Current challenges and issues of designing HVDC converter transformers

For the insulation of high-power transformers the oil/board insulating system has proven itself for many decades. Owing to constant development in the field of high-performance semiconductor elements, high-voltage direct current (HVDC) power transmission becomes more and more important on account of major advantages involved in these systems. This may become problematic in particular in case of converter transformers as the insulation is not exposed exclusively to a mere AC stress. Depending on the topology of the converter, the transformer insulation is subject to a composed voltage stress (alternating voltage with direct component). This, however, results in serious effects and challenges respecting electric stress of the insulating components of oil and board. In case of mere alternating voltage stress capacitive field distribution takes place according to permittivities. Conductivities in the context of AC electric field are not important, but cause negligible dielectric losses. In case of direct voltage stress they are decisive as field distribution ensues in dependence of their values. Compared to AC stress, this leads to a reduction of stress on the oil but to an increase of stress on the insulating board. During switching processes or polarity reversal a transition from the dielectric displacement field to the steady-state electric DC-field takes place. Polarization mechanisms, in particular interfacial polarization, and considerably differing polarization durations between the materials of oil and board must be taken into consideration. Additionally, the conductivity of insulation materials is dependent on many parameters, such as temperature, humidity and aging. The resulting stress on insulating board in case of steady-state field stress necessitates detailed studies which focus on conductivity with regard to conductivity changes resulting from stresses occurring during operation [1], [2].