I. Introduction
The search for a practical and reliable diagnostic method to assess the condition of large rotating machine stator insulation system has been ongoing for decades. Diagnosis of electrical apparatus by de testing has been a much appreciated method to get to know the state of the high voltage to ground insulation [1], [2]. There is a good reason for this regarding the pragmatic advantages of using de testing equipment. For rotating machinery winding insulation, time domain tests, such as step voltage and ramped voltage tests have been indeed used since decades with a certain degree of success [3], [4]. Findings drawn: from the measurement of the dielectric response, either in the time or the in frequency domain, become particularly useful when the dielectric response of the unaged insulation system is known, Dielectric properties are function of different parameters such as the magnitude of the applied voltage, temperature, humidity, the way the insulation component are laid out and its degree of ewing. This paper presents polarization/depolarization current (PDC) results obtained from hydrogenerator bars insulated with different modern epoxy-mica insulation systems. Some results using the same technique on bars with known defects are also presented.