Page 48 - Energize September 2021 HR

TECHNICAL

Identifying ageing and installation

issues in an HV bushing

Information from Megger

This article deals with the testing of 69 kV bushings which are installed on a 10 MVA, 69/13,09 kV Dyn1
two-winding transformer. After completing on-load tap changer (OLTC) maintenance on this 1969-vintage
transformer, the utility owner conducted electrical tests.

Overall transformer line frequency
power factor (LF PF) tests
Losses measured for the low-to-ground
(CLG) and high-to-ground (CHG)
insulation systems were higher than those
measured for the interwinding, or high-
to-low (CHL), insulation system of the
transformer. The temperature-corrected
(i.e., 20°C equivalent) LF PF test results
for CLG and CHG insulation components
were within acceptable limits (< 0,5%)
and even within limits established for new Table 1: Overall transformer LF PF test results - initial condition
transformers. However, the CHG LF PF
test result (at 0,43 %) was approximately
1,8 times the CHL LF PF result (0,24%)
(Table 1).
Based on recent experiences using
Narrowband Dielectric Frequency
Response (NB DFR) testing and successes
in finding hidden issues not observed by LF
PF tests, a NB DFR test was carried out. A Table 2: C1 test results for the high-side bushings, including 10 kV LF PF and 250 V NB DFR results,
NB DFR test is the measurement of PF (%) measured and temperature-corrected via the ITC method
or DF (%) at different frequencies between
1 Hz and 505 Hz. to determine a test specimen’s 20°C equivalent LF PF value when the specimen is power-
In addition, the testing specialist factor tested at a non-20°C temperature.
performed 10 kV C1 LF PF tests on the An ITC factor is a TC factor which is unique to every test specimen as it is based on the
transformer’s high-side winding bushings specimen’s specific condition. Note that the ITC factor for bushing H3 (about 0,6) is different
(Table 2). The H3 bushing produced than the ITC factors determined for bushings H1 and H2 (about 1,04). The non-uniformity
elevated LF PF test results (measured and which exists in these ITC factors is a clear indication that the H3 bushing has a different
temperature-corrected) which were notably insulation condition than H1 and H2 bushings.
greater than results for the sister bushings
mounted on the same transformer. Transformer bushing C1 test investigation
Accordingly, the individual temperature ITC factors are far more accurate than TC factors accessed in a look-up table. To underscore
corrected LF PF test results for bushings the problem with temperature-correction look-up tables, consider the following:
H1 and H2 earned the bushings good (G) • If the end-user had relied on correction tables, the TC factor would have been determined
ratings while the individual temperature based upon the average of the ambient and bushing temperatures. This would have
corrected LF PF test result for bushing H3 resulted in a TC factor close to unity for all three bushings – valid for bushings H1 and H2
resulted in an ageing (A) assessment. but not for H3.
The test results also display an • Bushing H3 would have been removed in this scenario due to a >1% PF value. However,
interesting anomaly in the ‘individual had the bushing been tested at 10°C instead of at approximately 30°C, its measured and
temperature correction’ values. A temperature-corrected PF test results by look-up tables may have been low enough to
temperature-correction (TC) factor is used pass acceptance criteria as a normal, service-aged unit.

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