POWER December 2014 - 41

Wind PoWer
bine off line, and it is never good to have arc
flashes up tower.
Magnetic Wedges. Most of the newer
and larger turbines installed during and since
2008 utilize larger, more robust generators
(2 MW and larger) with preformed, micainsulated
stator coils and well-designed mechanical
components. Additionally, most of
the older 1.5-plus-MW designs have been
upgraded with many of the weaknesses of the
earlier designs corrected. However, in many
of the newer larger designs, stator coil wedging
utilizes high-ferrous content composites
to smooth the flux fields and increase the efficiency
of the generator.
All of this is good in theory, but, due to
various reasons-including materials, manufacturing
processes, and application rigors-
these wedges have a tendency to loosen,
which leads to several bad results. They can
create a conductive dust circulating through
the windings that will lead to eventual electrical
failure. They can break into larger pieces
and pierce the ground wall, creating an immediate
and spectacular ground fault. And,
of course, coils can become loose without the
proper support. Additionally, the slot laminations
can be damaged by wedge movement
abrasion, preventing future remanufacturing
without major expense.
None of these failure modes (Figures 3 to
5) is easy to mitigate, and all require removing
the generator from the turbine. To date,
there is no effective way to test for this failure
mode beyond visual inspection and, other
than scheduling for replacement, there is not
a lot that can be accomplished anyway for a
reliable solution.
Failure Occurrences
Based on more than 2,000 generators that
have been repaired by Shermco since 2005,
some statistics have been developed that
try to quantify the frequency of various
failure modes. This is by no means an indication
of the overall failure rate; rather, it's
an attempt to identify the immediate cause
of what is failing.
Comparing these failure types with general
industry statistics shows that they are actually
fairly similar, with bearings being the
worst problem (Figures 6 and 7). Again, it is
suspected that the logistics of maintenance
has created some of these issues with lubrication
and other preventable failures, but good
training and solid maintenance practices are
improving maintenance practices, and generator
manufacturers are finding solutions to
their own reliability issues.
The Role of Maintenance
Proper and well-planned general maintenance,
up-tower repairs, and down-tower reDecember
2014 | POWER
6. Generator failures in wind industry and general industry. Source: Shermco
Industries, data from Dr. P.J. Tavener " Offshore Wind Turbines-Reliability, Availability and
Maintenance, " An Institution of Engineering and Technology publication (2012)
■ Industrial ■ Wind
50%
45%
40%
35%
30%
25%
20%
15%
10%
5%
0%
Bearings
Windings
Other
7. Occurrence of failure modes. The data represent 2,068 total failures. Source: Shermco
Industries
■ Total ■ Cumulative %
900
800
700
600
500
400
300
200
100
Bearings Stator
wedge
Rotor
Stator
manufacturing procedures all play vital roles
in extending the life expectancy of the current
fleet of turbines and reducing the overall
cost of energy production. The key elements
of cleanliness and lubrication are standard
across all industries, but they are especially
critical in a remote application, as quick response
is limited and a catastrophic failure is
very expensive.
Most turbine and generator manufacturers
have recommendations regarding maintenance
intervals, but they may be inadequate,
depending on the design and location. Annual
alignment of the drive train is also a normal
recommendation, but there are turbines
where alignment is a major issue, especially
if the base structure of the nacelle is not adequately
rigid. Alignment should also always
www.powermag.com
Rotor
leads
Collector
rings
Other
Cooling
system
be confirmed after initial installation of the
nacelle, even if it was properly performed at
the factory.
As mentioned in the introduction, budgets
for O&M of wind projects are often undervalued
during the development stage of a project,
when there is a lot of pressure to keep costs
at a minimum, without careful consideration
of long-term reliability. As utility rather than
developer ownership increases, more-experienced
asset management is addressing these
issues and realigning maintenance priorities
for long-term reliability.
The expanded use of condition-based
monitoring and computer-based maintenance
management systems is becoming
more prevalent, and predictive and proactive
maintenance planning is replacing the once41
100%
80%
60%
40%
20%
0%

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POWER December 2014

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