected by those skilled in the art and practicing the claimed subject matter, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word comprising does not exclude other elements or steps, and the indefinite article a or an does not exclude a plurality. A single processor or controller or other unit may fulfil the functions of several items recited in the claims. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. Any reference signs in the claims should not be construed as limiting the scope.