Wind power generation in the U.S. has been trending favorably upwards for wind farm owners. Primary contributing factors include the cost of wind turbine installations dropping by over one-third since 2010 as the capacity of turbines increased. Add-in the average capacity of turbines installed is now 2.32MW, up more than 200% since the late 1990s. Finally, capacity factors are also rising with an average of 42% reported over the period of 2014 to 2016, a significant increase of 31.5% over the period of 2004 to 2011. American Wind Energy Association (AWEA) reported that in 2019, the industry ended the year with just under 106 GW of operating wind power capacity and nearly 60,000 wind turbines.
Repowering wind farms
Repowering existing wind turbines with taller towers and longer blades are perhaps the most notable current industry trend. A repowered wind farm not only extends the life of the facility but leverages rising capacity factors found with modern technology along with more efficient power generation. One midwestern energy company, for example, has announced plans to spend upwards of $1 billion to repower 700 existing wind turbines with the promise of 19 to 28% more generation, depending on the farm site. Projections indicate that investment in repowering of existing wind turbine sites has the potential to grow to $25 billion by 2030.
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An incremental approach
Not all existing wind farm owners have the balance sheet or site factors that would encourage a full or partial repowering. Instead, those owners often pursue a more incremental approach to improving the reliability and capacity factor of their wind turbines. One approach is to consider an optimized blade replacement, typically at sites characterized by low wind speeds.
The development of more reliable and efficient gearboxes has also been the desire of many wind farm owners and the Department of Energy has invested heavily in their development. National Renewable Energy Laboratory (NREL) has also helped develop many new wind turbine components for increased turbine reliability. However, turbine blade pitch control valves, well known within the industry as having a limited life, continue to weigh down wind turbine reliability and capacity.
Controlling blade pitch for optimized process speed and product accuracy
Blade pitch control is a critical function within the overall wind turbine control system. Wind turbines operate at constant rotational speed, usually about 15-20rpm for large turbines. The gearbox increases the shaft speed to about 1,000–1,800rpm to match the generator rotational speed requirements, typically producing 60-cycle AC electricity at about 700V. The turbine controller starts the turbine when the wind reaches a given speed, usually about 8-16mph. A yaw drive keeps the turbine pointed into the wind to maximize electricity production.
The pitch control system, located within the turbine hub, rotates the three variable-pitch turbine blades in unison to precisely control the generator speed based on a feedback signal from the generator. On many of the hydraulically controlled units, there is one pitch control proportional valve per blade on a turbine so, for example, there are three valves used on a three-blade turbine. The pitch system also “stalls” the blades so that there is no lift generated by the rotating blades thus shutting the turbine down when the wind speed reaches about 55mph to protect the turbine from damage. A brake is usually engaged when the blades cease rotation.
Operational constraints from demanding environments
The pitch control system operates in a very demanding environment and the proportional control valve, one per blade, is arguably the device exposed to the harshest operating environment. Failure of only one of the three valves will force the wind turbine out of service. Data from operators confirm this observation with many field reports of pitch control valve failure within weeks of its first operation, with an unexpectedly large number of failures occurring within six months of service. Upon failure, a maintenance technician must travel to the turbine site and replace the pitch control valve in the hub. Performing this service can take one or more days, depending on the site location, technician availability, and weather conditions. Often the cause of the failure has been traced to circuit board failure due to inadequate vibration protection or the circuit board enclosure design does not prevent dirt and moisture ingress. The cost of a replacement pitch control valve is secondary to the cost of maintenance replacement evolution and the loss of energy generation.
Pitch control valves, therefore, must be designed with these specifications to operate 24/7 in an extremely rugged environment:
Able to withstand heavy vibration, shock, and rotational forces (up to 50G on three axes).
Valve electronics must be electrically isolated from the turbine nacelle.
Capable of withstanding extreme cold and heat ambient temperatures.
Complying with IP65 standards for protection against dirt and moisture, a major cause of valve failure.
Raise your wind turbine capacity cost-effectively with new technology
One pitch control valve that has proven itself in multiple wind turbine applications is Parker's D1FC and the D3FC direct operated proportional DC valve
with position feedback. The control valves receive an input signal (either 4-20ma or +/-10VDC) from the main turbine controller based on its monitoring of the generator output. Valve flow and performance specifications have been matched to the system requirements of the turbine so as to be compatible with the existing control parameters and co-exist with valves on the other axis.
In addition to IP65 designation, which inhibits moisture and dust infiltration, the D1FC and D3FC units are designed to meet IEC 682-6, -7, and -36 vibration standards so that sinus, random noise, and shock loads, respectively, are well accounted for in the design. The electronic driver card is installed with anti-shock mounting technology which minimizes vibrational effects. All fasteners are thread locked to guard against vibration as an additional measure of safety.