Page 63 - Energize May 2022

TECHNICAL

the findings and helped identify the
potential root cause of the failure.

Benefits
A combination of low-energy testing
procedures is always required after a
fault condition. On-site availability of
SFRA equipment and records of SFRA
measurements allow rapid identification
of a fault and its location. Other
Figure 7: High voltage short circuit test procedures help validate and evaluate
the magnitude of the damage. Oil
sampling is a good practice when an
active failure condition is continuously
affecting the regular operation of the
transformer. Nevertheless, faults can
be rapid and violent, not giving time to
the operator to prevent damage to the
transformer.
Continuous DGA sampling is
particularly recommended for heavily
loaded transformers, as in the case of this
furnace transformer. For the industrial
asset owner, loss of power supply is
directly proportional to financial losses
and non-recoverable production. In this
case, a spare unit was available, and
downtime was minimised.
Be proactive, not reactive. Keep
baseline records of the factory and routine
test results of your equipment, and make
sure that your electrical testing toolbox is
equipped with the right tools, especially
Figure 8: Phase C regulating winding as found during visual inspection with an SFRA portable device such as
Megger’s FRAX 101.
After a fault, several actions need to be considered:
• If the fault was external to the transformer, demagnetise the transformer after isolating References
it from the system. Remove residual magnetisation in the core to facilitate meaningful 1. ABB: “Transformer Service Handbook –
interpretation of AC test results, specifically SFRA, excitation, and transformer turns ratio 4th Edition”, Page 189, 2018
tests. 2. IEEE C57.152-2013: “Guide for
• If the fault is internal, use low-energy testing practices. Make sure combustible gases are Diagnostic Field testing of Fluid-filled
removed from the transformer. Power Transformers, Regulators, and
Reactors”
In this case study, electrical and dielectric testing confirmed that phase C winding failed
severely, and that the unit could not go back into operation without repair. A spare unit Contact Marie-Claude Rasendra, Megger,
was put in place to minimise downtime during the testing and repair of the faulty unit. The marie-claude.rasendra@megger.com,
damage in this transformer could have been identified solely by SFRA. Other tests supported www.megger.com

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