Seven-level inverter apparatus

1. In a seven-level inverter apparatus comprising:

a three-phase three-level inverter which converts a DC of a first DC power source of three levels into an AC;

three single-phase five-level inverters, each of which converts a DC of a second power source of three levels having the same voltage as the first DC power source into an AC, an output thereof being connected in series with an output of each phase of the three-phase three-level inverter; and

pulse width modulation control means provided for each phase which performs pulse width modulation control of a give voltage reference to supply gate pulses to switching devices composing the relevant phase of the three-level inverter and the single-phase five-level inverter of the relevant phase;

the seven-level inverter apparatus is characterized in that:

the pulse width modulation control means has

means which converts the voltage reference into a pulse width modulated voltage level of the relevant phase, and

state transition means which determines an output of a switching leg of the relevant phase of the three-level inverter, and outputs of outside and inside switching legs of the single-phase five-level inverter, based on transition of this voltage level; and

the state transition means

makes all of the three outputs of the switching leg of the relevant phase of the three-level inverter, and the outside and inside switching legs of the single-phase five-level inverter 0 or positive, when the voltage level is positive,

makes all of the three outputs of the switching leg of the relevant phase of the three-level inverter, and the outside and inside switching legs of the single-phase five-level inverter 0 or negative, when the voltage level is negative, and

makes all of the three outputs of the switching leg of the relevant phase of the three-level inverter, and the outside and inside switching legs of the single-phase five-level inverter 0, when the voltage level is 0.

2. The seven-level inverter apparatus as recited in claim 1 , characterized in that:

the state transition means,

when a voltage difference between the first DC power source and the second DC power source is not more than a prescribed threshold value, and there are a plurality of options of combination of the three outputs of the switching leg of the relevant phase of the three-level inverter, and the outside and inside switching legs of the single-phase five-level inverter;

selects a combination to perform switching of the switching leg in which switching has not been performed for the past longest time.

3. The seven-level inverter apparatus as recited in claim 1 , characterized in that:

the state transition means,

when a voltage difference between the first DC power source and the second DC power source exceeds a prescribed threshold value, and there are a plurality of options of combination of the three outputs of the switching leg of the relevant phase of the three-level inverter, and the outside and inside switching legs of the single-phase five-level inverter;

selects a combination in which the voltage difference becomes small by switching.

4. The seven-level inverter apparatus as recited in claim 3 , characterized in that:

the state transition means,

when there are a plurality of combinations in which the voltage difference becomes small by switching, selects a combination which discharges a charge of the DC power source with a larger voltage, when a current of the relevant phase is positive, and selects a combination which charges a charge to the DC power source with a smaller voltage, when the current of the relevant phase is negative.

5. The seven-level inverter apparatus as recited in claim 1 , characterized in that a load is an AC motor.