Page 35 - Energize June 2021

TECHNICAL

While replacing CVT-1, the testing specialist noticed a stain Lessons learned
on the surface of the adjacent phase CVT (CVT-2, Figure 6). • A CVT in good condition should have a very low PF value (at
CVT-2 was reporting an expected voltage in SCADA but given the 20°C) <0,2% at 1 Hz (Figure 5).
failure of CVT-1 and convenient access to the testing equipment, • The utility performs monthly visual inspections but did not notice
capacitance and PF tests and NB DFR tests were performed on the oil leak between C1 sections of CVT-2. Testing averted its
CVT-2 too (Figures 7 to 10). These tests revealed insulation failure likely failure.
in CVT-2. • NB DFR testing will be incorporated in the utility’s future routine
A disassembly of CVT-2 confirmed significant damage to its maintenance of CVTs.
C1 – B1 section (Figure 11). • The Megger Delta4000 and TDX120 provide both basic (capacitance
A puncture was found in the C1-B1 section, and oil was and PF) and advanced (NB DFR) diagnostic insulation testing.
missing from the top portion of C1-B1 section. Although its Atypical NB DFR results should be further investigated with a full
insulation integrity was compromised, its ratio was not affected. dielectric frequency response (DFR) test using the Megger IDAX300.n
CVT-2 was also replaced.
Contact Marie-Claude Rasendra, Megger,
marie-claude.rasendra@megger.com

Figure 9: NB DFR test results 1 to 500 Hz on CVT-2 (Note that all units
connected in series have been affected and the dielectric response
presents an atypical response for all four sections. The 1 Hz test result
of C1 section B1 revealed a different behaviour compared to the other
sections and a PF tip-up test (Figure 10), was performed to verify its
voltage dependence.).

Figure 6: CVT-2 as found during visual inspection (Note that a ratio test
performed at 10 kV and a digital multimeter indicated acceptable ratio
results.).

Figure 7: C1-B1 section capacitance and PF test results for stained
CVT-2.
Figure 10: Power factor tip-up test results for C1-B1 section of CVT-2.

Figure 8:10 kV line frequency PF test results for C1 sections Bi to B4 of
CVT-2 (Note the elevated PF for Ci-B1 section (0,25%) relative to the
other three sections (~0,08%)). Figure 11: Oil overflow due to puncture on top of C1-B1 section.

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