A large utlity was experiencing high failure rates of 100 kV HVCTs. Periodic offline tests were being performed as well oil samples were being sent to the laboratory dissolved gas analysis. This is both me consuming and labor intensive.

Since this utility had a large population of these HVCTs, they wanted to consider alternative online technologies to assist in determining the health of their assets. Three sets of HVCTs were chosen for evaluation and the results are presented below.

Sensors were installed in series with the C1 bushing capacitance to ground.

    

SET 1
Results of Offline Tests show significant gassing as well as high power factor.
The PD levels also correlate with the DGA analysis.

The Polar diagram also shows changes in the power factor of A and B Phases, which also correlates with the DGA and par al discharge measurement.

SET 2
Indicates significant correlation between offline tests, DGA and partial discharge results.

 
SET 3
Offline tests show no gassing and there is really no partial discharge. Signals shown are external noise. The offline power factor tests show quite high, but long term monitoring of the power factor and capacitance was not possible at this location.