Increased partial discharge was observed on a vertical, 25 MW, Kaplan, umbrella type unit. The decision was made to perform additional diagnostic measurements. A detailed slow-roll air gap and vibration measurements were performed and the loose stator core problem was detected. The loose core vibrations were related to the increased partial discharge.
First, slow-roll air gap measurement was performed with two, temporary mounted, air-gap sensors. One sensor was mounted on the stator core and the other on the rotor pole. Machine was rotated manually on lift oil with the sensors in the lower (first measurement) and upper (second measurement) plane of the stator core. The results obtained were the rotor and stator shapes in the measurement planes.
Second, vibration measurements in all relevant regimes were performed. Besides sensors being mounted on the bearings, one accelerometer was mounted on the stator frame, exactly at the joint between the two frame segments. The unit’s frame / core consists of three segments, in total. As indicated by the stator frame vibrations, the stator frame / core system passed through resonance during core temperature increase.
The main reason for the resonance was in the temperature dependent structure stiffness. The loose core segments were mutually loosely coupled when the core was cold having lower stiffness. As the core temperature increased they formed a more and more rigid ‘ring-shaped’ structure shifting the natural frequency of the system to the higher frequencies. This was detected from the stator frame vibrational response.
All of the measurements were performed using the CoDiS-PDS portable diagnostic system measurement device.
Additional vibration measurements regarding the stator frame and core are proposed with the emphasis on establishing vibrational modes. This should give an idea how to approach the repair process.