The insulation structure must withstand all normal and abnormal conditions when ac voltage is mixed with dc voltage of differing polarities over the operating temperature range. The presence of dc currents may also cause dc saturation of the core, leading to abnormal magnetizing currents and variations in sound.
A phase of a three-phase converter transformer bank typically comprises a high-voltage primary winding and two secondary ac/dc valve windings. Three such transformers together form the two secondary three-phase systems; one is connected in delta and the other in star. Each secondary system feeds a six-pulse bridge and the two bridges are connected in series to form a 12-pulse arrangement, as shown schematically in Fig. (1) Two such transformer banks are used with the secondary circuits connected in opposite polarity to form a ±215 kV dc transmission system.
Schematic diagram of ac/dc transmission system
Dry type converter transformer
Core and coil assembly of a converter transformer Siemens
Siemens Converter transformer
± 500 HVDC Converter Transformer TBEA