Electrical braking of synchronous generators for combined generator and water turbine bearings as well as stray-load losses determination

This study describes a new method for determining combined synchronous generator and water turbine bearings losses as well as stray-load losses of a generator, based on measurement of the speed change during electrical braking. In the electrical braking regime, the total braking torque of a hydrogenerator is made up of the torques defined by losses in the runner chamber of the hydraulic turbine, ventilation losses in the hydraulic-turbine generator, bearings losses in the generator and the water turbine, as well as electromagnetic and stray-load losses in the short-circuit. In this study, the torque-speed curve is represented by the corresponding equations which describe all the torques occurring during electrical braking. By applying these equations to the measured results and using some numerical calculations, combined generator and water turbine bearings losses as well as stray-load losses can be obtained. The proposed method is applied on a 190 MW generator at the Hydro Power Plant (HPP) Djerdap (in Serbia). The obtained results demonstrated a very high level of accuracy when compared with the values found in the manufacturer´s technical documentation (Electrosila). The proposed method has the best and most economical use in large synchronous generators and generators in older power plants.