technology is addressing two of the key issues that the wind power industry
faces as it matures: moving offshore and the use of bigger turbines. But the
clear focus is on system reliability, which has become the number one design
challenge that wind turbines must address.
In addition to a technology focus on gearboxes and generators,
suppliers are looking at the entire drive train and how to use technologies
such as gearbox health management tools, condition monitoring, ac brushless
servo motors, and precision cooling systems to provide robustness and
serviceability that the next generation of this alternative energy demands.
"Instead of focusing solely on the gearbox as in the past, OEMs
are focusing on the complete electromechanical drive train," says Dheeraj
Choudhary, business unit manager for Global Renewable Energy at Parker Hannifin. "If we look at the wind
turbine drive train from where the rotor shaft connects to the gearbox and on
the other side where electrical energy comes out of the turbine, there are
tremendous opportunities for boosting system reliability and serviceability."
Choudhary says that turbine design teams are now taking a
pragmatic approach on the need for reliability and serviceability in the drive
train of offshore wind turbines. The focus is on larger wind turbines (4, 5, 7
and up to 10 MW) developed for offshore operation, and on ways to keep the
drive train stable by deploying optimum conditioning and cooling for the
various key components such as bearing, gearbox, generator and converter.
"It is a well-known fact that if
the bearings and gearbox are well-lubricated, cooled with clean oil, have no
moisture or humidity present, and employ a good filtration package, the gearbox
is less likely to fail," says Choudhary. "If we can monitor the gearbox using a
condition monitoring package that includes trends from temperature, pressures,
vibration and cleanliness of the oil and moisture, we can reliably predict
failure or prevent failures by knowing what will happen in the gearbox over a
With the trend moving
toward larger turbines and offshore applications because of the availability
and consistency of the wind, an important focus is on the serviceability and
service intervals of these offshore installations. This is driving suppliers to
better control of the temperatures in the gearbox and to the optimization as
well as monitoring of the cleanliness of the oil in the system. To get service
people on board in an offshore turbine, it normally means you need a ship or
helicopter. As a result, suppliers are improving the lubrication systems and
increasing the lifetime of the filters, so it's a maximum of once a year to
service the unit; and sometimes the service interval is increased to 18 months.
Users can employ an online connection to assess when they need to
schedule maintenance on the turbines. Some of these systems, including
Parker's, constantly measure the pressure differential over the filter element
and communicate the information over industry-standard electronic protocols
that can be accessed remotely. With ability to program these devices, reaching
a threshold of pressure level indicates it is time to change the element.
Remote monitoring also allows for preventative and scheduled maintenance events
that reduce the cost of service.
Parker also offers systems to
monitor the condition of the gearbox lube oil with respect to the amount and
type of dirt in the bearings or gearbox. If everything is running smoothly and
there is no extensive wear on the bearings, it is normally OK to use an
18-month service interval, but performance is also dependent on the gearbox
itself, vibration, alignment, loads and other factors.
"We have also introduced
modularity into the system, so instead of using one large filter we are
specifying two or three optimum size and mesh filters on the gearbox
lubricating system to better perform service and further condition the oil
while keeping pressures consistently low in the system," says Olli Rantanen,
marketing manager at Parker's Filtration Group. "The lifetime of the components
is vital along with serviceability. We know that, in many gearboxes, people are
using a lot of connectors and hoses to connect different components which means
there is always a possibility for leakage, not to mention the drop in
efficiency of the package."
Parker's design approach to
these applications is to integrate multiple functions into a patented
uni-package which combines the process and the control parts of the total
system, plus optional functions including bypass filtration if that is needed.
A third important area is
condition monitoring. Normally if there is any oil condition monitoring at all
in these systems, it is metal residue monitoring - which means that only the
large metal particles are monitored, not for size but for count.
"We know that if a system is
starting to degenerate, it produces a lot of smaller particles before the big
particles start to appear," says Rantanen. "Instead of measuring particles that
are 100 microns or bigger we can start trending micrometer-sized particles
which the human eye can't see. ?You almost
need dedicated laboratory equipment to view particles of this size."
Parker has introduced new tools for this type of monitoring and
developed a system to do this online with the gearbox without taking samples to
a lab. The key is that measurements can be trended on a continuous basis and
specific commands can be used to prevent further damage or optimize the use of
the filtration system.
Trend to AC Brushless Servo Motors
"The key in wind turbine
applications is reliability and availability, which means the wind turbine must be available to
produce energy whenever there is wind," says Mauro Gnecco, North American wind
market manager for Moog
"The challenge for system suppliers is to make sure that components
are reliable and robust enough to support that environment which means
balancing features, performance and cost."
Electric pitch control systems are typically located in the
rotating part of the turbine (hub) and include one rotary actuator (electric
motor and gearbox) connected to each blade. Power and control electronics allow
the motors to develop the right torque and follow a motion profile provided by
the turbine controller. A power back-up system allows the motors to bring the
blades into a safe position, even in case of power loss, and a set of encoders
(one per blade) supplies blade position feedback.
With wind turbine applications, whether they are offshore or not,
the environment is harsh because system components are essentially outdoors
within a hub that is not weatherproof. Ambient operating temperatures range
from extremes of -40 to 50C. This means that the actuators and motors need
special attention paid to sealing, and the ability to meet performance
requirements throughout the full temperature range.
Moog offers new ac brushless servo motors rated for IP65. The
body of the motor is a single piece with two end caps to create fewer
connections and places for water to permeate the motor. ?The grease and lubrication systems have to be selected
to deal with the wide fluctuation in temperature ranges and, in very cold
temperatures, the motors are designed to self-heat via the controller on
"The rigorous environmental conditions apply to the controls as
well, plus all of the components in the control cabinet. Up in the moving rotor
as well, the system must be designed to withstand the vibration and shock of it
being continuously rotated," says Rob Nicholl, engineering manager at Moog Inc.
"Reliability is key because the motion system functions as the key safety
system of the wind turbine."
Moog is focusing on supplying highly reliable integrated electric
pitch systems to the wind industry. Even though they offer pitch systems using
different types of motor technologies (dc brush type motors, ac induction
motors and ac brushless servo motors), the focus is on systems
using ac brushless servo motors.
"Traditionally, dc motors have been applied to pitch systems but
there is a current trend to applying more ac synchronous motors to eliminate
the brush failure mode," says Nicholl. "Throughout the rest of industry and
aerospace applications, there is a general belief that ac motors are inherently
more reliable, even though they haven't been
widely accepted in the wind industry yet."
"The trend in the wind
industry we have seen is a continuing move toward larger-size turbines," says
Parveen Gupta, director of Wind Energy - U.S. sales for Bosch Rexroth
. "Until last
year, the most common size installed in the U.S. was 1.5 megawatts. But the
trend is larger and larger systems, and turbines in the 2 to 2.5 megawatt range
will be more or less a standard for onshore wind turbines over the next several
Gupta says a clear trend is more systems going offshore or
actually "near shore" where they are typically installed between three to five
miles off the coast. The advantage is a smooth flow of wind, the turbine is not
in anybody's backyard, and the noise issue is gone. For all those reasons, wind
turbines are moving to offshore locations but a major problem is regular
periodic maintenance which has the potential to become prohibitively expensive.
"Because of quality and reliability issues, people are talking
about using a direct drive system rather than a gearbox for large offshore
applications," says Gupta. "We see some benefits in that but; by the same token
with developments to increase reliability and improved quality, we believe you
can have a very good solution with a gearbox in the system even up to 5 or 6
With the demand for higher
power density and compactness
growing, Rexroth's differential gearbox, Redulus GPV-D, offers more compact
drives and minimizes the weight by providing a multiple power-split gearbox
Gupta says that unlike
conventional generator gearboxes in the multi-megawatt class, four or more
planetary gears do not revolve around the sun wheel in the input stage.
Instead, the Redulus GPV-D offers two input stages each fitted with three
planetary gears, and the advantages of a static determination paired with
freely adjustable sun pinions.
Another key design feature is
a thin outer diameter with only a slightly greater total length. With turbine
capacities increasing, it offers up to a 15 percent weight advantage over gearbox concepts currently in use without a decrease in reliability.
"There have been tremendous improvements in terms of planning and
doing preventative maintenance rather than repairs after a catastrophic
failure," says Gupta. "Along with the trend toward larger turbines typically
located offshore, there is a rise in condition monitoring solutions that enable
users to predict and plan for maintenance."
Lubrication is the key to the reliability and operating life of
gearboxes, and this applies in particular to gearboxes for wind energy plants
due to the difficult conditions in which they operate. This is why Rexroth is
offering main gearboxes featuring their own cooling lubrication system with
particle monitoring and water content sensors. Another development from Rexroth
is a blade control monitoring system that detects damage and ice build-up by
analyzing the natural frequencies of the elastic bodies.
Gupta says that while engineers are generally
concerned about the reliability of the gearboxes, often what actually limits
bearing life is lubricating fluid or oil which becomes too dirty or too hot.
The push to condition monitoring is one way to manage these important assets.