Hydrogenerators' Vulnerability Towards Bearing Insulation Systems, Shaft Voltage and Current

Due to their multiple and unique designs, it appears with time that hydro generating units have very different vulnerability to shaft and bearing currents. Moreover, the experience gained isn't easily transferred to other field engineers. Therefore, it is advised that at the very beginning of the design stage, the vulnerability to shaft currents of a specific design must be well known and understood, and with adapted instrumentation, its maintenance can be planned accordingly. This paper focuses on hydrogenerators' vulnerability to bearing failure by electrical discharge machining (EDM). The EDM bearing failure path will be recalled and explained. The vulnerability will be discussed based on six typical bearing insulation configurations. Based on experience and collected data including new statistics on a large fleet of hydrogenerators showing the evolution of the bearings insulation systems over the last 2 decades and risk of EDM is presented. Based on the new statistics presented, it is shown that bearing failure by EDM in hydrogenerators is still a source of interest due to the increase of shaft voltage sources and the vulnerability of bearing insulation systems.