Bidirectional DC-AC conversion circuit and a starting method thereof

1. A converter, comprising:

a bidirectional DC-AC converter circuit;

an input capacitor connected to an AC input of the bidirectional DC-AC converter circuit;

a bus capacitor connected to a DC output of the bidirectional DC-AC converter circuit;

a switch connected between a positive terminal of the input capacitor and an AC power supply;

a pre-charger circuit having an input connected to the AC power supply and an output connected to the DC output of the bidirectional DC-AC converter circuit; and

a controller configured to cause the pre-charger circuit to charge the bus capacitor while the switch is turned off and to cause the bidirectional DC-AC converter circuit to generate an inverting voltage from a voltage across the bus capacitor and apply the inverting voltage to the input capacitor before turning on the switch, wherein a phase of the inverting voltage is the same as a phase of a voltage of the AC power supply and wherein an amplitude of the inverting voltage is equal to an amplitude of the voltage of the AC power supply,

wherein the bus capacitor comprises a positive direct current bus capacitor and a negative direct current bus capacitor and a negative terminal of the positive direct current bus capacitor and a positive terminal of the negative direct current bus capacitor are connected to a neutral point;

wherein the bidirectional DC-AC converter circuit is a three-level converter and a DC output of the three-level converter is connected to a positive terminal of the positive direct current bus capacitor and a negative terminal of the negative direct current bus capacitor; and

wherein the controller is configured to control the pre-charger circuit such that an amplitude of a voltage across the positive direct current bus capacitor or negative direct current bus capacitor is equal to the amplitude of the AC power supply and to control the bidirectional DC-AC converter circuit such that the phase and amplitude of the inverting voltage are equal to the phase and amplitude of the AC power supply, respectively.

2. The converter according to claim 1 , wherein an amplitude of the voltage across the bus capacitor is not less than a peak-to-peak value of the voltage of the AC power supply.

3. The converter according to claim 1 , wherein the three-level converter is a T-type three-level converter comprising:

a first switching transistor and a fourth switching transistor connected in series between the positive terminal of the positive direct current bus capacitor and the negative terminal of the negative direct current bus capacitor;

a second switching transistor and a third switching transistor connected in series, the second switching transistor being connected to the neutral point; and

an inductor having a first terminal connected to a node connecting the first switching transistor, the third switching transistor, and the fourth switching transistor, and a second terminal used as an AC input of the T-type three-level converter;

wherein the controller is configured to turn on the second switching transistor and turn off the fourth switching transistor in a positive half cycle of the AC power supply to provide a first sinusoidal pulse width modulated signal to the first switching transistor and to provide a second sinusoidal pulse width modulated signal complementary to the first sinusoidal pulse width modulated signal to the third switching transistor and, in a negative half cycle of the AC power supply, to turn off the first switching transistor and turn on the third switching transistor to provide a third sinusoidal pulse width modulated signal to the fourth switching transistor and to provide a fourth sinusoidal pulse width modulated signal complementary to the third sinusoidal pulse width modulated signal to the second switching transistor.

4. The converter according to claim 1 , wherein the three-level converter is an I-type three-level converter comprising:

a first switching transistor, a second switching transistor, a third switching transistor, and a fourth switching transistor connected in series between the positive terminal of the positive direct current bus capacitor and the negative terminal of the negative direct current bus capacitor;

a first diode having a negative electrode connected to a node connecting the first switching transistor and the second switching transistor, and a positive electrode connected to the neutral point;

a second diode having a negative electrode connected to the neutral point and a positive electrode connected to a node connecting the third switching transistor and the fourth switching transistor; and

an inductor having a first terminal connected to a node connecting the second switching transistor and the third switching transistor and a second terminal used as an AC input of the I-type three-level converter;

wherein the controller is configured to turn on the second switching transistor and turn off the fourth switching transistor in a positive half cycle of the AC power supply to provide a first sinusoidal pulse width modulated signal to the first switching transistor and to provide a second sinusoidal pulse width modulated signal complementary to the first sinusoidal pulse width modulated signal to the third switching transistor and, in a negative half cycle of the AC power supply, to turn off the first switching transistor and turn on the third switching transistor to provide a third sinusoidal pulse width modulated signal to the fourth switching transistor and to provide a fourth sinusoidal pulse width modulated signal complementary to the third sinusoidal pulse width modulated signal to the second switching transistor.

5. A method of operating a converter including a bidirectional DC-AC converter circuit, an input capacitor connected to an AC input of the bidirectional DC-AC converter circuit, a bus capacitor connected to a DC output of the bidirectional DC-AC converter circuit, a switch connected between a positive terminal of the input capacitor and an AC power supply, and a pre-charger circuit having an input connected to the AC power supply and an output connected to the DC output of the bidirectional DC-AC converter circuit, the method comprising:

using the pre-charger circuit to charge the bus capacitor while the switch is turned off; and

using the bidirectional DC-AC converter circuit to generate an inverting voltage from a voltage across the bus capacitor and apply the inverting voltage to the input capacitor before turning on the switch, wherein a phase of the inverting voltage is the same as a phase of the AC power supply and wherein an amplitude of the inverting voltage is equal to the amplitude of the AC power supply,

wherein the bus capacitor comprises a positive direct current bus capacitor and a negative direct current bus capacitor and a negative terminal of the positive direct current bus capacitor and a positive terminal of the negative direct current bus capacitor are connected to a neutral point;

wherein the bidirectional DC-AC converter circuit is a three-level converter and a DC output of the three-level converter is connected to a positive terminal of the positive direct current bus capacitor and a negative terminal of the negative direct current bus capacitor;

wherein in step 1), the voltage across the positive direct current bus capacitor or the negative direct current bus capacitor is made equal to the amplitude of the AC power supply; and

wherein in step 2), the phase and amplitude of the inverting voltage are made equal to the phase and amplitude of the AC power supply, respectively.

6. The method according to claim 5 , wherein the voltage of the bus capacitor is not less than a peak-to-peak value of the voltage of the AC power supply.

7. The method according to claim 5 , wherein the three-level converter is a T-type three-level converter comprising:

a first switching transistor and a fourth switching transistor connected in series between the positive terminal of the positive direct current bus capacitor and the negative terminal of the negative direct current bus capacitor,

a second switching transistor and a third switching transistor connected in series, the second switching transistor being connected to the neutral point, and

an inductor having a first terminal connected to a node connecting the first switching transistor, the third switching transistor, and the fourth switching transistor and a second terminal used as an AC input of the T-type three-level converter;

wherein the step 2) comprises: in a positive half cycle of the AC power supply, turning on the second switching transistor and turning off the fourth switching transistor to provide a first sinusoidal pulse width modulated signal to the first switching transistor and to provide a second sinusoidal pulse width modulated signal complementary to the first sinusoidal pulse width modulated signal to the third switching transistor; in a negative half cycle of the AC power supply, turning off the first switching transistor and turning on the third switching transistor to provide a third sinusoidal pulse width modulated signal to the fourth switching transistor and to provide a fourth sinusoidal pulse width modulated signal complementary to the third sinusoidal pulse width modulated signal to the second switching transistor.

8. The method to claim 5 , wherein the three-level converter is an I-type three-level converter comprising:

a first switching transistor, a second switching transistor, a third switching transistor, and a fourth switching transistor connected in series between the positive terminal of the positive direct current bus capacitor and the negative terminal of the negative direct current bus capacitor;

a first diode having a negative electrode connected to a node connecting the first switching transistor and the second switching transistor and a positive electrode connected to the neutral point;

a second diode having a negative electrode connected to the neutral point and a positive electrode connected to a node connecting the third switching transistor and the fourth switching transistor; and

an inductor having a first terminal connected to a node connecting the second switching transistor and the third switching transistor and a second terminal used as the AC input of the I-type three-level converter;

wherein the step 2) comprises: in a positive half cycle of the AC power supply, turning on the second switching transistor and turning off the fourth switching transistor to provide a first sinusoidal pulse width modulated signal to the first switching transistor and to provide a second sinusoidal pulse width modulated signal complementary to the first sinusoidal pulse width modulated signal to the third switching transistor; in a negative half cycle of the AC power supply, turning off the first switching transistor and turning on the third switching transistor to provide a third sinusoidal pulse width modulated signal to the fourth switching transistor and to provide a fourth sinusoidal pulse width modulated signal complementary to the third sinusoidal pulse width modulated signal to the second switching transistor.