Guido Schmidt

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.

Ageing of corona protection material on rotating machines

The outer corona protection (OCP) is a field grading part of an insulation system, which is located in the stator slot area and the step iron area of an electrical machine. Recent publications show increasing numbers of ageing phenomena regarding the OCP, unrelated to a specific manufacturer. Although no direct correlation could be determined between the phenomenon and essential damage of machine insulation, it is necessary to establish a basic understanding of its main cause and the contributing parameters in order to estimate possible consequences for the machine operation and future designs.

Examination of anisotropic material characteristics in Outer Corona Protection (OCP) systems in large rotating machines

In large rotating machines the insulation system is extended by a field grading systems to provide an optimal electrical field distribution in the main insulation. In this study the anisotropic electrical material properties of the Outer Corona Protection (OCP) field grading system are analyzed. By modeling real OCP test samples and subsequent numeric calculation the systems field properties are characterized. Due to the consideration of complex spatial substructures, like the overlapping tape layers in the examined field grading system, the local field distributions will lead to an optimization of OCP resistivity configurations.