POWER December 2014 - 38

Wind PoWer
Generally speaking, with a wide range
of manufacturers, owners, third-party
O&M groups, and specialty service organizations,
the reliability experiences are
diverse and fractured. Unlike traditional
fossil generation facilities, where there are
just a few generator models in operation,
some wind sites have 400 or more distinct
turbines-often from multiple manufacturers.
There is an advantage in having the
risk spread over multiple machines, as turbine
maintenance outages often affect only
a small percentage of the project's output.
On the other hand, it is certainly more
complicated to manage even normal maintenance
activities on that many turbines,
especially considering that they are 300
feet in the air.
Even if all of the major components of a
turbine need to be replaced (and that would
be highly unusual), it is often only a twoday
job-unlike the weeks required for a
complete overhaul of a traditional generator.
Of course, there are decades of service
records, procedures, and maintenance strategies
in place for steam and gas turbine
generators. Wind O&M teams are often
still developing the experiences to define
and maintain strategies, a task complicated
by the multitude of manufacturers and
models compared to traditional generators.
Also, keep in mind that testing and servicing
these machines is often performed
without much supervision-after climbing
a 300-foot ladder. Well-experienced and
dependable personnel are critical in all aspects
of wind project maintenance.
Wind turbine original equipment manufacturers
(OEMs) have made great strides
in developing newer, larger designs that
are more robust, and they seem to be addressing
both maintenance and safetyrelated
issues in their newer designs. The
statistics show that there are still too many
failures, especially related to maintenance,
but as failure modes are better understood,
more-dependable machines are being built.
According to studies surveying traditional
industrial and utility applications, motors
and generators over 100 kW experience
service lives from 25 to 38 years, so at least
20 years for a wind turbine generator does
seem a reasonable goal.
Wind Generator Failure Modes
Understanding the common failure modes
and what might be done to extend the life of
a generator is critical to providing the reliability
needed by project owners. Most of
the generator designs utilized in wind turbines
are conceived and often manufactured
in Europe, utilizing insulation systems that
reflect historical materials and processes.
38
2. Rotor banding failure due to overspeed conditions. Courtesy: Shermco
Industries
Several common failure modes have been
identified, many of which can be traced to
identifiable root causes. However, specific
failures remain difficult to identify, as minor
failures can lead to catastrophic electrical
failures not directly related to the root
cause. In addition, it has been rare to receive
alignment, vibration, or power quality data
points on individual failed machines. Below
are examples of the common failure modes
for these generator types.
Rotor Insulation. Wound rotor insulation
damage mostly appears in two forms.
The most common is a failure of the supporting
materials-banding (Figure 2), ties,
wedges, and the like-to function adequately
based on overspeed conditions or loading
electrical forces.
Some earlier machines were initially designed
to operate synchronized to 50 Hz rather
than the 60 Hz utilized in North America,
and it has been thought that the resulting increase
in rotational speed was not fully taken
into account.
Electrical failures, the other common
mode, might be related to the same mechanical
weaknesses and the resulting vibration
and abrasion, but because wound rotors are
all energized by IGBT control circuits, there
should be some consideration that damage
was also due to the typical elevated electrical
stresses often generated by these drives,
especially if the excitation circuit has filters
that have failed and are unannunciated
through the SCADA system.
Some of the rotor windings also operate
at higher voltages than the stators-up to
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3,000 VAC-and it is critical that the insulation
system account for some level of
voltage endurance stresses as well as shortterm
dielectric properties. There is not yet
enough data compiled on the permanent
magnet style generators regarding rotor
failures, as most are still operating under
warranty from the generator OEMs, but
some of the older designs (<1 MW) have
shown that pieces of the magnets can come
loose, resulting in catastrophic failures of
the stator windings.
Rotor Leads. There have been many issues
with the rotor leads in doubly fed induction
generator (DFIG) designs. These
pass the excitation current from the collector
rings to the windings, and any power
generated by the rotor back out the same
way. They can be subject to high heat if a
bearing fails, as well as vibration, and, of
course, to poor electrical connection or supporting
structures at either end. The good
news is that most of these failures can be
repaired up tower, utilizing improved conductor
insulation and supporting materials.
Nonetheless, this is a very common failure
mode, and entire fleets have been retrofitted
to avoid unplanned events.
Bearings. Bearing failures are a large
cause of generator down time and are arguably
the easiest failures to prevent. Proper
lubrication, alignment, condition monitoring
(thermal and vibration), and the effective
management of induced shaft currents are the
key to bearing longevity.
However, maintaining a fleet of wind
turbines with dozens of critical bearings in
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POWER December 2014

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