DC transformer bushing replacement program in MH HVDC converter stations - implementation of RIP silicone rubber bushing design

摘要

Since 1970's Manitoba Hydro (MH) operates The Nelson River HVDC transmission lines accommodating two bipolar links: Bipole I (450 kV_(DC)) and Bipole II (500 kV_(DC)). The transformers' reliability is safeguarded through maintaining the fleet of spare units; however the risk of operating these units with some 40 year old DC bushings was a reason for serious concern, as these bushings with their insulation are most stressed components in the transformers. During the evaluation of the available bushing information, it was discovered that there was no useable design information for the bushings and insulation structure. In 2011, due to change of ownership of the bushing supplier and unavailability of special low-resistivity porcelain, the OIP bushings for DC applications were no longer offered and RIP bushings became the only obtainable option. MH realized that the relatively new second generation transformers would be left without replacement OIP bushing, exposing the 26 converter transformers to an increased risk and reduced asset life. The DC RIP bushing replacement program for 300, 450 and 500 kV_(DC) operating voltage was therefore prepared which subsequently had to overcome numerous problems: (i) the units were built by different manufacturers, hence their active participation was required, (ii) the old type bushings were no longer available and new bushings needed to be mechanically and electrically fitted in the old units, i.e. the electric stress distribution of new bushings had to meet design limits. Moreover, the bushing corona shield electrodes had to fit in the pressboard barrier systems of different transformer suppliers. In addition, the new bushing development was to be incorporated into all future transformers so that there was only one bushing design for each DC voltage class. The paper describes the design aspects of the DC bushing replacement, including differences in dielectric performance of different insulation materials and insulating systems employed in this program, as well as logistics of the replacement program. The bushing supplier and insulation supplier worked with three transformer manufacturers to ensure that the newly developed RIP bushings will work with old and new transformer insulation systems and meet new MH specification requirements. First 500kVdc voltage class bushings were already successfully tested at the bushing manufacturer factory and are awaiting installation in the field.