Routine diagnostic measurements of high-voltage machines

Describes a program of regular diagnostic measurements on the stator insulation of in-service high-voltage rotating machines. The results of 2639 measurements (over a period of 20 years) on 3.3 kV, 6.6 kV, and 11 kV machines have been accumulated in a database. Statistical analysis of typical insulation parameters, e.g., tan (δ), distributions provide reference values against which future measurement results can be evaluated. The insulation condition is assessed by normalizing and combining the measurement results to obtain a merit mark for the machine stator insulation. Repeated measurements on machines show clear trends in deteriorating merit mark with increase in insulation age. It is concluded that the diagnostic measurements and the merit mark system can, in the hands of the experienced test engineer, serve as a useful decision aid in alerting the operator of the machine to a potentially fatal deterioration of the stator insulation condition