POWER April 2017 - 39

OPERATIONS & MAINTENANCE
1. Causes of transformer failures between 1991 and 2010 (as a percentage
of total failures). Source: SPX Transformer Solutions
27%
28%
Electric disturbances
Lightning
Insulation
3%
2%
13%
4%
4%
4%
ing proposed, but I don't think the industry is
really at a point where a lot of those things are
generally being done to show payback. "
Transformer owners should more prudently
focus efforts on routine maintenance-examining
and changing out gaskets every three
years and bushings per the manufacturer's
recommendations, for example-but also pay
attention to proactive actions, such as conditionbased
maintenance, and especially monitoring.
Advances in Monitoring
Monitoring itself doesn't change the behavior
of the transformer, McIver noted. " The interesting
part about that is in terms of industry
philosophy. A lot of people pretty much just
run transformers to the failure point. They're
very big, very expensive devices and, as they
get older, there's not a good financial justification
to do dramatic changes to the device
while it's still in service. " So the monitoring
really just gives owners a measure of the transformer's
health and tells them which parts to
concentrate on and watch more closely.
Advances in monitoring include real-time
sensors, which can deliver valuable results.
One notable example is industrial measurement
provider Vaisala's real-time sensor for
moisture in transformer insulation oil. An elevated
moisture level in transformer oil is a
widely recognized root cause of failures, because
moisture essentially diminishes the ability
of oil to act as an insulator.
Moisture content in solid insulation after
initial factory drying is between 0.5% and 1%,
depending on requirements, but it starts to increase
gradually over the operating lifetime of
the transformer. Vaisala said that one critical
moisture source is residual moisture contained
in thick structural cellulose components, which
enters into transformer components from the
air. Small amounts of moisture may also be
generated as a byproduct of cellulose decomposition.
In a power transformer, the majority
April 2017 | POWER
6% 9%
Electrical connection
Overload
Foreign objects
Moisture
Line disturbance
Stress or fatigue
Other
of water is found in the solid paper insulation.
" Higher water content leads to accelerated
deterioration of the paper, reducing its degree
of polymerization, " the company said. " Moisture
and heat provide an optimal environment
for various compounds in the oil-such as acids
and metal ions-to react with the cellulose
molecules and break them down. "
When relative moisture saturation surges
beyond 20%, the dielectric strength of oil
starts to decrease rapidly, Vaisala said. Exceptional
high-loading events could also drive
excess moisture from the paper into the oil,
creating relative moisture saturation of 100%
and the formation of free water. That could
increase the risk of corrosion and rust particles
in oil circulation pipes and radiators, or
worse, form around active parts, resulting in
discharge that could cause catastrophic damage
to the transformer.
" Traditionally, industry understanding of
moisture dynamics in transformer oil has mainly
been based on samples taken once annually
that provide a limited 'snapshot' of oil health, "
explained Steven Jiroutek, regional segment
manager for industrial applications at Vaisala.
" However, growing use of online moisture
monitoring instruments in the global power
sector means more utilities can access 24/7
streaming data to monitor the condition of their
transformers. Integrating these instruments into
a comprehensive monitoring system and analyzing
real-time trends is enabling owners and
managers to make better, more cost-effective
maintenance and service decisions. "
Senja Leivo, a senior industry expert at
Vaisala, told POWER that while moisture sensors
have been on the market for nearly 20
years, Vaisala's device, designed for installation
directly into the oil circulation pipe of a
transformer, was launched recently and has
several installations globally. " If moisture levels
are found to be too high, the transformer
can be equipped with an online oil dryer or
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taken into more comprehensive offline overhaul, "
she said.
Space Weather
One glaring cause of transformer failure is
age. According to the Department of Energy's
Office of Electricity Delivery and Energy Reliability,
the average age of large power transformers
installed in the U.S. is between 38
and 40 years. The office said in an April 2014
report that more than 70% of the nation's large
power transformers are probably 25 years or
older, noting there are some units well over
40 years old-and some even older than 70
years-that are still operating on the grid. The
office worries that because more than 90% of
consumed power passes through these large
power transformers at some point, they are
one of the most vulnerable components on
the grid, faced with a wide range of potential
threats including severe weather, space weather,
and physical attacks.
In recent years, industry observers have
raised alarms about geomagnetic disturbances
(GMD), which occur when the sun ejects
charged particles that interact with and cause
changes in Earth's magnetic fields. These
charged particles can cause currents to enter
the grid through long conductors.
According to Dom Corsi, a senior transformer
engineer at Watertown, Mass.-based
Doble Engineering Co., GMDs can essentially
overheat a transformer's internal component
structure and affect actual current carry. " And
the other thing that happens is, because these
currents flow in the transformer and impact the
transformer by producing a bias on the core, the
transformer will saturate, " he told POWER in
February. " During the saturation events, there's
a significant change in the transformer's magnetizing
characteristics (Figure 2), which causes it
to draw reactive power from the grid. When the
transformer draws reactive power from the grid,
it can set up a system disturbance, which will
lead to an instability. If you can't compensate
for the instability, then you have the probability
of a cascading network event. "
Another impact is thermal, Corsi said. When
the transformer saturates, the transformer flux
spills out of the core. This affects the transformer
in two different ways: " One is the additional
main flux that spilled into the structural components
and windings, creating additional heating.
But also, because of the saturation, the harmonic
content of this flux results in induced currents
that also reach harmonics, which adds to the
heating component. "
In 2012, Earth experienced a near miss
by an extreme coronal mass ejection. Since
then, several agencies have been exploring a
number of grid resiliency solutions to counter
the potential GMD threat. The Federal
Energy Regulatory Commission (FERC) in
39
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POWER April 2017

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