as a Distinguished characteristics, Big High-Power Rotary Electric Machines (BHPREM) are not manufactured in series; besides, their individual Price Per unit is very high. That is why there has been a world-wide special interest in knowing the state of these machines at different period of their useful life, so to prevent unexpected damages to these machines. The working conditions of the main insulation in BHPREM are characterized in this paper the forms of interaction of the problems of main insulation in BHPREM concerning the parameters that can be measured throughout dielectric tests are also established. Besides, a general methodology for diagnosing the main insulation state of the BHPREM was obtained. This paper presents the concept of volume of equivalent void [1] and using this parameter for evaluation of Partial Discharges in solid insulation by indirect methods.