Converter with reactive power compensation

1. A voltage source converter for HVDC power transmission comprising:

at least one bridge rectifier having series connected diodes configured to interconnect, in use, a DC network on a DC side of the at least one bridge rectifier and an AC network on a AC side of at least one bridge rectifier; and

at least one auxiliary converter comprising a single-phase multilevel converter connected to the DC side of the at least one bridge rectifier, the at least one auxiliary converter configured to modify a DC voltage present on the DC side of the at least one bridge rectifier,

wherein the at least one auxiliary converter is controllable to synthesize a predetermined waveform to the DC side of the at least one bridge rectifier resulting in a sine wave, with minimal harmonic distortion, being generated on the AC side of the at least one bridge rectifier,

wherein the predetermined waveform is a waveform closely approximating an offset rectified sinusoid waveform on the DC side of the at least one bridge rectifier.

2. The voltage source converter according to claim 1 , wherein the at least one of the single-phase multilevel converter is configured as a chain-link converter.

3. The voltage source converter according to claim 2 , wherein the chain-link converter includes a chain of modules connected in series, each module including at least one pair of semiconductor switches connected in parallel with an energy storage device, and the semiconductor switches are configured to be controlled such that the chain of modules connected in series provides a continuously variable voltage source.

4. The voltage source converter according to claim 3 , wherein the chain-link converter includes a chain of modules connected in series, each module including a pair of semiconductor switches connected in parallel with an energy storage device in a half-bridge arrangement to define a 2-quadrant unipolar module that can develop zero or positive voltage.

5. The voltage source converter according to claim 3 , wherein the energy storage device comprises one of a battery, a fuel cell, and a capacitor.

6. The voltage source converter according to claim 3 , wherein each semiconductor switch comprises one of an insulated gate bipolar transistor, a gate turn-off thyristor, and an integrated gate-commutated thyristor.

7. The voltage source converter according to claim 3 , wherein the chain-link converter includes a chain of modules connected in series, each module including two pairs of semiconductor switches connected in parallel with an energy storage device in a full-bridge arrangement to define a 4-quadrant bipolar module that can develop positive or negative voltage.

8. The voltage source converter according to claim 7 , wherein the chain-link converter includes an energy source configured to bias the auxiliary converter.

9. The voltage source converter according to claim 8 , wherein the energy source comprises one of a battery, a fuel cell, and a charged capacitor.

10. The voltage source converter according to claim 2 , wherein the at least one bridge rectifier is connected to a primary winding of an AC transformer and the at least one auxiliary converter includes a pair of chain-link converters connected in parallel with the at least one bridge rectifier in a full-bridge arrangement with a secondary winding of the AC transformer.

11. The voltage source converter according to claim 10 , wherein the at least one auxiliary converter includes a switching arrangement that is controllable to switch the secondary winding of the AC transformer in and out of circuit as required.

12. The voltage source converter according to claim 11 , wherein the switching arrangement includes one or more mechanical and/or semiconductor switches.

13. The voltage source converter according to claim 2 , wherein the at least one bridge rectifier is connected to a primary winding of an AC transformer and the at least one auxiliary converter includes a chain-link converter connected in parallel with the at least one bridge rectifier in a half-bridge arrangement with a secondary winding of the AC transformer.

14. The voltage source converter according to claim 13 , wherein the at least one auxiliary converter includes a switching arrangement that is controllable to switch the secondary winding of the AC transformer in and out of circuit as required.

15. The voltage source converter according to claim 1 , wherein an auxiliary converter of the at least one auxiliary converter is connected in parallel with a bridge rectifier of the at least one bridge rectifier, the bridge rectifier and the auxiliary converter forming a single phase converter limb.

16. The voltage source converter according to claim 15 , wherein a plurality of single phase converter limbs are connected in series to define a two terminal DC network for multi-phase electrical power transmission.

17. The voltage source converter according to claim 15 , wherein a plurality of single phase converter limbs are connected in parallel to define a two terminal DC network for multi-phase electrical power transmission.

18. The voltage source converter according to claim 1 , wherein a plurality of single phase converter limbs are connected in series to define a two terminal DC network for multi-phase electrical power transmission.

19. The voltage source converter according to claim 1 , wherein a plurality of single phase converter limbs are connected in parallel to define a two terminal DC network for multi-phase electrical power transmission.

20. The voltage source converter according to claim 19 further including buffer reactors interconnected between each limb and a neighboring limb.