1. Introduction to Condition Monitoring of Transformer Insulation

Deregulation of electric supply around the world has led to a number of changes and new challenges for the electric utility industries. In the changed scenario electric power utilities are trying to enhance revenue streams and to reduce incremental cost per unit electrical power by various means. Power transformers are the most expensive and strategic equipment of any electrical power system. Over the years operating stresses of power transformers have increased due to load growth and increase in voltage level. Insulation failure of power transformers results in substantial costs of repair and huge financial loss due to inadvertent outage. In the context of maximum utilization of power transformers with long service life, it is necessary to determine the adequacy of the insulation system of a transformer before installation. Design acceptance tests are performed on transformers for this purpose, the most important of which is the impulse voltage withstand test. Minor local defects in transformer insulation often manifest itself as partial discharges (PDs), which may cause catastrophic failure in the long run if allowed to continue unnoticed. Thus PD tests have emerged as one of the significant tools for identifying local defects within transformer insulation. Large numbers of oil-immersed type power transformers, which are still in service all over the world, have outlived their designed service life. Studies have shown that the most common cause of failure in aged transformers is general aging of oil-paper insulation system. Age related degradation is accelerated in the presence of moisture in insulation and also at elevated operating temperatures. Thus it is economically beneficial for power utilities to assess the overall condition of transformer insulation system with the aim to minimize the risk of failures and to avoid forced outages. Practicing engineers are using chemical techniques for assessing the overall condition of transformer insulation for a long time. The most important chemical techniques for condition monitoring of transformers are: (i) Dissolved Gas in Oil Analysis, (ii) Determination of Degree of Polymerization of Paper and (iii) Furan in Oil Analysis. In the past, these methods have enabled power utilities to anticipate developing problems and plan maintenance activities accordingly. In recent past, new non-invasive diagnostic methods have been proposed which are complimentary to classical electrical measurements. These methods are based on measurement of dielectric response of transformer insulation in time domain, viz. measurement of polarization and depolarization currents and recovery voltage measurement, or in frequency domain, viz. frequency domain spectroscopy (FDS). Large mechanical forces act on the transformer windings and mechanical structure when subjected to fault currents during its service life. These mechanical stresses can lead to winding deformation which could ultimately lead to major failure. Frequency response analysis is a diagnostic approach which looks for changes in the frequency response signature of a transformer which would be indicative of winding deformation. From the point of view of asset management, an important issue before the operational and planning staff of power utilities is to make a rational decision on the remaining life of aged transformers. The best-known and most widely used relationship in this context is the one between life expectancy and thermal loading of transformers using temperature rise as the main index of aging. But other methods of remaining life analysis based on measurement of degree of polymerization (DP) and also furanic concentration in oil are being investigated to offer reliable information taking into account complex thermo-chemical degradation of transformer insulation.