Method and device for damping voltage harmonics in a multilevel power converter

A method for reducing lower order harmonics of a multi-level power converter comprising at least one phase leg comprising a plurality of chain-link connected cells each comprising a capacitor, the method comprising, for each phase leg of the converter:

• obtaining, from a current control, a present reference voltage for use during a present half switching duration, a half switching duration being a time period corresponding to one switching opportunity of the cells;
• dividing the half switching duration into a plurality of time intervals;
• and
• at the beginning of each time interval:
• predicting the reference voltage waveform for the remainder of the half switching duration based on the obtained present reference voltage;
• predicting the leg output voltage waveform for the remainder of the present half switching duration for the case that switching for one cell is performed in the leg during the time interval;
• predicting a flux error, being a time integral of a difference between the leg output voltage and the reference voltage, at the end of the present half switching duration for the case, based on the obtained present reference voltage, the predicted reference voltage waveform and the predicted leg output voltage waveform, wherein the predicted flux error is a cumulative flux error of the present half switching duration and a preceding half switching duration;
• and
• determining not to perform switching for any cell in a present time interval of the plurality of time intervals when the predicted flux error passes zero during the first half switching duration.

A control unit for a phase leg of a multi-level power converter, the control unit comprising:

• processor circuitry;
• and
• a storage unit storing instructions executable by said processor circuitry whereby said control unit is operative to:
• obtain, from a current control, a present reference voltage for use during a present half switching duration, a half switching duration being a time period corresponding to one switching opportunity of the cells;
• divide the half switching duration into a plurality of time intervals;
• and
• at the beginning of each time interval:
• predict the reference voltage waveform for the remainder of the half switching duration based on the obtained present reference voltage;
• predict the leg output voltage waveform for the remainder of the present half switching duration for the case that switching for one cell is performed in the leg during the time interval;
• predict a flux error, being a time integral of a difference between the leg output voltage and the reference voltage, at the end of the present half switching duration for the case, based on the obtained present reference voltage, the predicted reference voltage waveform and the predicted leg output voltage waveform, wherein the predicted flux error is a cumulative flux error of the present half switching duration and a preceding half switching duration;
• and
• determine not to perform switching for any cell in a present time interval of the plurality of time intervals when the predicted flux error passes zero during the first half switching duration.

A computer program embodied on a non-transitory computer readable medium for reducing lower order harmonics of a multi-level power converter, the computer program comprising computer program code which is able to, when run on processor circuitry of a control unit for a phase leg of the power converter, cause the control unit to:

• obtain, from a current control, a present reference voltage for use during a present half switching duration, a half switching duration being a time period corresponding to one switching opportunity of the cells;
• divide the half switching duration into a plurality of time intervals;
• and
• at the beginning of each time interval:
• predict the reference voltage waveform for the remainder of the half switching duration based on the obtained present reference voltage;
• predict the leg output voltage waveform for the remainder of the present half switching duration for the case that switching for one cell is performed in the leg during the time interval;
• predict a flux error, being a time integral of a difference between the leg output voltage and the reference voltage, at the end of the present half switching duration for the case, based on the obtained present reference voltage, the predicted reference voltage waveform and the predicted leg output voltage waveform, wherein the predicted flux error is a cumulative flux error of the present half switching duration and a preceding half switching duration;
• and
• determine not to perform switching for any cell in a present time interval of the plurality of time intervals when the predicted flux error passes zero during the first half switching duration.