POWER April 2015 - 40

OPERATIONS & MAINTENANCE
1 Even wind power requires oil. These typical dissolved gas analysis syringes are
used to collect insulating oil from pad-mount transformers. The oil should be tested periodically
for predictive maintenance. Courtesy: Shermco Industries
2. Safety first. Tight transformer enclosures
can make it difficult to take good
samples, even when they are de-energized.
External valves and gauges can improve safety
and ease of sampling. Courtesy: Shermco
Industries
is almost always disruptive in several ways.
Wind turbines often cut in and out several
times a day, except in ideal wind conditions,
based on both minimum and, unintuitively,
maximum wind speed. This erratic load level
can create multiple in-rush currents and,
more importantly, thermal cycling beyond
the design of the transformer. The resulting
expansion and contraction of components
has been shown to have a direct effect on the
life of fuses, winding wires, and other metallic
parts, including insulated bushings. This
thermal cycling can also lead to the loosening
of connections, both internal and external,
which can result in arcing, insulation deterioration,
and lead failures.
Ground faults in the collection system
cables can also be very detrimental to the
transformers; therefore, surge arresters and
special grounding transformers should be
utilized in the system to help alleviate some
of these destructive conditions.
Many wind turbines now utilize a fully
converted AC/DC-DC/AC output, allowing
for variable speed generation with constant
voltage and frequency supplied to the transformer.
These electronic components normally
deliver less-than-ideal waveforms that can
be very destructive to the transformer if it is
not designed to manage those waveforms. In
some turbine designs, there are filtering circuits
to smooth out the waveform, but these
are often not monitored, and a failure mode
defaults to bypassing the filter.
Both the generator and the convertor can
create harmonics and other distortions at
every cycle, which lead to overheating and
40
eddy currents in the windings, again shortening
the life of the insulation. Low-voltage
ride-through events also contribute to thermal
stress on all of the components. Switching
events from circuit breakers on both the lowand
high-voltage side of the transformers can
also create surges and transient overvoltages.
Lightning strikes anywhere on the system
can, of course, create overvoltage conditions
as well.
Another consideration is the sizing of the
transformers. Often, during the development
phase of a wind project, the cost/value of
transformers can be under-calculated, leading
to a smaller transformer than what is ideal. It
has been suggested that the transformer be
sized at least 15% above the nameplate rating
of the turbine to account for overspeed
production as well as voltage variations. This
would mean that a typical 1.5-MW turbine
would probably need at least a 1.65 kVA
rating, but that would be an unusual size. In
most standard product lines, the size jumps to
2 kVA, so it has only been since all of these
issues have come to light that the specific designs
and sizes needed for wind turbines have
become readily available. That being said,
lead times are still often long, and remanufacturing
companies probably have difficulty
sourcing the properly sized cores to rebuild.
Most wind projects do have programs to
sample and analyze oil from their transformers
(Figure 1). Generally speaking, most of
the failures that have been discussed are not
due to moisture or other general degradation
of the insulation but to arcing and other events
that can create explosive gasses (typically hywww.powermag.com
drogen
and acetylene) that are both dangerous
and destructive. Most failures are contained
within the transformer housing, but there have
been cases of fires and explosions.
Employee safety should be the most critical
factor in making decisions regarding the
safe condition of a transformer. At-risk transformers
can be especially dangerous when
the low-voltage side of the cabinet is opened
while energized for switching (Figure 2).
Many new transformer designs, as well as
after-market upgrades, are available with the
various gauges and sample ports located outside
of the enclosure. This allows sampling
and monitoring to be done more safely and
without de-energizing the transformer.
Speaking of switching, it is very common
for these " strings " of transformers to be daisy-chained
to allow for ease of construction.
Normally, no switchgear is utilized in making
the connections. A transformer internal
load break switch is used to de-energize the
turbine for service while leaving the daisy
chain intact for the balance of the " string. "
This is not the primary purpose of the switch,
of course, and the repeated use can result in
both destructive surge electrical conditions
and premature mechanical wear. It is also not
always a safe way to disconnect a turbine, but
it is probably considered cost-efficient.
There have been many improvements
and suggested upgrades to help with these
transformer issues. Electrostatic shielding
between the primary and secondary windings
POWER | April 2015

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POWER April 2015

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