UHVDC converter transformer re-usage for LCC to VSC upgrade

1. A voltage source converter (VSC) system of a high-voltage direct current (HVDC) system, the VSC system comprising:

a plurality of line-commutated converter (LCC) transformers reused from an LCC HVDC system, each LCC transformer being operable to transform alternate current (AC) voltage, the LCC transformers including a plurality of tap changers reused from the LCC HVDC system and set such that the voltage output by each LCC transformer is lower than a maximum voltage that the LCC transformer is capable of outputting;

a plurality of VSC units coupled in series, each LCC transformer respectively coupled to a single converter unit of only the VSC type, each VSC unit operable to convert between the AC voltage and direct current (DC) voltage; and

a plurality of bypass breakers, each bypass breaker coupled in parallel with at least one of the VSC units and operable to be closed to bypass the at least one VSC unit.

2. The VSC system of claim 1 , wherein the plurality of VSC units include a plurality of six-pulse VSC units.

3. The VSC system of claim 1 , wherein the plurality of VSC units comprise a plurality of modular multi-level converter units.

4. The VSC system of claim 3 , wherein the plurality of VSC units comprise a plurality of full-bridge modular multi-level converter units.

5. The VSC system of claim 3 , wherein the plurality of VSC units comprise a plurality of half-bridge modular multi-level converter units, and wherein the VSC system of the HVDC system further comprises a plurality of diodes coupled to the plurality of VSC units, respectively.

6. The VSC system of claim 1 , wherein the plurality of LCC transformers comprises a plurality of three-phase LCC transformers.

7. The VSC system of claim 1 , wherein the plurality of LCC transformers comprises a plurality of two-winding LCC transformers.

8. The VSC system of claim 1 , wherein the plurality of LCC transformers comprises a plurality of three-winding LCC transformers.

9. The VSC system of claim 1 , wherein the VSC system is part of a rectifier station.

10. The VSC system of claim 1 , wherein the VSC system is part of an inverter station.

11. The VSC system of claim 1 , wherein the bypass breakers are reused from the LCC HVDC system.

12. A method of upgrading a line-commutated converter (LCC) high-voltage direct current (HVDC) system to a voltage source converter (VSC) HVDC system, the LCC HVDC system comprising a plurality of LCC transformers, a plurality of LCC units, and a plurality of bypass breakers, the plurality of LCC transformers include tap changers, the method comprising:

disconnecting the LCC units from the LCC transformers;

removing the LCC units;

replacing the removed LCC units with a plurality of VSC units;

coupling the VSC units in series with each other;

coupling the VSC units to the plurality of LCC transformers, so that the VSC HVDC system comprises the plurality of VSC units coupled in series, and each LCC transformer respectively coupled to a single converter unit of only the VSC type; and

adjusting the tap changers of the LCC transformers to lower an output voltage of the LCC transformers.

13. The method of claim 12 , further comprising connecting at least one of the VSC units to one of the bypass breakers of the LCC HVDC system.

14. The method of claim 12 , further comprising replacing the bypass breaker.

15. The method of claim 12 , wherein the plurality of VSC units comprise a plurality of half-bridge modular multi-level converter units, the method further comprising coupling a diode to at least one of the VSC units.

16. A method of upgrading a line-commutated converter (LCC) high-voltage direct current (HVDC) system to a voltage source converter (VSC) HVDC system, the LCC HVDC system comprising a first LCC converter unit coupled between a first node and a second node, a second LCC converter unit coupled between the second node and a third node, a third LCC converter unit coupled between the third node and a fourth node, and a fourth LCC converter unit coupled between the fourth node and a fifth node, a first LCC transformer coupled between an AC network and the first LCC converter unit, a second LCC transformer coupled between the AC network and the second LCC converter unit, a third LCC transformer coupled between the AC network and the third LCC converter unit, and a fourth LCC transformer coupled between the AC network and the fourth LCC converter unit, a first bypass breaker coupled between the first node and the third node, and a second bypass breaker coupled between the third node and the fifth node, the method comprising:

replacing the first LCC converter unit with a first VSC converter unit and coupling the first VSC converter unit to the first LCC transformer, the first node, and the second node;

replacing the second LCC converter unit with a second VSC converter unit and coupling the second VSC converter unit to the second LCC transformer, the second node, and the third node;

replacing the third LCC converter unit with a third VSC converter unit and coupling the third VSC converter unit to the third LCC transformer, the third node, and the fourth node;

replacing the fourth LCC converter unit with a fourth VSC converter unit and coupling the fourth VSC converter unit to the fourth LCC transformer, the fourth node, and the fifth node;

coupling a third bypass breaker between the first node and the second node;

coupling a fourth bypass breaker between the second node and the third node;

coupling a fifth bypass breaker between the third node and the fourth node;

coupling a sixth bypass breaker between the fourth node and the fifth node; and

adjusting tap changers of the first, second, third, and fourth LCC transformers to lower an output voltage of these LCC transformers.

17. The method of claim 16 , wherein one of the third, fourth, fifth or sixth bypass breakers is the first bypass breaker of the LCC HVDC system.

18. The method of claim 17 , wherein another one of the third, fourth, fifth or sixth bypass breakers is the second bypass breaker of the LCC HVDC system.

19. The method of claim 16 , wherein the first, second, third, and fourth VSC converter units comprise a plurality of half-bridge modular multi-level converter units, the method further comprising coupling a diode to at least one of the VSC converter units.