Synthesis of Experiences using Resistive Temperature Detectors (RTD) as PD Sensors for Detecting and Locating Electrical Defects inside Generator Stator Windings

It is generally accepted within the industry that terminal partial discharge (PD) measurements are less sensitive to electrical discharges deep within the stator windings. Typically, on large generators, electrical defects beyond 10-15% from the line-end of the stator winding cannot be detected. This equates to first 2 to 3 coils from the line-end. Therefore, other measures are necessary to expand the zone of detection. Large generators and motors are typically equipped with 12 or more resistive temperature detectors (RTDs) embedded into the stator winding. The RTD acts as an RF-antenna placed into a winding that is sensitive to the high frequency component of an electromagnetic pulse caused by PD. The RTD is both capacitively and magnetically coupled to the PD pulse originated by a discharge. Calibration tests show that the RTD is useful in assessing PD levels within a few adjacent slots from its location. Using case examples, comparison between two forms of PD measurements is presented - (a) Conventional terminal PD measurements using three 80pF epoxy mica capacitors on the line-side of each stator winding-phase; (b) Deep winding PD monitoring using embedded resistive temperature detectors (RTDs) as PD sensors. For the latter case, few RTDs in each stator winding-phase were selected to cover high and low potential slots in the winding. Detecting PD at lower potential slots using RTDs allows to detect faults caused by loose bars in the slot, “vibration sparking”. This report synthesizes the overall PD monitoring experience using collection of experiences from hydro and turbo generators. The focus is placed on demonstrating how RTDs have helped catch faults close to the winding neutral. Defect location capability of RTD PD sensors is demonstrated and results compared with corona probe measurements, which is traditionally used for defect location.