Analysis of resistance characteristics of multilayered field grading material structures on rotating machines

The proper resistance range for field grading constructions, such as the Outer Corona Protection (OCP) and the End Corona Protection (ECP), on rotating machines appears to be an essential parameter for correct performance of the entire insulation system. The insulation system of high voltage stator coils or bars usually is assembled using tape material with a certain overlap. Furthermore field grading systems with highly complex structure, such as e.g. the so called “Double-OCP-System”, were established to handle thermo-mechanical stresses occurring on large GVPI turbine generators. The complexity as well as the variety of designs makes it difficult to define the targeted system resistance value. Furthermore electrical properties measured on tape material cannot be simply used to describe the behavior of entire system. The approach in this work is to understand how the targeted resistance of complex field grading structures can be approximated based on properties of tape material. Or in reverse, how the tape material must be designed to achieve the targeted overall resistance on a global structure.