OBSOLESCENCE
it last longer? And most importantly, is it
possible to be warned of impending failure?
The plant engineer faced with these
issues will no doubt have been diligently
following the manufacturer’s instructions
for periodic maintenance and checking, to
ensure that the equipment remains safe
and fit for purpose. But the maintenance
instructions may go no further than
that. A different set of issues arises if
the instructions have been lost and the
manufacturer is no longer in business:
then the plant engineer and site staff will
have to know everything that there is to
know about maintaining that piece of
equipment, or find a contractor who does,
possibly at short notice.
It’s not ideal. There must be many
engineers who are faced with these
problems and say: “I think I could deal with
this, if only I knew where to start and what
to look for”.
What is not clear from the outline
above is that a very significant part of the
management of ageing electrical plant
is the need for accurate, consistent and
complete recordkeeping. This is because
in practically every case the environment
and circumstances of use will have a
profound effect upon the useable life
of the asset. One-off events such as
lightning strikes or fault currents may
result in a sudden reduction in residual life.
Continual immersion, high temperatures,
salt spray corrosion and prolonged
operation close to maximum rating will
result in progressive and possibly more
predictable degradation.
Consider a transformer as an example.
Transformers are generally one of three
types: mineral oil-cooled, biodegradable
fluid-cooled or cast resin air-cooled. The
first type is the most common and will be
used for this example (a damaged unit is
shown on p42). The transformer is likely to
have a tap changer to adjust the voltage,
which may be done on- or off-load. The
windings are insulated with paper, which
is impregnated with the cooling oil. The
external connections are via bushings
Margrit Hirsh/Shutterstock
which are generally
porcelain. There
may be a pump to
circulate the cooling
oil. A number of
failure scenarios
spring to mind:
• Slow degradation
of the cooling oil and
the paper insulation
due to the operating
temperature
• Distortion of
the windings
and possibly
the core due to
fault currents or
lightning strikes.
This may result in
immediate internal
damage due to reduced
clearances causing flash-over or
shorted turns, or a slower degradation
due to partial discharge
• Failure of the tap changer mechanism
due to frequent (or infrequent) use
• Overheating due to cooling pump
failure
• Partial discharge failure of
the bushings, perhaps due to
manufacturing defect, poor installation
or mechanical damage, surface
contamination or lightning strike
• Water ingress affecting any part of the
transformer.
It must now be clear that there is value
in keeping a detailed inventory of assets
with a full history including incidents,
environmental conditions, any changes
of location or configuration, maintenance
activities, tests and test results.
TESTING
Testing may invoke a range of activities
depending on the transformer’s condition
and suspected degradation mechanisms.
Perhaps the best known is analysis of
the cooling oil to detect the products
of degradation of the paper insulation
and internal arcing. Useful results
can be obtained
by appropriately
trained site staff
during routine
maintenance, but a
more detailed analysis
can be provided by
a laboratory and is
advisable from time
to time for high voltage
transformers or, for
other transformers, to
establish a baseline
and confirm trends.
Other tests include
acoustic analysis and
thermography while
on line, and winding
resistance, insulation,
swept frequency analysis,
capacitance and tan delta tests
kirov1976/Shutterstock
while off line. The value of a critical
visual inspection should not be overlooked!
Where does this lead? The desired
outcome is to establish a trend and
show rate of progression of degradation.
Studies indicate that there is a wide
variation in typical transformer lifetimes
depending on size and construction, as
well as use and environmental factors,
such that large transformers may last 30
to 50 years, whereas small transformers
may last as little as 10 years. Whether to
refurbish or replace is then an economic
argument tempered with considerations
of convenience.
But few ageing electrical assets are as
uncomplicated as transformers. Variable
speed electrical drives are a case in point.
They combine electrical, electronic,
software and mechanical failure points
in a highly proprietary and integrated
package. Moreover, the technology used
has changed rapidly over the last 30
years, both in terms of drive electronics
and control hardware and software. That
is not to imply that knowledge of their
failure mechanisms and possibilities
of life extension are beyond the grasp
of a site electrical engineer; only that
Winter 2021 www.operationsengineer.org.uk 41
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