Monitoring Stator Insulation in Critical Motors: Choosing Diagnostic Tests

The petrochemical industry has increasing interest in the condition monitoring of stator winding insulation during service. The factory acceptance specifications developed from the American Petroleum Institute (API) standards often require a partial discharge (PD) and/or power factor tip-up (PFTU) test for brand new windings as a factory acceptance test before installation. Petrochemical industry users often employ PD detection systems for the condition assessment of critical motors, but the use of the PFTU test as a periodic evaluation technique is less popular. Specialized resources are needed to perform these tests at the factory and in the field, imposing different technical demands and requiring expert data interpretation to ensure value for the end user. PD and PFTU may serve as complementary techniques and are most useful when data are trended through the machine winding service life. Both types of measurement depend on stator winding geometry, insulating materials, insulation system construction, and operating environment. This article explains the PFTU and PD tests as applied to both factory acceptance testing and field testing on installed machines, including setup and execution of measurements, and explores technical considerations to define an appropriate test regime to monitor insulation condition. Examples of PD and PFTU test data from factory acceptance of new stator windings and on-site measurements of installed machines are discussed.