Operating an inverter for coupling an electric machine, designed for operation on AC voltage, with an AC voltage network

1. A method for operating an inverter which couples an electric machine designed for operation on AC voltage to an AC voltage network, wherein the inverter is coupled to a network-side converter connected to the AC voltage network by means of a DC intermediate circuit,

the method comprising controlling the network-side converter with a network control signal and controlling conversion of electric energy by the machine-side converter with a machine control signal from a control unit:

determining a torque-related machine current component from electric machine currents of the electric machine,

detecting an actual value for a magnetic flux,

determining an actual value of a machine torque of the electric machine by linking the detected magnetic flux to the torque-related machine current component,

determining a torque difference between the actual value of the machine torque and a setpoint value of the machine torque of the electric machine,

filtering the torque difference with a bandpass filter tuned to a machine-side natural frequency to determine a filter output signal,

processing the filter output signal with a Park transformation to determine a d-component and a q-component,

determining mean values for the determined d-component and q-component,

regulating the mean values to zero with a PI controller,

determining a compensation signal with an inverse Park transformation, and

superimposing the compensation signal determined with the inverse Park transformation on the network control signal or the machine control signal, or both.

2. The method of claim 1 , further comprising determining frequency ranges for interharmonics from a network frequency and a machine frequency, in particular a machine speed.

3. The method of claim 2 , wherein the method is carried out only for the determined interharmonics.

4. The method of claim 1 , wherein a separate PI controller is provided for the d-component and the q-component.

5. The method of claim 1 , wherein the Park transformation and the inverse Park transformation are performed with a vector rotator using a reference angle.

6. The method of 5 , wherein the reference angle is determined by taking into account at least one of a network angle, a network frequency, a machine frequency and a flux angle of the magnetic flux.

7. A computer program product residing on a non-transitory computer readable medium and comprising a program having program code sections, wherein the program code sections, when read into a memory of a processor of a control unit and executed by the processor of the control unit, causes the control unit to:

control the network-side converter with a network control signal and controlling conversion of electric energy by the machine-side converter with a machine control signal,

determine a torque-related machine current component from electric machine currents of the electric machine,

detect an actual value for a magnetic flux,

determine an actual value of a machine torque of the electric machine by linking the detected magnetic flux to the torque-related machine current component,

determine a torque difference between the actual value of the machine torque and a setpoint value of the machine torque of the electric machine,

filter the torque difference with a bandpass filter tuned to a machine-side natural frequency to determine a filter output signal,

process the filter output signal with a Park transformation to determine a d-component and a q-component,

determine mean values for the determined d-component and q-component,

regulate the mean values to zero with a PI controller,

determine a compensation signal with an inverse Park transformation, and

superimpose the compensation signal determined with the inverse Park transformation on the network control signal or the machine control signal, or both.

8. A control unit for operating an inverter coupling an electric machine operating on AC voltage to an AC voltage network, wherein the inverter comprises a network-side converter connected to the AC voltage network and a machine-side converter connected to the electric machine and electrically coupled to the network-side converter by way of a DC intermediate circuit,

the control unit comprising a network-side connector configured for connection to the network-side converter and providing a network control signal to control the network-side converter, and a machine-side connector configured for connection to the machine-side converter and providing a machine control signal to control the machine-side converter,

the control unit further configured to:

determine a torque-related machine current component from electric machine currents of the electric machine,

detect an actual value for a magnetic flux,

determine an actual value of a machine torque of the electric machine by linking the detected magnetic flux to the torque-related machine current component,

determine a torque difference between the actual value of the machine torque and a setpoint value of the machine torque of the electric machine,

filter the torque difference with a bandpass filter tuned to a machine-side natural frequency to determine a filter output signal,

process the filter output signal with a Park transformation to determine a d-component and a q-component,

determine mean values for the determined d-component and q-component,

regulate the mean values to zero with a PI controller,

determine a compensation signal with an inverse Park transformation, and

superimpose the compensation signal determined with the inverse Park transformation on the network control signal or the machine control signal, or both.