Page 50 - Energize January 2022

TECHNICAL

Misleading line frequency insulation

results on a contaminated transformer

Information from Megger

1978 vintage, 20 MVA, 69/13,09 kV, Traditional analytic guidelines suggest that these temperature-corrected LF PF test
Dyn1 transformer, operating for a results are indicative of an insulation in acceptable condition. Modern learning, however,
A major North American utility, was cautions that LF PF test results alone no longer provide a sufficient screen.
taken out of service to repair a pump in the Accordingly, as part of the power factor test, 1 Hz PF was also measured and corrected
cooling system. The technical maintenance to 20°C equivalent values (Figure 1). The 1 Hz PF test results are much greater than
team completed the required repairs and those recommended for service-aged transformers, as flagged by the software with an
conducted a complete protocol of routine Investigation (I) rating.
commissioning tests, including a 10 kV line
frequency (LF) power factor (PF) test. The C1 insulation systems of the HV bushings were also tested.
The conditions during testing were:
The investigation • Ambient temperature (T A) = 15°C
The insulation condition of the • Transformer top oil temperature (T OT) = 30°C
transformer was evaluated in the • Bushing temperature (T avg = (T A + T OT)/2) = 22,5°C
field after pump repairs. The overall • All HV bushings’ line frequency power factor (LF PF) test results were higher than desired,
capacitance and line frequency power with power factor values double or nearly triple the nameplate values.
factor (LF PF) test at 10 kV was performed
using Megger’s DELTA4000 power factor These test results warranted investigation.
(dissipation factor) test set.
The LF PF test results obtained at An additional diagnostic clue that something is amiss with these bushings is their
30°C top oil temperature (T OT), and temperature sensitivity. The typical variation of LF PF between 20 and 22,5°C should be close
then properly corrected to their 20°C to zero. For example, bushing H2 measures 0,72% PF at 22,5°C. At a mere 2,5°C difference,
equivalent values using the individual the expectation is that bushing H2 would also measure approximately 0,72% PF at 20°C.
temperature correction (ITC method) However, ITC reveals very different dielectric behaviour (Figure 2) of these bushings, i.e., an
1
are presented in Figure 1. extreme temperature sensitivity.

Figure 1: Transformer overall LF and 1 Hz PF test (PowerDB screen shot)

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