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Ambient and test conditions can affect the analysis results of the insulation system testing in high voltage (HV) electrical machines. In this context, insulation resistance (IR) and polarization index (PI) are two common techniques for winding insulation assessment in offline mode. This paper investigates the influence of main parameters that affects IR/PI tests results. Temperature, humidity, and pressure are the wellknown environmental parameters which could affect dielectric parameters of insulation such as IR and PI. On the other side, test voltage level and voltage polarities connections are two prominent factors in sampling the resistance during the test. Moreover, the insulation resistance value depends on the type of fault and its severity. The variations of insulation resistance with considering the mentioned parameters are evaluated in this paper, to study the possible correlation between IR/PI and changes in other parameters. It was found that with taking the effective parameters into account, the accurate and reliable insulation test interpretation can be achieved in HV electrical machines. 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.

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