Page 51 - Energize January 2022

TECHNICAL

In Figure 2, the column Δ%PF refers
to the difference between nameplate
and measured percentage PF values
corrected to 20°C. As done with the
overall PF testing performed on the
transformer, 1 Hz PF test results were
collected for the bushings as well
(Figure 3).

Figure 4: HV bushing removed from the transformer – surface contamination on its lower insulator

Not surprisingly, the 1 Hz PF test results confirmed the need to investigate all HV
bushings. The asset owner ultimately decided to replace the HV bushings. The transformer
oil was drained and the HV bushings were removed from the unit. Figure 4 shows a layer of
contamination on the bottom section of the bushings.
Figure 2: HV bushings LF PF values (DF or Based on these findings, the owner decided to filter the oil to ensure the removal of
tangent delta) – measured and corrected by ITC
all in-tank particulates. New bushings were installed and tested. Excellent LF (Figure 5)
and 1 Hz PF (Figure 6) values were obtained.

Test conditions were:
• Ambient temperature (T A) = 12°C
• Transformer top oil temperature (T OT) = 37°C
• Bushing temperature (T avg = (T A + T OT)/2) = 24,5°C

With the HV bushings replaced, the oil filtered, and under similar test conditions, the overall
LF PF test was carried out and results show clear improvement (Figure 7).
As with the LF PF values, 1 Hz PF test results did improve as shown in Figure 8, but
Figure 3: HV bushings 1 Hz PF (DF or tangent assessment indicates that the unit requires further investigation. Why would the 1 Hz PF
delta) – measured and corrected by ITC values still be high after HV bushings replacement and oil filtering?
To answer this question, the asset owner requested a definitive analysis of the
transformer’s insulation system, using the IDAX300 test instrument, to obtain a complete
dielectric frequency response (DFR).
Full-spectrum DFR shows that the interwinding solid insulation contains a typical
percentage moisture (1,7 %) for a service-aged transformer but the liquid insulation reports
high conductivity (11,8 pS/m).
Further internal inspection of the unit revealed that the tank walls and paper insulation
retained contamination in areas making it difficult to remove. It was recommended to flush the

Figure 5: New HV bushings LF PF (DF or tangent
delta)

Figure 6: New HV bushings 1 Hz PF (DF or Figure 7: Overall LF PF (DF or tangent delta) Figure 8: 1 Hz PF results after HV bushings
tangent delta) results after HV bushings replacement. replacement

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