Significance of cellulose power transformer condition assessment

Power transformers are designed to withstand system abnormalities such as over voltages and external short-circuits. Transformer monitoring and diagnostics are the effective techniques in preventing the eventual failures and contribute to ensure the plant´s reliability. Transformer life mainly depends on the integrity of its solid insulation (cellulose). The solid insulation in transformers degrades with time at rates which depend on the temperature and the amount of moisture, oxygen and acids in the insulation system. Moisture and oxygen cause the paper insulation to decay much faster than normal. Moisture in a transformer accelerates the aging process and causes severe deterioration in the mechanical and electrical properties of insulation system. The aging of paper insulation is irreversible. The degree of polymerization value is a main relation between insulation deterioration and formation of aging products. To predict a failure or an unusual behavior, an accurate interpretation of monitoring, diagnostics and maintenance data is required. The monitoring of insulation system and verification of core and winding mechanical integrity allow optimization of the lifecycle management of an asset. This paper presents an investigation and condition assessment of 152 MVA, 512 kV, 1-phase, 2-winding generator step-up transformer having a history of combustible gases.