Third Time's The Charm at Oregon Wind Farm
Encouraging results from recent up-tower testing at a wind farm (Figure 1) in Oregon indicate that our conductive-microfiber bearing protection ring is effectively preventing generator bearing damage. Specially designed by our engineering team to protect wind turbine motor bearings, the ring appears to have solved a chronic bearing failure problem once and for all.
The generator bearings at the top of one turbine tower first failed in May 2006, only 11 months after the tower was brought on line. The company that owns and operates the wind farm replaced the bearings and slip rings, but the new bearings failed only five months later — October 2006. Once again, new bearings and slip rings were installed.
The third bearing failure came 11 months later, in September 2007. This time, in addition to replacing yet another set of insulated (ceramic-coated) bearings and slip rings on both ends of the generator, the owner decided to try our conductive-microfiber bearing protection ring (Figure 2) and our shaft collar with its specially enhanced highly conductive surface on the drive end. All components were installed by our regional distributor on September 12. The generator’s two standard carbon block, spring-loaded brushes, which rub on the slip ring at the non-drive end, were also replaced at that time.
Nearly three months later, on December 4, 2007, our crew used a probe and oscilloscope to measure shaft voltage on the generator with and without our bearing protection ring and collar engaged. All measurements were taken on the same circuit (Figure 3). Wind speed ranged from 10.2 to 13.4 mph. Real-time data from these field tests show that the conductive-microfiber bearing protection ring and collar were reducing shaft voltage by an average of 84.5%.
The first measurement, taken during full-power operation with a wind speed of 12.1 mph, established a baseline voltage (the system’s ground noise level) of 2.60 volts (peak-to-peak) from the 5.824” shaft of the tower’s doubly fed, asynchronous generator [Figure 4]. The oscilloscope setting was 10.0 V/div, 400 ms/div.
The crew then conducted eight more measurements in two series [Figure 5]. The Series 1 readings measured the shaft voltage with all components engaged. Our bearing protection ring and collar were on the drive end of the shaft, and the standard carbon block brushes were on the non-drive end. For the Series 2 readings, our bearing protection ring was disengaged and our shaft collar was removed, leaving the non-drive-end carbon block brushes as the only shaft-current mitigation.
High-frequency currents induced on the shafts of wind turbine generators through parasitic capacitive coupling can reach levels of 60 amps and 1200 volts or greater. If not diverted, these currents discharge through the generator’s bearings, causing pitting and fluting that result in premature bearing failure and catastrophic turbine failure. Our patented conductive-microfiber bearing protection ring technology effectively steers these harmful currents away from the bearings and channels them safely to ground.
The ring surrounds the generator shaft with millions of conductive microfibers of a very small diameter (less than 10 microns). Strong and stiff, yet flexible, these fibers provide a high density of contact points — parallel paths of least resistance from the motor shaft to ground. Capable of conducting currents of many tens of amperes and discharging from tens to thousands of volts with frequencies in the MHz range, the fibers significantly reduce voltage build-up on the generator shaft. The ring is especially suitable for use at high frequencies because its fibers tend to compensate for variations in the roughness of the shaft surface and/or microscopic misalignment of the ring and shaft.
When the microfibers lose mechanical contact with the rotating shaft, electric contact is quickly re-established somewhere along the ring, due to local field emission. When the gap between the shaft and the fibers is relatively large (greater than 5 microns), this is accomplished through the phenomenon known as a gaseous or electric “breakdown,” a cascading effect of secondary electrons obtained by collisions and impact ionization of the gas ions accelerating across the gap. With a smaller gap (5 nanometers to 5 microns), field emission is a form of quantum tunneling known as Fowler-Nordheim tunneling, a process in which electrons “tunnel” through a barrier in the presence of an electric field. Thus the ring fulfills all the functions of conventional spring-loaded carbon brushes with neither the direct frictional wear nor the hot-spotting/thermal wear common to such brushes. And because multiple microfibers dissipate heat better than single-conductor devices, the ring can tolerate higher current densities. Furthermore, the microfibers are not adversely affected by oil, grease, dust, moisture, or other contaminants.
More specifically, our wind turbine bearing protection ring is engineered to safely divert up to 120 amps of continuous high frequency shaft current at frequencies as high as 13.5 MHz and discharges of up to 3000 volts (peak). Maintenance-free for a minimum of five years, effective at any RPM, and available for any size generator, the ring is suitable for up-tower retrofits and preventive maintenance programs as well as for OEM installation. A new split-ring model makes on-site retrofits even faster and easier. Our shaft collar, coated with highly conductive silver paint, enhances the ring’s function.
Results of the Series 1 tests of the turbine with the bearing protection ring installed (Figure 6) show an average shaft voltage of 6.41 volts (peak-to-peak). Results of the Series 2 tests where only the spring-loaded carbon block brushes were used (Figure 7) show an average shaft voltage of 41.35 volts (peak-to-peak). The difference between these figures, 34.94 volts, indicates that the bearing protection ring and collar successfully divert approximately 84.5% of the damaging current that remains on the tower’s generator shaft when the only bearing protection is from the carbon block brushes at the non-drive end (Figure 8). Furthermore, the voltage wave form with our ring and collar engaged was a smooth wave with no detectable discharge to the bearings, while the wave form without our ring and collar showed a bearing-current-discharge pattern with voltage peaks an average of 6.5 times higher.
These measurements show that our maintenance-free bearing protection ring significantly lowers shaft voltages and thus mitigates the destructive impacts of shaft current discharges to bearings in wind turbine generators. By contrast, carbon block brushes apparently provide minimal protection, continue to allow current discharges in the generator’s bearings, and require frequent maintenance and replacement.
Damaged bearings can cause generator failures, which leads to costly repairs and unplanned downtime. In fact, a failed 1.5 MW generator can account for over $48,000 of lost revenue if down for a month, and repair costs can add up to as much as $50,000. Consequently, the return on investment for preventing such failure by installing our bearing protection ring at the factory or in the field as part of a preventive maintenance program can be quite high indeed.
Author's Bio: William Oh is General Manager of Electro Static Technology, an Illinois Tool Works company and a leader in the development and application of passive ionization technology.