Method of controlling three-phase, three-level inverter

Claim 1

A method of controlling a three-phase, three-level inverter, in which a plurality of on/off modes are in turn switched in order to control an instantaneous space voltage vector determined by a combination of the electrical potentials of both terminals (P,N) and the neutral-point terminal (O) of the capacitor (3) of said three-phase, three-level inverter comprising a DC voltage source (1), a capacitor (3) both terminals (P,N) of which connected to said DC voltage source (1) and having a neutral-point terminal (O) positioned between both of these terminals, and a plurality of switch elements (4a-4b,4c-4d,5a-5b,5c-5d,6a-6b,6c-6d) connected to the section between both terminals (P,N) and the neutral-point terminal (O) of said capacitor (3) and three output terminals (Tu,Tv,Tw) of the inverter, comprising the steps of: judging whether the current of the output terminal (e.g.Tu) of an inverter of the phase such that the electrical potential of the output terminal equals to that of the neutral-point terminal (O) of said capacitor (3) among three-output terminals (Tu,Tv,Tw) of said inverter is in a positive direction in which current flows into a load connected to this output terminal (e.g.Tu) or in a negative direction in which current flows from the load when another on/off mode having the same instantaneous space voltage vector as the instantaneous space voltage vector in a tentatively selected on/off mode exists; actually selecting the tentatively selected on/off mode if the result of said judgment is positive and actually selecting another on/off mode instead of the tentatively selected on/off mode when the result of said judgment is negative in the case where the electrical potential of the neutral-point terminal (O) of said capacitor (3) is above a preset upper-limit value; and actually selecting the tentatively selected on/off mode if the result of said judgment is negative and actually selecting another on/off mode instead of the tentatively selected on/off mode when the result of said judgment is positive in the case where the electrical potential of the neutral-point terminal (O) of said capacitor (3) is below a preset lower-limit value.