UHVDC converter transformer re-usage for LCC to VSC upgrade

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.

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.