A voltage source converter has a half bridge (18) with two current valves (19, 20) connected in series and an arrangement configured to carry out voltage measurements for determining a value of the DC voltage between opposite poles (21, 22) of a DC side of the converter. Each current valve comprises a semiconductor device (23, 24) controlled by an associated gate drive member (29, 30), each forming gate drive parts of one gate drive unit (28) in common to both current valves. The gate drive unit (28) comprises an isolated two-way communication link (33) between the gate drive members. The arrangement is included in the gate drive unit and configured to measure the entire DC voltage between said opposite poles (21, 22). A converter control device (31) calculates and sends control signals to the gate drive unit based on the result of the voltage measurement.