Stator Winding Insulation - Part I Delamination and voids

Stator winding insulation is a laminated system consisting of numerous layers of mica-paper tape on a fibre-glass backing material impregnated and consolidated with a synthetic resin, usually epoxy or polyester-based. The stator windings of rotating machines built prior to 1970 are likely to contain a bituminous resin instead of epoxy or polyester. Further, these older insulation systems tend to employ large flake mica rather than mica-paper. Consequently, the results derived from any CM tool that is focused on stator winding insulation must be viewed in the context of the differences in these materials. While the older, thermoplastic resins flow at relatively low temperatures, around 70?C, causing resin migration leading to void formation and embrittlement of the main ground-wall insulation, the high mica content provides significant resistance to PD deterioration. Further, the propensity of bitumen-based insulation to soften at elevated temperatures per- mits the ground-wall to conform to the walls of the stator core slot and render a degree of self-healing. The lack of this property in modern, thermoset insulation systems based on synthetic resins has resulted some problems to be discussed below. Consequently, the use of age as a reliable index of machine health is not supported.

Voids or delaminations in the ground-wall insulation of stator windings may result from the manufacturing process and/or operating stresses. The presence of voids in new stator windings, although not desirable and should be minimised, does not necessarily imply that the winding be rejected or that it is not fit for the design life intended. Application of various diagnostic tests, such as PD and dielectric loss, as well as potentially destructive overvoltage tests aid in the production of stator windings with minimal void content. During the life of the machine, these initial delaminations, or those initiated by thermal, mechanical or electrical stress may result in the growth of voids prone to PD. The probability that a void will be subject to PD is governed by a number of factors such as void dimensions, electrical stress, pres- sure, temperature and the presence of initial electrons to cause discharge inception.

A PD is so called because it represents a gas breakdown between the two dielectric surfaces or between a conductor and a dielectric surface. Details of the physics of the process can be found in Pedersen?et al.?(1991). PD, similar to any gas breakdown, results in the emission of heat, light, sound as well as the production of electrons and ionic species. Depending on the energy of the discharge, erosion of the void walls will result causing growth of delamination and potentially failure in the long term. However, from a practical perspective, stator winding failure caused by internal void discharge is very uncommon and is not considered by manu- facturers and users of rotating machines to be a significant problem. There are two principle reasons why void discharge does not lead to rotating machine failure:

●?Presence of mica, a material extremely resistant to electrical discharge attack, forces PD erosion to occur only in the organic binding resin component. Consequently, the electrical breakdown path must follow a very circuitous route from the initiation site to the grounded core steel before failure can result. Typically, the initiation point is located at the edge of the copper conductor stack. Thus, the time-to-failure for such a process is very long, in the order of decades.

●?Application of on-line PD monitoring equipment as well as advances in the interpretation of the data produced by these tools. Use of PD monitoring equipment has enabled machine users to better determine the condition of stator winding insulation in operating machines and to take corrective action at early stages. Although there is little maintenance available to remediate the presence of voids in the ground-wall insulation, actions such as maintaining the integrity of the slot support system or reducing the operating stresses on the machine may arrest the process of PD delamination.

Thus, although void discharge has been and continues to be, a subject of intense study by academic and other research organisations, failure of stator winding insulation from this mechanism is of low probability. The use of PD measurement may be of value, however, because PD is a symptom of other stator winding deterioration mechanisms that can cause failure. These mechanisms will be discussed below, therefore PD can provide a useful history of machine winding insulation.