An electrical discharge has found its way to ground through the bearing. The result is a “fluting” pattern on the race. There are many “band-aids” to remedy this situation. You must detect it before fixing it. The easily detected vibration appears in the chart to the right.
Most of the recent end-of-warranty inspections in the US market seem focused on gearbox health. This is rightfully so because the component has a rather high failure rate and associated costs. But what about the generator? How do you inspect the generator which is also a high-ticket item to repair? Moreover, what do you look for? You cannot exactly “bore scope” a generator as you can most of the gearbox. This is where vibration condition monitoring detects the three most common generator problems: misalignment, electrical discharges, and lubrication.
Misalignment between gearbox and generator is the first common problem. Paul Berberian of Easy Laser, a company that manufacturers wind turbine alignment tools, explains, “Misalignment is easy to detect and correct, but there are no standards per se because everything moves around so much. The gearbox moves back axially and differently from tower to tower. Secondly, temperature variations make for a wide margin of allowable misalignment.” In addition, environmental temperatures vary upwards of 100 degrees from season to season, and component temperatures vary due to non-wind days, seasons, and periods. The thermal growth and physical movement make for a loose alignment tolerance. Therefore, seeing misalignment in vibration readings is common, along with prematurely wearing high-speed-shaft bearings. Generator input bearings then become collateral damage.
There are three different types of directional misalignment. A moderately misaligned gearbox and generator will add stress on associated components. It is common to see post-alignment vibration readings significantly drop on the bearings on both sides of the coupling. But the location still must be monitored to see if permanent damage was caused by the corrected condition. Yes, it is possible to cause significant damage with an easy fix of an alignment check.
Motors and generators suffer from the same ailments, such as stray currents. These discharges find the path of least resistant to ground, often through bearings.
A vibration spectrum (velocity readings) will show a high-amplitude peak at the generator’s running speed. The higher this amplitude at 1x generator/hss (high speed shaft) running speed, the higher the level of misalignment. Vibration signatures show before (tall) and after an alignment in the accompanying plot. The “before” peak at generator running speed is quite high, likely affecting efficiency and jeopardizing reliability.
Electrical discharge in the generator is another common issue. The phenomenon occurs as stray electrical currents find a path to ground and do so through the generator bearing. Detecting electrical discharge in a generator provides an example of the electrical line frequency in a vibration spectrum.
The accompanying photo (below) also shows what happens when an electrical discharge is not addressed. It electrically flutes the bearing race, as shown by the clearly visible bearing outer race defects (BPFO: Ball Pass Frequency Outer race). If left to discharge, the bearing likely seizes.
By: David Clark/Condition Monitoring Consultant
Lubrication is the final common generator issue. Over lubrication and under lubrication both contribute to bearing failure. The chart, When generator bearings become discharge paths, is from another generator bearing in which the cage defect (FTFI labels) for this bearing shows multiple harmonics in vibration, albeit at low levels. Now would be the time to check the maintenance interval and lubrication before amplitude and damage increase. A follow-up reading to monitor changes would determine if the damage increases over time. BPFI indicates inner race defects for this generator bearing. The amplitudes are approaching an alert level in vibration. It’s time to lubricate and monitor for an improvement or decline.
All three common generator-related issues – misalignment, electrical discharge, and lubrication – are detectable in vibration measurements. In fact, wind-turbine manufacturers would be wise to monitor for these common failures prior to or during commissioning so the unit will not have under-warranty issues related to these failure modes. Obviously, owners would be wise to monitor for these common failure modes for the next 18 years out-of-warranty when they will have to foot the bill. Even wiser would be to inspect the generator as well as the gearbox during end-of-warranty inspections. WPE
(for full charts see November print issue of WPE)
Filed Under: Generators, O&M
