RESIDUAL LIFE ASSESSMENT AND LIFE EXTENSION TECHNIQUE OF TRANSFORMER

摘要

The vertically integrated power utilities, existed over later half of twentieth century in India, has transitioned to horizontal organisations of 'Gen Cos', 'Trans Cos' and 'Dist Cos'. The deregulation allowed private sector participation within laid down frame work in all the activities of Generation, Transmission and Distribution. The power equipment have mostly exhausted their serviceable life. The continuous running of these assets itself a challenge. Utilities thru System Management are estimating the residual life of the equipment and making the system available with assured continuous power supply free of ripples, hormonics, dynamic over voltages etc. Power Transformer is a major and vital component of supply system and its reliability and availability shall be of comparatively higher than others in a S/S. Most of the Grid S/Ss configurations so planned that their components either physically duplicated or provided with duplicate operational accessories except for power Transformer. Any transformerr fault ends up partly/full loss of S/S power depending upon network condition. A power utility suffering unplanned loss of power transformer depending upon network criticality loses revenue as also put to penalty for loss of power/system stability as per Regulatory Policies. Most of the existing power transformers have reached near their useful life completion under normal operation or subjected to accelerated ageing on their exposure to operational stresses (full faults, insulation degrading, mechanical stresses, etc.) and such fleet is required to be evaluated for residual life and life extension. Condition assessment is performed thru analysis of historical data and performing dielectric tests, partial discharge tests, vibration tests, oil analysis and checking serviceability of major accessories-OLTC contacts, PRV, Bushings, LAs, Valves, etc. These studies facilitate assessment of residual life and any modification/replacement carried out with latest devices result into life extension. A Trade-off between life enhancement and investment or go for replacement with new one and use this transformer at some other location needing low operational duties is left for decision under Asset Management Plan. The paper includes types of gases & their allowable quantities, types of fault inside the transformer winding causing liberation of those gases, deposit and damage of winding insulation and advanced diagnosis tools to ascertain present winding condition before un-tanking the transformer for repair. Power Transformer manufacturing facilities at works and site are characterised for Controlled Environment (Cleanliness and Orderliness), Heavy Lifting Equipment for detanking of core and coil assembly, Manufacturing of winding and application of insulation and dried and oil impregnated operation followed by packing in positive dry air pressure maintain low moisture content, Core and coils subsequent to assembly and placed in the tank for final drying and during On-site Drying process, the moisture is reduced to below 1%. High Voltage testing including applied voltage & induced voltage test and PD measurement, of assembled transformer. Other add ons (old/new for capcity uprating) are fitted at Client site/repair site as the situation arises In Brazil, Italy, Spain and Nigeria, stationary transformer manufacturing/servicing hubs have been in operation by some manufacturers and a few other suppliers/manufacturers have mobile shops for servicing/repair of transformers.