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. The method also includes transmitting the control signal from each of the at least one DC offset correcting device to a control device of the MMC. The method also includes, the control device mapping the control signal(s) from the at least one DC offset correcting device to the at least one phase leg. The method also includes, based on the mapping, the control device sending switching commands to the semiconductor switches of MMC cells in each of the at least one phase leg to compensate for the DC offset.