Space charge has been a general concern related to the reliability and the lifetime of HVDC components. Recently, the researches on space charge characteristics and its impacts in solid insulating materials have attracted many attentions and been widely reported under DC electric fields. It has been found that the enhanced electric field due to space charge can accelerate ageing and lead to a permanent failure eventually. However, the insulation may withstand more complex stresses in converter transformers i.e. both AC voltage and DC voltage are applied on the second winding of converter formers. The space charge dynamics under this superimposed electric field is not well understood. In the presence paper, the space charge dynamics in the pressboard impregnated with different ageing statuses oil under pure AC voltage (50 Hz), pure DC voltage and AC/DC superimposed voltage have been investigated using the pulsed electroacoustic (PEA) technique with a fast data acquisition system. Space charge profiles were measured in insulation system consisted of either single layer oil impregnated pressboard or oil/oil impregnated pressboard. Two types of mineral oil (the fresh and the service aged) were used in this work. The results show that the amount of space charge under pure AC voltage is generally very small compared with DC voltage. However, when a small DC offset is superimposed with AC stress, homo charge injection (corresponding to the polarity of the DC offset) can be clearly observed. The charge amount and charge movement are evidently increased under the superimposed stress, particularly, in the fresh oil/pressboard insulation system. It has been found that the charge amount is generally larger under the superimposed stress than the sum of the pure AC and DC space charges, due to the non-linear space charge injection of the applied field. The impact of the superimposed AC/DC stress needs to be considered when designing, testing and operating converter transformers.
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