HVDC-transformers-a technical challenge

Design and manufacturing of HVDC-transformers and smoothing reactors for high DC-voltages are still very special devices and the number of manufacturers is limited. The paper analyses the components of HVDC-transformers and reactors in more detail and shows the influence of DC-operating conditions. The core looks very similar to conventional AC-transformers since it has to fulfil the same physical tasks. The designer has to recognise additional features like DC-currents which can oversaturate the core and probably overheat it and change completely the noise level and noise spectrum. The windings of HVDC-transformers look also very similar to AC-transformers. Additional losses by current harmonics are not so special for transformer designers. A speciality are valve windings. Very high test-voltages in combination with low number of turns can be a challenge for the designer. The insulation structure between windings and to ground as well as valve side leads and bushings are different from AC-transformers. Field plots for AC-, DC- and polarity reversal describe the technical problem in more detail. The process of quality control with the elements of material selection and control, component specification and testing of components needs more attention. Especially the process of adjustment of design of valve leads and valve bushings under the influence of various stresses and on site conditions is discussed in detail. The testing of HVDC-transformers is based on standards and customer specifications. The manufacturing process of HVDC-transformers differs partly from conventional transformers. Some examples are described in the paper.