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Electrical pitting

Electrical pitting – Bearing failure

Electrical erosion refers to the removal of material from bearing contact surfaces by unintended electric current or voltage. There are two distinct types of electrical erosion: fluting and pitting. Electrical pitting is often called excessive voltage due to the common cause of an excessive voltage through a bearing. Electrical pitting can be seen on both rolling elements and raceways. Pits are caused by current flowing through the bearing to ground due to a potential difference. In wind turbines, the excessive voltage is often caused by lightning strikes. Ensuring adequate grounding and using insulated bearings where necessary can help protect bearings from electrical erosion. Although electrical erosion is a severe failure mode, it is not common in wind turbine bearings.

Also referred to as: Excessive Voltage, Spark Erosion, Lightening Damage, Beading

Causes

Electrical erosion is caused by unintended currents flowing through Hertzian contact points. This usually occurs between the rolling elements and raceways of a bearing. Electrical pitting is commonly caused by excessive voltage such as from a lightning strike or occasionally a static build up. This can cause current to flow through turbine bearings if there is not an adequate grounding path. As the current flows between bearing components, sparks may create pits by removing material. The material is removed due to localised melting and welding together before being pulled apart and vaporised or washed away. The properties of the material around the localised melting zone are often affected due to tempering/rehardening which can cause points of high stress concentration leading to micropitting or crack initiation.

Appearance

Pits caused by electrical erosion tend to have a conical shape with a melted surface appearance. The craters, which have a diameter up to ~100um, tend to be ordered in a bead like procession in the rolling direction of the bearing. The pits are usually duplicated on both the rolling element and the raceway due to the contact areas melting and welding together before being torn apart.

Progression

All forms of electrical erosion are considered severe, however there can still be some progression. Over time, the pits can develop fluting or progress into severe adhesive wear, micropitting and macropitting. This is caused by the change in material properties around the pits due to tempering/rehardening and the stress concentrations as the Hertzian contact patch passes over the surface damage.

Detectability

MethodDetection EfficiencyNotes
Visual inspectionDifficult to observe visually due to access limitations on generator bearings. If the micro-craters are large enough and there is access to other bearings then it may be possible to observe visually.
Borescope inspection✓✓Difficult to observe with a borescope due to access limitations on generator bearings. However, if accessible a borescope will be able to detect electrical erosion.
Vibration analysis✓✓Noise and vibration of high intensity can be an early indication of electrical erosion so it can usually be detected using vibration data.
SCADA dataSCADA data does not usually aid detection of electrical erosion.
Oil debris sensorElectrical erosion does not shed much debris so cannot usually be detected using oil debris sensors.
Oil sample analysisElectrical erosion does not shed much debris so cannot usually be detected using oil sample analysis.

Discussion

Electrical erosion can be a very severe failure mode if found in a bearing due to the removal of material from the critical contacting surfaces and the high intensity vibration usually experienced. In wind turbines, electrical erosion is most commonly seen in generator bearings, main bearings and pitch bearings. Also, the current discharge can damage the lubricant properties causing further damage to components.

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.

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 aluminium oxide outer or inner ring coating to increase electrical resistance
  • Replace deep grove ball bearings with hybrid bearings that use non-conductive ceramic balls

Severity Rating

RankDescriptionDetectionRecommended Action
1N/AN/AN/A
2N/AN/AN/A
3Conical shaped pits, up to 100um diameter and ordered in a bead like procession. Duplicated on both the rolling element and the raceway.Borescope, vibrationMonitor regularly for signs of progression or violent vibration and consider scheduling replacement soon. If persistent, improve mitigation against electrical erosion.
4N/AN/AN/A
1
Not applicable
2
Not applicable
3
Example of rank 3 electrical pitting (a bearing failure)
4
Progresses to other failure modes
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