Study of stress grading systems working under fast rise time pulses

Stress grading coatings of cable and coil terminations are considerably affected by the high dV/dt present in PWM voltage source converters. As repetitive steep transients can result in the development of hot spots and enhanced electric fields in the stress grading (SG) system, premature failures of power apparatus can occur. Stress grading systems need to be improved to work under such conditions and to obtain the desired stress relief. In this work, a stress grading system that can help to control the electric stress as well as the hot spots under fast rise time pulses is studied. It is important to design and test these systems considering the real PWM waveform in which both the fast pulses and the fundamental low frequency are included. With that purpose a fast single-phase low power two-level PWM generator was used to evaluate the stress grading system. Also this generator was used to measure some of the dielectric properties of the material.