Page 49 - Energize March 2022

TECHNICAL

of doing things and new options available Test results
for testing and monitoring today which Only the DRM test results flagged a deficiency in Phase B of the seventeen-position resistive-
have not previously been available, says type on-load tap changer (OLTC), which was not observed in static measurements. Looking
Woodley. at the TTR measured values, it was observed that all phases and all taps had a measured
Highlighting one good source of ratio well within the generally agreed acceptance value of 0,5%. Advanced OLTC testing was
guidance for asset leaders to look to, Wills carried out. During the transition from one tap position to the next, the current dropped.
identifies CIGRE’s technical brochure, The DRM analysis identified an issue with one of the transition resistors of Phase B.
Guide for Transformer Maintenance, which
aims to help asset owners define and Action taken
apply best practice in areas such as testing Based on the DRM analysis, the owner inspected the OLTC, finding a loose connector. The
and component selection. broken contact was welded back in place and the OLTC was reinstalled. A verification test
was performed after repair. Subsequent testing on each transition for all three phases found
What are the benefits of a more in- that the issue had been rectified.
depth transformer testing regime?
A stringent testing regime makes the Conclusions
difference between planned maintenance Routine electrical and dielectric testing on the transformer did not provide a clear indication
and unplanned maintenance, Wills of mechanical damage in the seventeen-position resistive-type OLTC. True DRM allows for
summarises. With testing in place, it’s the identification of faulty components affecting the resistive characteristic of the transition
more likely operators will be able to resistors in the OLTC. Megger suggests that a make-before-break test is included as part of
sustain a reliable network at reduced cost. the static winding resistance test. The normal WRM test did not indicate any problem, nor
They can minimise the extent to which did any of the other tests performed, except for the DRM test. DRM testing is useful for
they have to resort to portable equipment power transformer OLTC assessment.
and reduce overall downtime. “If you have
the knowledge to retire a transformer just CASE STUDY 2: Reducing failure risk of HV instrument transformers in the transmission
before it’s going to go wrong, that’s ideal. But system.
having to do so after it goes wrong creates
panic, further maintenance and a potential Background
emergency situation,” Woodley concludes. A utility’s SCADA system indicated a loss of voltage in one phase. Since 765 kV CVTs provide
the voltage information to all control, metering and protection systems at the location in
Case studies question, the CVT (CVT-1) corresponding to that specific phase was immediately suspected.
The following international case studies
demonstrate the operational and business Summary
benefits which can be generated through Ten kV capacitance and power factor (PF) tests at line frequency confirmed insulation
the introduction of new transformer failure of C1 sections B2 and B3 of CVT-1. For further insights, 250 V narrowband dielectric
testing regimes. frequency response (NB DFR) tests, comprised of PF measurements made between one and
500 Hz, were performed on all four sections of CVT-1.
CASE STUDY 1: Introducing dynamic The customer reported this failure as an over-pressure event and replaced CVT-1 with
resistance measurement – pinpointing CVT-1R. CVT-1R was similarly tested. While replacing CVT-1, the testing specialist noticed
concealed problems inside a resistive- a stain on the surface of the adjacent phase CVT (CVT-2). CVT-2 was reporting an expected
type OLTC before physical inspection. voltage in SCADA but given the failure of CVT-1 and convenient access to the testing
equipment, capacitance and PF tests and NB DFR tests were performed on CVT-2, which
Summary revealed insulation failure. A teardown of CVT-2 confirmed significant damage to its C1 –
The team at the Electric Power B1 section. A puncture was found, oil was missing from its top portion, and its insulation
Transmission Network in Karbala, Iraq, integrity was compromised. CVT-2 was also replaced.
performed routine electrical tests on a
three-phase YNd1, 25 MVA, 132/11,5 kV Takeaways
transformer including winding resistance A CVT in good condition should have a very low PF value (expressed in %) at 20°C – less than
measurements (WRM), transformer turns 0,2% at 1Hz. Monthly visual inspections did not pick up the oil leak between C1 sections of
ratio (TTR), short circuit impedance (SCI), CVT-2. Testing averted failure. NB DFR testing was incorporated in the utility’s future routine
excitation current, and line-frequency CVT maintenance.
power factor. The team also performed
advanced tests such as magnetic balance, This article was initially published as a Utility Week report in association with Megger in
frequency response of stray losses September 2021.
(FRSL) and OLTC dynamic resistance
measurement (DRM) tests. Contact Marie-Claude Rasendra, Megger, marie-claude.rasendra@megger.com, https://megger.com

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