POWER February 2022 - 13

O&M
How Measuring Total Gas
Pressure Detects Transformer
Air Leaks
From generation to transmission and
distribution, the power industry is changing.
With distributed energy resources
(DERs) increasingly participating in
wholesale capacity, energy, and ancillary
services markets, new regulations are
helping spur the development of innovative
technologies that can enhance our
electrical grid.
While some new technologies are
already online and making a difference
as the economy continues to move toward
electrification, aging infrastructure
is still utilized to generate, transmit, and
distribute the electricity that powers our
modern lives. Unfortunately, that older
equipment has higher failure rates, which
can lead to interruption for customers
as well as higher inspection, maintenance,
repair, and restoration costs. This
fascinating dynamic offers a myriad of
different and interesting opportunities
to revolutionize how the industry operates-and
highlights the significance of
power transformer monitoring.
Power transformers are one of the
most important elements of the electric
power system because they are used to
transmit electrical energy between the
generator and the primary distribution
circuits. Transformers are often utilized
in distribution networks to enable the
interconnection of different voltage levels
to transfer energy over long distances.
The lifecycle of these instruments is
about 40 years, but since transformers
are among the most expensive components
in the network, monitoring is critical
to detect any problems as early as
possible, thereby avoiding sudden failures
and electricity outages.
By detecting problems at an early
stage, power utilities are able to schedule
a planned service break to make
necessary repairs or carry out the maintenance
necessary to avoid a costly
failure. With the increased number of
sealed transformers, ambient air leaks
have become a significant issue as
oxygen and moisture accelerate the aging-and
degradation-of paper used in
the insulation. A power transformer's
residual life relies heavily on the condition
of the insulation paper within its
windings, so keeping the transformer
dry and free of oxygen is crucial.
February 2022 | POWER
Limitations of Traditional
Monitoring Methods
New power transformers tend to feature
sealed designs (Figure 1), and many old
transformers are retrofitted with sealing
systems to keep oxygen and moisture
out. Air leaks often result from the embrittlement
of gaskets or the rubber bag,
and the issue of air leaks as a source of
oxygen has grown in importance in recent
years. However, the increased use
of sealed transformer designs challenges
asset managers to confirm that the
tanks remain sealed from oxygen and
atmospheric humidity intrusion.
Measuring oxygen concentration in
oil is one way to confirm air ingress,
but measuring oxygen directly will not
necessarily reveal if air gets in. That's
because oxygen sensors cannot detect
air leaks if there are reactions in a
transformer that consume oxygen. Plus,
keeping an eye on only oxygen requires
an experienced dissolved gas analysis
(DGA) specialist to review and interpret
the measured results, since there is not
an internationally recognized standard
for understanding how oxygen concentration
in transformer oil impacts the
operational life of power transformers.
While the seven key fault gases are
used for transformer diagnostics, oxygen
and nitrogen often remained in the
specifications because oxygen accelerates
the aging of the transformer's solid
insulation. To maximize a transformer's
lifetime, its oxygen levels should remain
as low as possible.
Traditionally, air leaks have been detected
by measuring oxygen and nitrogen
from standard DGA oil samples, but
the risk of sample contamination with
ambient air poses a significant problem.
Taking samples at the transformer site,
and then transporting and handling the
samples can introduce oxygen to such
samples, which can lead to inaccurate
interpretations of the data. Even though
the contamination can be identified by
observing nitrogen level fluctuations in
DGA results over extended periods of
time, oxygen measurement alone will
provide mixed results.
Some DGA monitors (Figure 2) have
integrated oxygen measurement and
nitrogen levels are calculated based on
those oxygen values. However, those
calculations assume that the oil is saturated
to ambient air with a constant
oxygen-nitrogen ratio, failing to consider
that such calculated nitrogen values are
only valid for free-breathing transformers-not
for transformers with sealed
designs. When compared against laboratory
reference results, those DGA monitors
show significant bias when used
for sealed transformers, and the typical
large fluctuation in data makes determining
the possible ingress of air into the
transformer tank difficult.
But at the end of the day, decisionmakers
don't need to know the exact
concentration of oxygen or nitrogen gases.
Instead, they need to know whether
their sealed transformer is exposed to
air ingress, with air being the source
of oxygen. Recognizing the significant
shortcomings of traditional monitoring
techniques, Vaisala, a global technology
leader and expert in weather, environ1.
Vaisala's OPT100 Optimus, a maintenance-free multi-gas dissolved gas analysis (DGA) monitor,
safeguards power transformers 24/7. The OPT100 brings long-term stability, reliability, and
measurement performance for detection of any formation of fault gases. Courtesy: Vaisala
www.powermag.com
13

http://www.powermag.com

POWER February 2022

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