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FRAX-101
Power transformers are specified to withstand
mechanical forces from both transportation and in-service
events, such as faults and lightning. However, mechanical
forces may exceed specified limits during severe incidents
or when the insulation’s mechanical strength has
weakened due to aging. FRAX is a powerful tool when
trying to detect these problems.
Method basics
A transformer consists of multiple capacitances,
inductances and resistors, a very complex circuit that
generates a unique fingerprint or signature when test
signals are injected at discrete frequencies and responses
are plotted as a curve.
Capacitance is affected by
the distance between
conductors. Movements in the
winding will consequently
affect capacitances and
change the shape of the
curve.
The SFRA method is based
on comparisons between
measured curves where
variations are detected.
One SFRA test consists of
multiple sweeps and reveals if
the transformer’s mechanical
or electrical integrity has been
jeopardized.
Practical application
In its standard application, a
“finger print” reference curve for each winding is captured
when the transformer is new or when it is in a known good
condition. These curves can later be used as reference
during maintenance tests or when there is reason to
suspect a problem.
The most reliable method is the time based comparison
where curves are compared over time on measurements
from the same transformer. Another method utilizes type
based comparisons between “sister transformers” with the
same design. Lastly, a construction based comparison can
under certain conditions be used when comparing
measurements between windings in the same transformer.
These comparative tests can be performed 1) before and
after transportation, 2) after severe through faults 3) before
and after overhaul and 4) as diagnostic test if you suspect
potential problems. One SFRA test can detect winding
problems that requires multiple tests with different kinds of
test equipment or problems that cannot be detected with
other techniques at all. The SFRA test presents a quick
and cost effective way to assess if damages have occurred
or if the transformer can safely be energized again. If there
is a problem, the test result provides valuable information
that can be used as decision support when determining
further action.
A reference measurement on a mission critical transformer
is therefore a valuable investment as it will make analysis
easier and more reliable when an incident occurs.
Analysis guideline
As a general guideline, shorted turns, magnetization and
other problems related to the core alters the shape of the
curve in the lowest frequencies. Medium frequencies
represent axial or radial movements in the windings and
high frequencies indicate problems involving the cables
from the windings, to bushings and tap changers.
The comparisons are simplified using the built-in
analysis tools, e.g. interpretation according to Chinese
standard DL/T 911-2004, that help identify differences
between the curves.
Example
The figure below shows a single phase transformer after
a service overhaul where, by mistake, the core ground
never got connected (red), and after the core ground was
properly connected (green).
This potential problem clearly showed up at frequencies
between 1 kHz and 10 kHz and a noticeable change is
also visible in the 10 kHz - 200 kHz range.
The Application
