Measurement variations of insulation resistance/polarization index during utilizing time in HV electrical machines – A survey

• Variations of insulation resistance index and polarization index during utilizing time in HV electrical machines is studied.
• It shows the results of resistance measurement during a period of time are variable and depend on different factors.
• The dependency of IR/PI variations on the current components, insulation conditions, and constant time is evaluated.

The concept of insulation system is chiefly concerned with the stator winding lifetime of the high voltage (HV) electrical machines. Along with the insulation testing, insulation resistance (IR) and polarization index (PI) techniques are suitable for different types of HV electrical machines and transformers. Since the power plant industries have to move toward higher reliability, the IR/PI accurate analysis needs to be optimized. This precise study is necessary, since the results of resistance measurement during a period of time are variable and depend on different factors. This point of view indicates the important role of the IR/PI variations assessment over the utilization time. Therefore, this paper is an attempt to evaluate the dependency of IR/PI variations on the main profiles as; current components, insulation conditions, and constant time. For this purpose, deviation of the resistance values in the mentioned tests are presented and discussed to achieve the optimum trend of IR/PI results interpreting. A comprehensive review of literature was done and industry experts were surveyed for their ideas and experience under a research project. The main results are summarized in this paper.