Method and apparatus for controlling power unit of frequency changer

The invention discloses a method used for controlling power units in a frequency converter and a device thereof; wherein, the method comprises the steps as follows: quantity of each phase of power unit in the frequency converter where default occurs is gained; the maximum synthesis voltage vector value output by the frequency converter can be worked out by the level number of the frequency converter; set of corresponding voltage vectors is determined under a prearranged angle coordinate system; amplitude limit of the output synthesis voltage vectors is carried out according to the maximum synthesis voltage vectors; corresponding voltage vector is selected in the set of the vector according to the synthesis voltage vector value or the voltage vector value after amplitude limit; sequence corresponding to the switch state is gained according to the selected voltage vector; a middle value is selected out of the switch state sequence, alternatively, the switch state which is closest to themiddle value is sequentia