The smoothing transformer, a new concept in DC side harmonic reduction of HVDC schemes

Direct connection schemes have been a subject of recent investigation, offering operational flexibility and substantial reductions in AC components. In these schemes, the use of active DC filters has been suggested to replace the conventional tuned passive filter design. This paper presents the smoothing transformer as a new means for reducing DC harmonics at characteristic and noncharacteristic frequencies using only passive components. A realistic smoothing transformer design is examined using the New Zealand HVDC power system operating in the direct connection mode. The steady-state and transient performance of the smoothing transformer design is compared with that of the existing DC smoothing reactor and filter bank.