A Case Study of Low-Frequency Axial Vibrations in Synchronous Condensers

Abstract With the ongoing efforts of integration of renewable energies in electric grids, synchronous condensers play an increasingly important role in ensuring a reliable and flexible power supply. A large number of these machines has already been installed or will be installed in the near future. One particularity of these machines in their most widely used horizontal shaft configuration is the lack of a well-defined static thrust force. Low static radial forces on journal bearings have been reported to result in high radial vibration amplitudes associated with oil whirl or oil whip, yet no such mechanism is known for thrust bearings. In this case study, we show that low axial loads on a combined radial and thrust bearing can lead to low-frequency axial vibrations of high amplitude, even resulting in a thrust bearing damage. With below 10 Hz, the frequency of these vibrations is below the range usually monitored with acceleration sensors, making it hard to detect using state-of-the-art monitoring equipment and thus a potential undiscovered thread to numerous machines already running in the field. We further introduce a method of mitigation that has proven to be successful in avoiding these vibrations and their unwanted effects on machine behavior and bearing lifetime with minimum modifications of the affected combined bearings.