Application of some novel non-destructive diagnostic tests for condition assessment of stator coils and bars following voltage endurance tests

With ongoing development of novel nonpartial discharge-based test methods that show promise as diagnostic tools in the cable industry, there is an increasing interest to determine if these tests have the ability assess the condition of generator stator insulation systems. One reason for this is that after data interpretation of modern online PD measurements has identified a problem and some maintenance is performed, there is still a need to conduct some diagnostic tests to benchmark the condition of the machine. Also, utilities are recognising that there are potential limitations with simple "go" "no-go" DC or AC hipot tests. Some of the new cable diagnostic test methods such as dielectric spectroscopy, low frequency tanδ, recovery voltage method and leakage current measurements are now being considered. Before these test methods can be employed directly on generator insulation systems for condition assessment, it is important to first examine issues related to test procedures, identify the merits, and determine limitations (if any) of these test methods in a laboratory environment. This paper describes the preliminary results of work that was done at Powertech Labs using two of these diagnostic test methods. Bars/coils of different vintage and manufacture that were previously tested using conventional test methods (such as PDA, DF and VE) as well as, virgin bars/coils were recently benchmarked using dielectric spectroscopy and Powertech\´s LIpATEST.