scope:

This guide provides technical information regarding factors that can affect the life of an impregnated paperinsulated transmission cable system, and it reviews available methods to evaluate the remaining life of such systems and preventive maintenance to extend their service life.

Purpose

This guide provides technical background, discussion, and advice to those needing to evaluate the remaining life of impregnated paper-insulated transmission cable systems. Guidance is given for both high-pressure fluid-filled (HPFF) (see Association of Edison Illuminating Companies CS2-97 [B12]¹) and self-contained liquid-filled (SCLF) (see Association of Edison Illuminating Companies CS4-93 [B13]) cables, which use a dielectric liquid as the filling and pressurizing medium (dielectric fluid), and for high-pressure gas-filled (HPGF) (see Association of Edison Illuminating Companies CS2-97 [B12]) cables, which use nitrogen gas as the filling and pressurizing medium. The information presented here is intended to provide a complete and concise summary and overview, with frequent references to the technical literature for those who wish to investigate specific subjects in more detail. Emphasis is placed on practical, realistic, and economical methods for performing field and laboratory sampling and testing, and later analysis to determine the actual condition of a cable installation.

This guide describes the various aging mechanisms that act on cable components. This includes aging of the outer protective coverings and the fluid-containing sheaths and pipes. It also includes thermal-mechanical aging of SCLF cable sheaths and their mechanical aging due to creep and fatigue, and thermal-mechanical aging of HPFF insulation due to flexural fatigue resulting from cyclic loads. Thermal and electric stress aging due to cable heating effects and the applied voltage are covered as well.

Methods are presented to assist in evaluating the extent of cable aging. Both intrusive and nonintrusive techniques are described; however, emphasis is placed on the latter. Treatment of the subject includes a description of traditional evaluation methods used in the past, supplemented by newer and emerging methods. Emerging methods include dissolved gas and metals analysis of dielectric fluids, as well as tests for other byproducts of the decomposition of cellulose, which can be used to gauge insulation aging and diagnose incipient failure mechanisms. Finally, advice is provided on preventive maintenance methods, for use in extending the actual service life of a cable system well beyond its economic life.

It is intended that this guide be revised periodically to reflect new developments in diagnostic technology and experience gained with long service-operating cables.

¹The numbers in brackets correspond to those in the bibliography in Annex A.