PDC measurements to assess machine insulation

DC testing is probably the most commonly used maintenance and diagnostic tests periodically conducted on machine stator insulation systems. With the commercial availability of more sophisticated equipment it is now possible to continuously monitor both charge and discharge current during a step voltage test, also known as polarization/depolarization current (PDC) test. This test is related to the dielectric response of the insulation system. This paper presents theoretical considerations on the dielectric response of the various types of machine winding insulation systems encountered in the field and the usefulness of using both the charge and discharge currents to assess the condition of the insulation system.