A method for controlling a converter, which comprises:
- a series connection of at least two main valves (2, 3) between two poles (4, 5), a positive and a negative one, of a direct voltage side of the converter, said main valves each comprising a controllable semiconductor device and a rectifying member connected in anti-parallel therewith, an alternating voltage phase line being connected to a midpoint (10), called phase output, of the series connection between two main valves while dividing the series connection into two equal parts,
- a series connection of at least two intermediate link capacitors (7, 8) between the two poles (4, 5) of the direct voltage side of the converter, which series connection through a midpoint (9), called intermediate link midpoint, is divided into two equal parts,
- a resonance circuit (16) comprising a series connection of an inductor (17) and an auxiliary valve (18) between the phase output (10) and the intermediate link midpoint (9), said auxiliary valve comprising at least two controllable semiconductor components (20) arranged in opposite polarity with respect to each other, in which the resonance circuit further comprises for each main valve at least one capacitive member (15), which is connected in series with said inductor (17) and auxiliary valve (18) and in parallel with the main valve (2, 3),
- means (31) for measuring the phase current (iph) in the phase output, and
- a control arrangement (24) for controlling turn on and turn off of the controllable semiconductor devices of the main valves and the controllable semiconductor components of the auxiliary valve, in which the control arrangement, when carrying out the commutation process, is adapted to deliver control signals to main valves and auxiliary valve participating in the commutation process for turning them on and off at different times in a determined order for changing the phase potential on the phase output from the potential of one of said poles to that of the other pole, and in which the control arrangement is adapted to, when a measured value of the phase current exceeds a predetermined value, when starting a said commutation process with the phase current flowing through a controllable semiconductor device of a main valve, carry out the commutation process without assistance of the resonance circuit,
- characterized in that, when carrying out two subsequent said commutation processes for changing the phase potential on the phase output from the potential of the first one of the poles to that of the second of the poles and then back to that of the first pole and when the time integral of the difference voltage by which the phase potential deviates from the potential of the first pole is desired to be made lower than obtainable by a method in which a conventional commutation is first done from one pole to the other and then without any delay a conventional commutation is made back to the first pole, the second commutation process is started at a time lying in a period before an end time for completing the first commutation process, and that the limits for the time interval between said start time and end time is made dependent upon the magnitude of the measured phase current.