Bidirectional power valve and control method therefor and hybrid multi-terminal HVDC system using the same

1. A bidirectional power valve for current occurring in a high voltage DC conductor, the bidirectional power valve comprising:

a first power diode arrangement of a first conducting direction;

a second power diode arrangement of a second conducting direction that is opposite the first conducting direction relative to a common connection of the first and second power diode arrangements;

a mechanical disconnector connected in series with the second power diode arrangement, wherein the second power diode arrangement and the mechanical disconnector are connected in parallel with the first power diode arrangement; and

a power valve controller configured to control opening of the mechanical disconnector in response to a commutation of the high voltage DC conductor current direction from the second conducting direction to the first conducting direction and to control closing of the mechanical disconnector in response to a commutation of the high voltage DC conductor current direction from the first conducting direction to the second conducting direction.

2. The bidirectional power valve according to claim 1 , wherein the first power diode arrangement comprises a plurality of first power diode units connected in series.

3. The bidirectional power valve according to claim 2 , wherein the second power diode arrangement comprises a second power diode unit.

4. The bidirectional power valve according to claim 3 , wherein a peak inverse voltage of the second power diode unit is substantially equal to or greater than the sum of forward-biased voltages of the plurality of first power diode units and less than a predetermined multiple of the sum.

5. The bidirectional power valve according to claim 1 , further comprising a current sensor configured to sense a direction of the high voltage DC conductor current and to send a signal representing it to the power valve controller.

6. The bidirectional power valve according to claim 5 , wherein the mechanical disconnector is controlled to be opened where the signal from the current sensor indicates the high voltage DC conductor current direction is lined with the first conducting direction of the first power diode arrangement.

7. A hybrid multi-terminal high voltage direct current system, comprising:

a high voltage DC conductor;

a bidirectional power valve comprising a first power diode arrangement of a first conducting direction, a second power diode arrangement of a second conducting direction that is opposite the first conducting direction relative to a common connection of the first and second power diode arrangements, and a mechanical disconnector connected in series with the second power diode arrangement in series, wherein the series-connected second power diode arrangement and the mechanical disconnector are connected in parallel with the first power diode arrangement;

a power valve controller configured to control opening of the mechanical disconnector in response to a commutation of the high voltage DC conductor current direction from the second conducting direction to the first conducting direction and to control closing of the mechanical disconnector in response to a commutation of the high voltage DC conductor current direction from the first conducting direction to the second conducting direction;

a LCC-based rectifier;

a first VSC-based inverter; and

a second VSC-based inverter;

wherein the first VSC-based inverter is connected to the LCC-based rectifier or the second VSC-based inverter via the high voltage DC conductor and the bidirectional power valve; and

wherein the first conducting direction of the first power diode arrangement of the bidirectional power valve points towards the first VSC-based inverter.

8. The system according to claim 7 , further comprising a system controller configured to control blocking/deblocking of the LCC-based rectifier and to switch the first VSC-based inverter to operate as a rectifier.

9. A direct current path commutation method, comprising:

forward biasing a first power diode arrangement of a first conducting direction while a voltage across the first power diode arrangement is connected across a series-connected opened mechanical disconnector and a second power diode arrangement of a second conducting direction, wherein the first power diode arrangement is arranged in parallel with the series-connected opened mechanical disconnector and second power diode arrangement;

closing the mechanical disconnector;

forward biasing the second power diode arrangement, wherein the first conducting direction of the first power diode arrangement and the second conducting direction of the second power diode arrangement are opposite to each other; and

closing the mechanical disconnector and forward biasing the second power diode arrangement after forward biasing the first power diode arrangement.

10. The method according to claim 9 , wherein, after the mechanical disconnector is closed and the second power diode arrangement is forward biased, the method further comprising:

forward biasing the first power diode arrangement; and

opening the mechanical disconnector.

11. The method according to claim 10 , wherein the mechanical disconnector is opened after the first power diode arrangement is closed.

12. A method of operating a hybrid multi-terminal high voltage direct current system, the method comprising:

forward biasing a first power diode arrangement of a first conducting direction while a voltage across the first power diode arrangement is connected across a series-connected opened mechanical disconnector and second power diode arrangement of a second conducting direction, wherein the first power diode arrangement is arranged in parallel with the series-connected opened mechanical disconnector and second power diode arrangement;

supplying power from a LCC-based rectifier to a first VSC-based inverter through the first power diode arrangement;

closing the mechanical disconnector;

blocking the LCC-based rectifier and switching the first VSC-based inverter to operate as a rectifier; and

forward biasing the second power diode arrangement and supplying power from the first VSC-based inverter operating as the rectifier to a second VSC-based inverter via the closed mechanical disconnector and the second power diode arrangement, wherein the first conducting direction of the first power diode arrangement and the second conducting direction of the second power diode arrangement are opposite to each other.

13. The method according to claim 12 , wherein, after forward biasing the second power diode arrangement and supplying power from the first VSC-based inverter to a second VSC-based inverter, the method further comprises:

de-blocking the LCC-based rectifier and switching the first VSC-based inverter to operate as an inverter;

forward biasing the first power diode arrangement; and

opening the mechanical disconnector.

14. The method according to claim 13 , wherein the mechanical disconnector is opened after the first power diode arrangement is closed.