OMICRON Magazine

CASE STUDY – TEMPORARY PD MONITORING ON A ROTATING ELECTRICAL MACHINE A two-week temporary PD monitoring session was recorded with MONTESTO 200 on an aging 6.3 kV synchronous generator with an output of around 10 MVA in an industrial plant. Coupling capacitors were permanently installed to enable periodic online measurements and temporary monitoring on each phase of the generator and connected to the terminal box. The MONTESTO 200 acquisition unit was connected via plug-and-play to the same terminal box. The PD charge and voltage trend were recorded over the specified two-week time period to determine the insulation’s condition, which could then be compared with other generators of the same type, history, and age in the plant. The Phase-Resolved Partial Discharge (PRPD) patterns for all three phases as well as the 3PARD diagram with automatic clustering were generated for selected time stamps in the trend chart. The clusters that appear in the related PRPD patterns were individually selected to evaluate possible PD defects, which were separated from any disturbance signals. The probable type of the PD defect could be determined with a software feature specifically used for rotating electrical machines, called Pattern Classification. The monitoring software was then used to automatically generate a customized report with relevant data and charts. The results showed a slight but steadily increasing charge trend. PD patterns showing surface discharge defect can be seen on the end winding, which can mainly be explained by dirt and impurities. The increase in the charge values shows that the PD activity is increasing. Therefore, a continuous aging process is taking place. Based on the reported data, the user was able to document the stator insulation condition in terms of defect type and trend. The user decided that the documented results showed no immediate risk of machine failure. After a few months, a decision was made to repeat the PD monitoring sessions to observe the evolution and plan any necessary maintenance. The resulting PRPD patterns for the three monitored phases are generated for each of the six clusters automatically identified in the 3PARD. This helps you identify defects in the insulation. Possible causes of the documented PD activity are indicated using the Pattern Classification feature for rotating machines. Phase U Phase V Phase W 3PARD Full PRPD Vertical scale: Logarithmic, 100 pC to 100 nC Pattern Classification Evaluation: Abrasion of slot corona protection tape/paint – S3, phase U Cluster 1 Pattern Classification Evaluation: Abrasion of slot corona protection tape/paint – S3, phase W Cluster 2 35