The world’s first transformer, which Siemens developed and manufactured for high-voltage direct-current (HVDC) transmission systems with what is currently the highest transmission voltage of ± 1,100 kilovolts (kV). The workflows for manufacturing and testing this product were challenging due to the transformer’s extreme dimensions (LxWxH: 37.5 m x 12.0 m x 14.5 m) and shipped to China where the transformer will enable a high-voltage direct-current (HVDC) transmission with low losses over a record distance of 3,284 kilometers with a transmission capacity of 12 gigawatts.

An HVDC converter converts electric power from high voltage alternating current (AC) to high-voltage direct current (HVDC), or vice versa. HVDC is used as an alternative to AC for transmitting electrical energy over long distances or between AC power systems of different frequencies. HVDC converters capable of converting up to two gigawatts (GW) and with voltage ratings of up to 1,100 kilovolts (kV) have been built, and even higher ratings are technically feasible. A complete converter station may contain several such converters in series and/or parallel.

Almost all HVDC converters are inherently bi-directional; they can convert either from AC to DC (rectification) or from DC to AC (inversion). A complete HVDC system always includes at least one converter operating as a rectifier (converting AC to DC) and at least one operating as an inverter (converting DC to AC). Some HVDC systems take full advantage of this bi-directional property (for example, those designed for cross-border power trading, such as the Cross-Channel link between England and France). Others, for example those designed to export power from a remote power station such as the Itaipu scheme in Brazil, may be optimised for power flow in only one preferred direction. In such schemes, power flow in the non-preferred direction may have a reduced capacity or poorer efficiency.