Abstract: In a method of compensating for a DC offset of a high-voltage AC output from a Modular Multilevel Converter (MMC) including at least one phase leg, the MMC is connected to a three-phase high-voltage AC grid via a grid transformer. The method includes, in at least one DC offset correcting device, measuring the DC offset by in each of the at least one DC offset correcting device: obtaining a high-voltage AC signal in the MMC, removing high-voltage AC components from the obtained high-voltage AC signal by means of a passive higher-order filter to obtained an analogue filtered signal, converting the analogue filtered signal to a digital signal by means of an analogue-to-digital converter, removing remaining AC components from the digital signal by means of a digital filter to obtain the DC offset, and in a controller comparing the obtained offset with a reference value and forming a control signal based on said comparing.

Abstract: In a method of compensating for a DC offset of a high-voltage AC output from a Modular Multilevel Converter (MMC) including at least one phase leg, the MMC is connected to a three-phase high-voltage AC grid via a grid transformer. The method includes, in at least one DC offset correcting device, measuring the DC offset by in each of the at least one DC offset correcting device: obtaining a high-voltage AC signal in the MMC, removing high-voltage AC components from the obtained high-voltage AC signal by means of a passive higher-order filter to obtained an analogue filtered signal, converting the analogue filtered signal to a digital signal by means of an analogue-to-digital converter, removing remaining AC components from the digital signal by means of a digital filter to obtain the DC offset, and in a controller comparing the obtained offset with a reference value and forming a control signal based on said comparing.