As part of our ongoing annual surveys at a customers data centre and following some issues with partial discharge on the HV cabling feeding the cast resin transformers on the packaged substations, we were asked carry out ultrasonic surveys of the transformers and cable termination chambers.

PROCESS
All 16 site cast resin transformers were surveyed using the airborne ultrasonic detector and their sound signatures were carefully listened to. There was no audible noise from any of the transformers and all but one of the transformers were free from any of the telltale ultrasonic signatures of discharge. The sound files from the suspected faulty transformer were recorded, analyzed and compared with known examples. From our nonintrusive surveys and analysis it was confirmed that the discharge heard and seen in the Spectralyser software was arcing between the the insulation of the HV cables from the bushings on the termination chamber to the HV connections on the resin cores.

REPORT
Due to the severity of the issue and risk of failure the site staff were notified immediately and we explained that the best course of action would be to isolate the transformer to allow a thorough visual inspection and investigation into the cause of the discharge. With the permission of the bank the site staff transferred the load to the alternative supply and removed the transformer from service. A detailed report of our findings and analysis of the ultrasonic recordings was issued.

REMEDIAL WORKS
With the transformer isolated a full inspection of the transformer was able to be completed. It was found that the HV cabling from the bushings in the transformer enclosure to the cast resin windings had been incorrectly installed. The cables were cleated and correctly supported but the spacing requirements had not been met allowing discharge between the HV phase conductors. The damaged cables were replaced during the following week and the transformer was tested and put back into service.

CONCLUSION
If the discharge had gone undetected the cable insulation would have failed resulting in severe damage to the transformer and ancillary equipment in the switch room. As a direct result of our visit a series of inspections were carried out on the remaining 15 transformers and several were found to have cables installed in the same manner. Although no other transformers had the same damage to the cables it was decided that the cables should be replaced in ALL the transformers on the site to prevent a reoccurrence of the fault found during our ultrasonic surveys. The transformers are now part of our annual inspections to allow ongoing safe and reliable service.