Cells control in a multilevel converter

A control device for a multilevel converter operating to convert between alternating current and direct current, the converter being connected to a transformer controllable via a tap changer and comprising three phase legs with cells, the control device comprising

• a central control element configured to
• regulate, using tap-changer control, a modulation index of the converter to above 0.80, and
• provide a voltage reference for the phase leg being separate from voltage references provided for other phase legs,
• a carrier providing block providing a group of carrier waves for a phase leg, where the carrier waves of the phase leg are identical but displaced in time from each other with a time delay,
• a group of cell control blocks, where each cell control block is configured to compare a corresponding carrier wave with the voltage reference and control a corresponding cell based on the comparison,
• a switching control block configured to
• stop the cell control blocks from switching the cells when waveform values of a power transmission converter waveform are detected to be in a selected interval around a peak value of the power transmission converter waveform, and
• allow the cell control blocks to control switching of the cells when the waveform values of the power transmission converter waveform are detected to be outside said selected interval, and
• a reference voltage adjusting block configured to
• obtain measurements of a phase arm current,
• filter out harmonic components of the phase arm current and
• adjust the voltage reference based on the filtered harmonic components.

A computer program product for controlling cells in a phase leg of a multilevel converter operating to convert between alternating current and direct current,

• said computer program product being provided as a data carrier comprising computer program code configured to cause a control device to, when said computer program code is loaded into the control device
• regulate, using tap-changer control, a modulation index of the converter to above 0.80,
• provide a group of carrier waves for the phase leg, where the carrier waves of a phase leg are identical but displaced in time from each other with a time delay,
• provide a voltage reference for the phase leg being separate from voltage references provided for other phase legs of the converter,
• stop switching of the cells when waveform values of a power transmission converter waveform are detected to be in a selected interval around a peak value of the power transmission converter waveform,
• control switching of the cells based on comparison of the respective carrier waves with the voltage reference when the waveform values of the power transmission converter waveform are detected to be outside said selected interval, and
• obtain measurements of a phase arm current, filtering out harmonic components of the phase arm current and adjusting the voltage reference based on the filtered harmonic components.