It does happen, although rarely, in typical industrial environments that an electric machine will die of old age. The reason is that machines are often exposed to contamination issues, other environmental issues, power issues or a combination of events that degrade the insulation slowly or rapidly.
There is also a rule of reliability that should be considered in any maintenance process: the act of intrusive maintenance is the number one cause of failure of industrial equipment. In effect, the maintenance itself is the cause of significant failures. Does this mean the best action is to then eliminate maintenance? Unfortunately, many businesses go from the one extreme to another. Worse, they see an immediate success that later becomes a disaster.
With these ideas in mind, we will consider a generator armature that failed unexpectedly immediately following activities performed by a repair facility. We are often called in to evaluate such situations, so evaluated the condition of the machine and the mode of failure.
First, we noted the way the failure occurred in the armature and the history. The insulation system was 36 years old and had been taken out of service every five years and sent to a repair vendor for cleaning and baking. The specification calls for an IEEE Std 43 insulation resistance and polarization index test before returning the armature for service. In this case, there were a large number of these machines at multiple locations. While some had failed in the past for a variety of reasons, this one failed immediately upon return from the repair facility, at least within the first 24 hours of operation.
Second, we reviewed the test sheets associated with the repair. It was noted that the insulation resistance value was 13 GigOhms and the polarization index was about 12:1. When checked, the previous test was about 5 GigOhms and the polarization index was about 8:1. The test sheets identified that test results were adjusted for temperature.
Third, we removed sections around the damaged coils and evaluated the insulation system. It was noted immediately that there was ‘carbonized’ insulation between the conductors throughout and that the insulation system was dry and brittle.
Discussion: while many understand that they need to watch for low insulation resistance readings and low polarization index, they often overlook the other side of the equation. As an insulation system ages, it becomes dryer and more brittle. This changes how the insulation system reacts during operation in which the dipoles of the insulation must be able to align. As the insulation (dielectric) ages, the chemical changes that degrade the system’s pliability and dielectric condition, the insulation resistance and polarization index actually increase. It then becomes very important to track insulation resistance and indicate when there is a significant change in insulation resistance and polarization index over time. This may be an indicator of the insulation system aging.
In the case of this study, the insulation system had been aging over time. The act of taking it out of service, subjecting the winding to steam cleaning and baking, triggered the weakest component to fail. There were issues with the oven being set at too high a temperature, but not high enough to damage the insulation (high drying temperatures actually increase the drying time… but that is another blog). The failure, in this case, was the lack of understanding by both the repair vendor and associated engineers and trades involved that an increase in the corrected insulation resistance and polarization index indicated a fault.
Also noted was that a visual inspection would have identified the potential failure, and did, but that the instrument results trumped the visual inspection results. It must be remembered that the human senses are also important instruments when evaluating machines and that all testing methods have strengths and weaknesses. The visual inspections should have carried the same weight as the instrument results.
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