Page 62 - Energize May 2022

TECHNICAL

An oil sample was taken from the transformer and sent to a lab for a DGA test. Results All evidence clearly pointed to
are shown in Table 5. a fault inside the transformer. The
remaining question was, where? To
localise the fault in the overall active
part of the transformer, SFRA testing
was carried out using Megger’s FRAX
101.
Table 5: Gas concentration from a transformer oil sample SFRA open circuit test results
(Figures 5 and 6) indicate a short circuit
The “latest before failure” DGA test results are not representative of a gassing issue, in phase C (blue traces). The fault had
especially for a furnace transformer. Numbers fall under condition 1 as specified by existing also affected the other two phases,
standards. This is a case where only an on-line gas monitor or a ‘lucky’ oil sample may have resulting in a completely asymmetrical
detected the incoming arcing fault before failure. response.
The post failure DGA analysis indicates a D2 position in Duval’s Triangle 1, as well SFRA HV short circuit test results
as in Pentagon 1. D2 is an indication of “high energy discharge including arcing, with confirm that phase C was affected
expected metallic fusion and presence of carbon particles in the oil.” These results are in this transformer but also that this
presented in Figure 4. damaged phase affected the response
of phases A and B.
It was obvious that the transformer
required major work before returning it
to service. To evaluate the extent of the
damage identified by this series of tests,
the asset owner ordered an internal
inspection.
During the inspection, the fault was
observed on the phase C regulating
winding as shown in Figure 8. The A and
B phase windings, as well as the core,
were undamaged.
The asset owner decided to have
Figure 4: Duval’s fault analysis of the oil sample taken after the failure of the transformer
the unit repaired. The unit was taken to
its manufacturer’s repair shop for full
restoration and returned to service.
The root cause of failure is not 100%
clear. This furnace transformer was in
heavy use for many years, with the full
power frequently switched on and off.
The condition of the transformer was
never investigated thoroughly before
the fault occurred.

Conclusions
During the service life of a power or
Figure 5: High voltage open circuit – Tap position 1
distribution transformer, it is almost
impossible to prevent transient
events and through-faults which can
severely damage the active part of the
transformer. After many years in service,
especially for furnace transformers, it
is beneficial to thoroughly evaluate the
operational condition. For service-aged
transformers, the operational profile
may need to be modified so that the
transformer continues to withstand
Figure 6: High voltage open circuit – Tap position 13 operational stresses.

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