Special breaker properties are needed in order to transfer high currents from one current path to another, a process that is called commutation.

In the direct-current switchgear of the Shuanlong converter station, the circuit-breaker was integrated, which has enabled uninterrupted switchover from grounding electrode mode to metallic return transfer mode and vice versa with no requirement for the operator to cut down the transmission power of the system.

Till now, this only functioned with rated DC currents below 5000 A. However, Siemens has now successfully stretched this limit with its circuit-breaker in order to improve the operation, stability and reliability of this HVDC system in China.

Siemens Division Energy Management Transmission Solutions Business Unit CEO Tim Dawidowsky said: "We’ve successfully tested our DC commutation circuit-breaker at the Shuanlong converter station in operation at full load at 5000 A DC.

"We’ve thus proven the switching capability of this breaker in what is currently the world’s largest HVDC transmission system with no need for current flow reduction."

Since last July, Siemens HVDC system Xiluodu-Zhejiang has been transmitting 8GW of power at a DC transmission voltage of 800kV over a distance of 1,670km from the Xiluodu hydropower plant in Sichuan province to Zhejiang, a highly industrialized province of China.

The company claimed that this makes it the highest capacity HVDC link in the world currently.

The circuit-breaker was tested during the high-water season to ensure that the hydropower plant could deliver the required power.

Image: Siemens DC commutation breaker has been successfully tested. Photo: Courtesy of Siemens AG.