Insulation diagnosis based on partial discharge signals from power apparatus is widely accepted as a condition bases maintenance strategy in the smart power grid operation. Field applications show that the amplitude of partial discharge may or may not increase with the severity. Lab investigation was carried out to study the behavior of two major kinds of partial discharges in transformer, i.e., the surface discharge and the internal discharge. The lab tests identify the difference in the amplitude distribution of above two discharges. This characteristic can be used to infer the discharge model in field measurement. The recorded partial discharge signals show that the amplitude of the surface discharge increases with the severity of defect, but the internal discharge shows a different pattern.
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