The invention discloses a control method of a three-level frequency converter, and belongs to a control method of a frequency converter. The control method comprises the steps that a. instant per-unit values of three-phase reference
voltage used for controlling output
voltage of the three-level frequency converter are determined, and the reference
voltage is corrected according to a requirement of the three-level frequency converter on midpoint level balance control or not and according to a midpoint level deviation value; b. three-phase
pulse sequence switching flags are calculated according to the instant per-unit values of the three-phase reference voltage respectively; c. the action time of three-phase virtual voltage space vectors is calculated; d. a three-level virtual voltage
space vector diagram is superimposed by the three-phase virtual voltage space vectors, and a finally output
pulse sequence is recombined and sorted according to the action time of the three-phase virtual voltage space vectors and by combining with the three-phase
pulse sequence switching flags; and e. a frequency converter control pulse is generated by hardware, the calculated action time is loaded into a hardware unit, and the hardware unit automatically generates the control pulse as required. The control method has the advantages that a PWM (pulse-width modulation) method of the voltage space vectors is simplified.