MVDC power transmission system for sub-sea loads

1. A system for the transmission of a direct current (DC) at a medium voltage level, the system comprising:

a system DC link, wherein the system DC link is configured to carry power from a source to a load module, the load module comprising:

a DC-to-DC voltage step-down converter,

a DC-to-AC inverter, the DC-to-AC inverter coupled downstream to the DC-to-DC voltage step-down converter; and

a system AC link for carrying power from the load module to a motor system on a load side of the system AC link.

2. The system of claim 1 , wherein the DC-to-DC voltage step-down converter comprises a bipolar voltage step-down converter.

3. The system of claim 1 , wherein the DC-to-DC voltage step-down converter comprises a unipolar voltage step-down converter.

4. The system of claim 3 , wherein the DC-to-AC converter is coupled to a transformer on a source side of the system AC link.

5. The system of claim 1 , further comprising a controller, the controller being configured to drive a voltage delivered from the DC-to-DC voltage step-down converter to a command value.

6. A system for the transmission and distribution of a direct current (DC) at a medium voltage level, the system comprising:

a system DC link, wherein the system DC link is configured to carry power from a source to a load module, the load module comprising:

a DC-to-DC voltage step-down converter, and

a DC distribution system on the load side of the DC-to-DC voltage step-down converter, the DC distribution system comprising:

at least two DC-to-AC power inverter modules coupled to the output of the step-down converter; and

at least two system AC links for carrying power from each DC-to-AC power inverter module to a motor system on a load side of the system AC link.

7. The system of claim 6 , wherein each DC-to-AC power inverter module comprises a voltage source inverter VSI and a DC-to-DC converter, the DC-to-DC converter being coupled between the DC link downstream the DC-to-DC voltage step-down converter and the VSI.

8. The system of claim 6 , wherein each DC-to-AC power inverter module comprises a current source inverter.

9. The system of claim 6 , wherein the DC-to-AC power inverter module is coupled to a transformer on a source side of the system AC link.

10. The system of claim 6 , further comprising controllers, the controller of the DC-to-DC voltage step-down converter being configured to drive a current delivered from the DC-to-DC voltage step-down converter to a command value.

11. A method of transmitting and distributing direct current (DC) power at a medium voltage level, the method comprising:

converting an on-shore alternating current (AC) source voltage to a medium voltage direct current (MVDC);

transmitting the MVDC to a load module via a sub-sea DC cable link;

converting the MVDC to a medium voltage alternating current (MVAC) via the load module; and

distributing the MVAC to a motor system via a sub-sea AC cable link.

12. The method according to claim 11 , further comprising driving a sub-sea pump via the MVAC motor system.

13. The method according to claim 11 , wherein converting the MVDC to a medium voltage alternating current (MVAC) via the load module comprises:

down converting the MVDC to a lower MVDC; and

converting the lower MVDC to the MVAC.

14. The method according to claim 11 , wherein converting the MVDC to a medium voltage alternating current (MVAC) via the load module comprises controlling a DC-to-DC step-down converter such that the step-down converter delivers a desired constant output voltage to at least one DC-to-AC inverter.

15. A system for the transmission of a direct current (DC) at a medium voltage level, the system comprising:

means for transmitting on-shore medium voltage DC power to an off-shore sub-sea load module, the load module comprising:

means for down-converting a medium voltage DC;

means for converting the down-converted medium voltage DC to a medium voltage AC; and

means for distributing medium voltage AC power from the load module to a sub-sea motor system.

16. The system according to claim 15 , wherein the means for down-converting a medium voltage DC comprises a unipolar or bipolar voltage step-down converter.

17. The system according to claim 15 , further comprising means for converting the medium voltage AC power to a desired voltage AC power.

18. The system according to claim 15 , further comprising means for controlling a voltage delivered from the down converting means to a command value.

19. The system according to claim 15 , wherein the means for converting the down-converted medium voltage DC to a medium voltage AC comprises a half bridge configured for controlling current delivered to the sub-sea motor system.

20. The system according to claim 15 , wherein the means for converting the down-converted medium voltage DC to a medium voltage AC comprises an inverter configured for controlling voltage delivered to the sub-sea motor system.