Power converter

1. A power converter to convert DC power into AC power to be supplied to a load to supply active AC power to the load, the power converter comprising:

an inverter circuit connected between positive and negative terminals of a DC power supply;

three single-phase bridge circuits each connected in series between an AC terminal of one of different phases of the inverter circuit and the load; and

a power conversion controller to generate, based on phase voltage commands, gate signals to control operations of the inverter circuit and the three single-phase bridge circuits, wherein

when a semiconductor switching element of one of the single-phase bridge circuits fails, the power conversion controller sets an output voltage of the single-phase bridge circuit of a phase to which the faulty semiconductor switching element belongs to zero to continue operation to supply the active AC power,

when the phase to which the faulty semiconductor switching element belongs is referred to as a faulty phase,

the power conversion controller generates the gate signals to turn on two of the semiconductor switching elements on a negative side in both legs of the single-phase bridge circuit of the faulty phase when the semiconductor switching element on the negative side has a short-circuit fault,

the power conversion controller generates the gate signals to turn on two of the semiconductor switching elements on a positive side in both legs of the single-phase bridge circuit of the faulty phase when the semiconductor switching element on the positive side has a short-circuit fault,

the power conversion controller generates the gate signals to turn on two of the semiconductor switching elements on the negative side in both legs of the single-phase bridge circuit of the faulty phase when the semiconductor switching element on the positive side has an open fault, and

the power conversion controller generates the gate signals to turn on two of the semiconductor switching elements on the positive side in both legs of the single-phase bridge circuit of the faulty phase when the semiconductor switching element on the negative side has an open fault.

2. The power converter according to claim 1 , further comprising:

bypass switches connected in parallel to the corresponding single-phase bridge circuits of the respective phases, wherein

the power conversion controller controls to turn on one of the bypass switches connected to the single-phase bridge circuit of the faulty phase when the semiconductor switching element of the single-phase bridge circuit fails.

3. The power converter according to claim 2 , wherein

when the semiconductor switching element fails, the power conversion controller turns off all the gate signals to the single-phase bridge circuit of the faulty phase.

4. The power converter according to claim 2 , wherein

the power conversion controller controls to turn on one of the bypass switches, of the bypass switches, which is connected to the single-phase bridge circuit of the faulty phase, when two or more of the semiconductor switching elements of the single-phase bridge circuit fail.

5. The power converter according to claim 1 wherein

the inverter circuit uses semiconductor switching elements formed from a narrow bandgap semiconductor, and

the single-phase bridge circuits use semiconductor switching elements formed from a wide bandgap semiconductor.

6. The power converter according to claim 1 wherein

a switching frequency of the single-phase bridge circuits is higher than a switching frequency of the inverter circuit.

7. The power converter according to claim 1 wherein

a DC-side capacitor voltage of the single-phase bridge circuits is half a phase-voltage step width of the inverter circuit or less.

8. The power converter according to claim 1 wherein

the power conversion controller divides the phase voltage commands of sinusoidal voltages into first voltage commands to command the inverter circuit and second voltage commands to command the three corresponding single-phase bridge circuits, and

when the sum of the first voltage commands is a positive value, the power conversion controller superimposes a common voltage component on each of the three second voltage commands so that the sum of the three second voltage commands becomes a non-positive value, or

when the sum of the first voltage commands is a negative value, the power conversion controller superimposes a common voltage component on each of the three second voltage commands so that the sum of the three second voltage commands becomes a non-negative value.

9. The power converter according to claim 8 , wherein

the inverter circuit outputs fundamental wave components of the sinusoidal voltages.

10. The power converter according to claim 8 , wherein

the first voltage commands are a single pulse voltage whose absolute value is ½ of a voltage of the DC power supply and whose polarity is a positive or negative voltage each repeated once in a fundamental wave period of the phase voltage commands.

11. The power converter according to claim 10 , wherein

with respect to a phase and a phase angle α of the phase voltage commands, the single pulse voltage is a zero value when the phase is in ranges of zero to α, π−α to π+α, and 2π−α to 2π, is a positive value when the phase is in a range of α to π−α, and is a negative value when the phase is in a range of π+α to 2π−α, and

the phase angle α is determined by formula (1) below where ν php is an amplitude of the phase voltage commands, and ν mdc is a DC voltage of the inverter circuit

[Formula 1]

α=cos −1 (∥ php π/2∥ mdc )  (1).

12. The power converter according to claim 1 , wherein

the inverter circuit is a three-level inverter.