Page 49 - Energize September 2021 HR

TECHNICAL

Given the LF PF test results, the
utility performed NB DFR tests on the
high-side bushings. 1 Hz and 505 Hz
measurements from this testing are
provided in Table 2. These tabular results
affirm the good condition and rating of
bushings H1 and H2 but escalate the
assessment of the H3 bushing to an
‘investigate’ rating. NB DFR test results
for bushing H3 (the blue trace in Figure
1) revealed the following two worrisome
electrical characteristics so the utility
replaced the bushing:
• A bushing in good condition will
have an ITC temperature-corrected
PF value ≤1% at 1 Hz. The ITC
temperature-corrected 1 Hz PF test
Figure 1: NB DFR measured test results for the high-side 69 kV bushings
result of bushing H3 was 7,92 %
(Table 2).
• The 10 kV LF PF test result, 1,1%
(Table 2), and the LF PF test result
at 250 V, a typical NB DFR test
voltage, taken from Figure 1, 0,8%,
are different. This implies a voltage
Table 3: Pre-installation C1 test results for new H3 replacement bushing, 10 kV LF PF and 250 V NB DFR
dependence of the LF PF test results.

Before installing in the transformer, LF
PF and NB DFR testing was performed
on the H3 replacement bushing to
confirm its integrity. LF PF and NB DFR
test results indicated the bushing’s
good (G) condition (Table 3 and Figure
2). After replacing the H3 bushing, the
utility repeated the overall LF PF tests
on the transformer to observe what
effect the H3 bushing replacement had
on the overall HV winding insulation
test characteristics (Table 4). A notable
improvement was observed. Figure 2: Pre-installation NB DFR test results for H3 replacement bushing
With the problem corrected in the
overall winding insulation test results, the
utility staff then complemented their now
routine procedure with a 10 kV LF PF test
on the installed H3 replacement bushing
(Table 5).
The 10 kV measured LF PF test
result for the H3 replacement bushing
was acceptable. However, the ITC
temperature-corrected LF PF result is
about 1,5 times the measured value
while the 1 Hz measured and ITC
Table 4: 10 kV overall transformer LF PF test results after H3 bushing replacement
temperature-corrected results are nearly
the same; and the 505 Hz PF test result
is both non-typical and notably higher
than the 505 Hz PF test results for the
H1 and H2 bushings.
NB DFR tests for the installed H3
replacement bushing revealed a non- Table 5: Post-installation C1 test results for H3 replacement bushing, 10 kV LF PF and 250 V NB DFR

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