Root Cause Analysis of Arcing in Retaining Rings of Turbogenerators

Abstract

Retaining rings are an important, and the most highly stressed, component of the entire turbogenerator system. Arcing in retaining rings is a very serious problem as it can easily escalate to a full-blown failure. In this paper, we investigate this arcing problem in the retaining rings. We determine the most likely mechanism by which arcing occurs and the type of events that lead to it. Specifically, we try to test two different mechanisms that could have led to the indications of arc. The first one is sparking due to high field effects (or high voltage gradient across the contact junction) and the other is a make-and-break contact arcing owing to the presence of inductance in the system. Experiments performed to measure the contact resistance between the retaining ring and mild iron piece indicate that even very high fault currents cannot produce enough voltage drop to cause sparking. Transient three-dimensional finite-element simulations show that a break in a relatively small contact region, which can be part of a bigger contact area, can generate localized voltage spikes that are high enough to lead to arcing. Interestingly, this can happen at relatively low currents. Furthermore, hardware experiments in the laboratory confirm the possibility of generation of arcs due to a make-and-break contact at low currents.