Shaft Current Measurement and Failure Prediction in Hydrogenerators

Circulating currents in the bearings of electrical machines are known to cause electrical discharge or local overheat that could lead to a bearing damage. When it comes to hydrogenerators, there is still today a lack of information, measurements, and statistics, and many challenges remain. After studying this phenomenon in a large fleet of hydrogenerators from tens to nearly 500 Megawatts, it appears that hydrogenerators have different levels of vulnerability to bearing damage caused by shaft current. Shaft voltage coming from the stator core air gap and high frequency currents coming from excitation systems equipped with thyristors’ bridge have been identified to be the main source of bearing damage in hydrogenerators. As it could be challenging due to the absence of standards, measurement techniques have been experimented and discussed for voltage and current measurement in hydrogenerators. An optimized measurement system, cost effective and 100% non-intrusive, is proposed. A case study on two units is presented using this measurement system. Finally, based on experience and measurements, a comprehensive path to a bearing failure is presented and a design-based prediction method is proposed to evaluate the risk of a specific design based on the probable presence of an armful shaft voltage and the vulnerability of the bearing.