General Gearbox problem in Wind Industry

Something from me to be shared based on my search from the experts:

According to Andy Milburn, gear and bearing consultant, Milburn Engineering, Inc.,
Seattle, gearboxes in wind turbines are prone to failures stemming from their
application in wind energy systems. The following are the most common problems:

Highly variable load and speed: Wind is an intermittent energy source. It alternates
between gusting and still. Therefore, the load that these gearboxes are trying to
transmit is a lot more variable than it is in a plant operation. "There are some
applications where the load in plants is pretty variable," says Milburn. "But I think wind
turbines are probably the worst of the lot."

Low gearbox safety factors: The drive system in wind turbines is designed to be
compact. "They try to make things as small as they can, so that means the safety
factors that they’re using are low compared to a typical industrial application where
they don’t have to be too concerned about weight," says Milburn.

Flexible foundation: Typically, a plant gearbox and motor are mounted on a large
concrete foundation or a steel structure that’s bolted to a concrete foundation. "Up in
the nacelle, we don’t have that luxury," says Milburn. "The nacelle is flexing and the rotor itself is causing lots of loads in the whole structure. This causes misalignment
between the generator and the gearbox."

Periods of no rotation: The gearboxes operate only around 30% of the time, so they
are often idle. "Most rotating machinery doesn’t like to sit at rest," says Milburn. "They
run into problems with lubrication then."

Operating as a speed increaser: Wind system gearboxes operate as a speed
increaser instead of a speed reducer. "Most gearbox applications use a high-speed
motor that drives something that’s turning slower," says Milburn. "In this case, you’re
taking the blade that’s rotating slowly, and you’re increasing the speed up to the
generator speed. So that has some affect on things in terms of lubrication."

Extreme operating environment: The turbines have to operate in extremely cold or
extremely hot settings. Although the gearbox is in a nacelle and protected from rain it
can still be subjected to extreme temperatures.

High operating temperature: Manufacturers are resistant to adding large radiators to
wind turbines. "They allow these gearboxes to run pretty hot, and that means the oil
viscosity gets low," says Milburn. "When they’re rotating slowly, you don’t get a thick oil
film between bearings and gears, so you get metal-to-metal contact — and that’s a
problem."

Design details: Design flaws may cause failures, even if they occur slowly over time.

Cost of Wind Turbine O&M

Cutting costs by preventing failure instead of running wind turbines until they break lies at the heart
of a new approach to operations and maintenance.

Mass production of wind turbines on an industrial scale is not yet a decade old, and few turbine
models have operated in large numbers for more than a handful of years. Robust operational data
remains a relatively scarce commodity, and even expert technicians have been reliant on little
more than anecdotal data regarding what component is likely to fail when on which turbine, or
how trouble can be averted before it happens.

More recently, however, owners, operators and developers have been looking to the cost of
running the projects, and are recognising the value in preventing failure instead of running turbines
until they break.
Two components of the cost of operation and maintenance (O&M) of wind turbines are vitally
important and need to be minimised: those for scheduled maintenance and for unscheduled
maintenance. If component failures lead to unscheduled stoppages, then the additional cost of loss
of electricity sales is introduced. That is why considerable efforts are being made to control and
forecast such failures.
There are three ways of expressing O&M costs. The simplest way is to assume that the total
annual charges represent a percentage of the installed cost, often quoted between 3% and 5%.

which can be expressed as a cost per yearly power output, or per hourly electricity generated,
which is used here.
The inexactitude of the science of calculating O&M costs begins with the variety of ingredients
that make up the whole in addition to scheduled and unscheduled maintenance. The diagram
below shows data taken from an analysis four years ago from sister publication Windstats,
showing that the costs of O&M ranged from €15-26/MWh and, while this is an average, there
are significant variations both above and below the estimates quoted. The latest data from the
International Energy Agency, reporting from 12 different countries, gives a similar range of €7-
26/MWh.

My 2 cents....