Sensitivity Check of an Internal VHF/UHF Sensor for Transformer Partial Discharge Measurements

The failure of a transformer can result in high costs due to repair (or replacement) and outage costs. Failures in a power transformer are mostly due to defects in the insulation. Therefore, transformers should be regularly monitored for insulation defects by PD measurements. The UHF technique (and in general on-line PD measurement techniques) could play an important part in insulation condition assessment, since it is not so cost demanding (especially compared to DGA), it can be performed on-line and can localize and identify the defect with minimum noise interference. However, the UHF technique requires dielectric windows fitted on the tank of the transformer, in order to place the sensors, and especially old transformers do not incorporate these on the tank, by design. Therefore another way of measuring in the UHF range should be implemented. In this paper a new internal sensor will be presented which can be installed through the oil-drain valve of the transformer and its sensitivity and application will be tested.