Pitch System Performance
By Cory Mittleider on 5/1/19
Pitch bearings a.k.a. blade bearings connect the blade root to the rotor hub. The blade bearings allow the blade position to be optimized at different stages of operation. This includes optimized angle of attack to control drivetrain load usually to maximize output, but the pitch system is also used as the emergency brakes to stop the system in case of failures. Properly functioning pitch bearings are critical to maximized productivity of WTGs.
There are different systems used by different turbine manufacturers. For the sake of this article I’ve focused on a system found in a very common 1.5MW turbine platform.
Typical Pitch Bearing: 2-row 4-point contact ball bearing
Blade Angle Faults
Blade Angle Asymmetry or BAA faults are indicated when the pitch angle of the three axes are different from one another.
Blade Angle Implausibility or BAI faults are indicated when the actual pitch angle, and the desired pitch angle are different.
There are several components in the pitch bearing system that can contribute to poor pitching performance. We will look at many of these items in this article.
Pitch Motor
Motor Bearings
Encoder
Brake
Shaft
Pitch Drive
Pitch Bearing (Blade Bearing)
Pitch Motors
The pitch motor is connected to a gearbox which turns the pitch bearing by engaging gear teeth on the inner or outer ring of the pitch bearing. There is a holding brake on the motor which holds the blade in position when not actively pitching.
DC Pitch Motor
Pitch Motor Bearings
The bearings in these motors are prone to failure. They are relatively small, they are subjected to great deals of heat, and are “sealed for life” bearings. There are different opinions on the cause of these failures. Regardless of cause the result is a motor that doesn’t run as expected, and eventually the rotor will drop and hit the stator causing motor damage, and encoder damage. All of these will contribute to Blade Angle Faults
You can read more about the special bearings use here and also some of the other upgrade options here to increase the life of these bearings.
Special Bearing
Pitch Motor Encoder
There is an encoder on this motor as feedback to the system to determine position of the blade. If this encoder stops working properly it will certainly trigger faults. Read more about encoder upgrade options here
Pitch Motor Encoder
Pitch Motor Brake
There is a brake on these motors that holds the blade in position when not being actively pitched. Brakes are a wear item and do need to be adjusted periodically. If the brake is not properly adjusted it could slip or if a brake rectifier or coil is failing it may not release the brake when pitching causing additional load slowing pitch response time. Either failure can cause Blade Angle Asymmetry faults.
Pitch Motor Shaft
The pitch motors engage the gearbox by using a keyed round shaft connection hub. At some sites this interface is wearing rapidly due to impacting load and reversing load on the application. This failure starts as the key rocking back and forth in the keyway. The motor shaft keyway eventually accumulates enough damage that a fault is thrown.
Pitch Drive Gearbox
The pitch motor is mounted on the high speed side of a planetary gearbox. This gearbox has a spur pinion on the low speed shaft which engages the geared ring on the pitch bearing to turn the blades. The gearbox acts as a speed reducer and a torque increaser to provide the required torque to pitch the blades.
Pitch and Yaw Gearboxes
Pitch Drive Input Hub
The same sites that have problems with pitch motor shaft failures described above will see corresponding damage to the input hub on the pitch drive gearbox. When the pitch motor is repaired / replaced, this hub also needs to be replaced. To solve this problem some sites are retrofitting their pitch drives with splined input sections.
Pitch Drive Internal Failures
There are multiple stages of planetary gears inside a typical pitch gearbox. If there are any internal failures of bearings, gears, carriers, shafts, etc… there will be an increase in operational torque, or possibly even jamming. These will lead to Blade Angle Faults.
Pitch Bearings
There are several failure modes that can be observed in 2 row ball bearing.
Some of these are:
Corrosion
Cage damage
Raceway edge damage
Damage to balls
Ovalization of bearing rings
All of the above mentioned failure modes will lead to an increase in system load. If a higher load is needed on one axis than the others this will likely lead to Bearing Angle Asymmetry faults. There are solutions available for all of these problems available now.
Conclusions
Blade Angle Asymmetry faults are very common and can be caused by many factors. These faults can cause large amounts of down time. Some of the failures are solved at a low cost repairs, and some are indications of larger repair needs like pitch bearing replacements. Trending of the BAA faults, as well as pitch motor currents can give you a good idea of pitch system health over time, and help you identify turbines that are facing more urgent needs.
Malloy can help you solve many of the problems described above. Please contact us to talk more about your application and your existing failure modes so we can discuss what can be done to solve them for your site.
Resources
If you have any questions please give me a call at 605-357-1076
-Cory Mittleider