Patent ID: 9312786
Filing Date: 2016-04-12
CPC Classification: H02M,Y02B

Claim Text:
1. A method of controlling cells in a phase leg of a multilevel converter operating to convert between alternating current and direct current, the method being performed by a cell control unit of the converter and comprising regulating, using tap-changer control, a modulation index of the converter to above 0.80, providing 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, providing a voltage reference for the phase leg being separate from voltage references provided for other phase legs of the converter, stopping 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, controlling switching of the cells based on comparisons 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 obtaining 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.