Modern wind turbines use generators coupled with converters to maintain the constant 60 Hz output required for grid connection. Insulated Gate Bipolar Transistors, or IGBTs, in the converter perform this task day in and day out but there is one drawback, the high-speed switching frequencies of the IGBTs induce a charge on the generator rotor shaft. If the generator does not have a way to shunt this current to ground, it will build up enough voltage to form an arc through the rotor shaft bearings. This arcing will quickly cause lubrication breakdown and create microscopic craters on the raceways and rolling elements, which will eventually cause the bearing to fail through the process of electrical erosion. Topics such as this issue will be the focus of the 2nd Annual Wind Turbine Symposium this September.
Evidence of electrical erosion includes darkened or burnt grease, rolling elements with a dull grey appearance and grey bands of fluting on the bearing raceway as shown in Figure 1. Fortunately, there are ways to detect this damage as it occurs and retrofit solutions to prevent the damage from occurring in the future.
Figure 1: Fluting on the outer raceway of a deep groove ball bearing used in a wind turbine generator
To prevent electrical erosion of bearings and other issues, generator manufacturers incorporate a slip ring and brush system to provide a low resistance path to ground for any charge that may build on the rotor shaft. This grounding system may degrade over time if the ground ring becomes oxidized, the brushes become worn or the brush holder spring does not have enough pressure to hold the brush firmly against the ring. Maintenance of the system is critical and often prescribed bi-annually.
As electrical erosion damage progresses to an advanced state, the bearing will begin to operate at a high temperature and dispel darkened grease. At this point it is important to shut the turbine down immediately to prevent catastrophic damage to the generator. A better plan is to use vibration data to detect bearing damage with enough time to plan for an up tower bearing replacement as shown in Figure 2. Vibration data can be collected by either an on board CMS or a portable system.
Figure 2: A non-drive end generator bearing failure vibration signature as viewed with Fleet MonitorTM
Several solutions exist for generators where electrical erosion is a persistent problem. Solutions are listed in order of increasing expense and effectiveness:
- Improving grounding system maintenance practices and frequency
- Resurface ground ring
- Upgrade grounding brush and/or brush holder
- Replace bearings with aluminum oxide outer or inner ring coating to increase electrical resistance
- Replace deep grove ball bearings with hybrid bearings that use non-conductive ceramic balls
To learn more about generator bearing failure modes and other wind turbine drivetrain reliability issues please plan to join us at the Romax InSight Second Annual Technical Symposium this September 12th and 13th in Breckenridge, Colorado.
Request an invitation for the two-day symposium here: