The stators of large generators in electric power stations are equipped with end windings that connect the stator to the electric grid and guide the generated stator current through the generator. These end windings frequently exhibit white dusting and greasing at various locations during outage inspections.
The main contributor to this early ageing, as often pinpointed by the maintenance companies, is the local presence of vibrations, caused by a combination of complex electromagnetic and mechanic forces (Lorentz forces, Maxwell forces, thermal forces). The assessment of the vibration behaviour of generator stator end windings requires a perfect comprehension of the present forces and how they generate the vibrations.
Laborelec developed a methodology to assess the vibration sensitivity and behaviour of generator stator end windings. The methodology consists of three steps: visual evaluation, offline assessment of the vibration sensitivity and online monitoring of the vibration behaviour. The visual inspection will map the locations where damage is present and will identify what mechanism will most likely generate this damage, related to vibrations or to other causes. Then the offline assessment is performed by means of impact testing with the application of specific experimental modal analysis techniques on the impact test results. This assessment will reveal which vibration mechanism is potentially present and generating the damage. If a vibration suspicion is confirmed, one can install an online vibration monitoring system. The online monitoring is performed with specific instrumentation combined with signals from critical generator process parameters and focusing on specific frequencies. Combining these three techniques, it is possible to have a full assessment of the vibration behaviour and to validate the presence of vibrations as main contributor to the early ageing of the generator stator end windings.
Olivier le Fevere de ten Hove, 36 years old, is an expert in vibrations and mechanics of large power generation turbines. He has more than 10 years of experience in vibration analysis and vibration troubleshooting. Next to permanent monitoring, he has specialized in specific vibration testing (impact testing, modal analysis, operational deflection shape measurements) using various equipment. He has also conducted trim balancing operations on various turbine types. He has specialized in the dynamic behaviour of generator stator end windings and has been involved in several root cause analysis cases. Since 2007, he has conducted several studies related to generator stator end windings issues, in Europe and in the Middle East.