Life assessment of transformer using thermal models

Power transformers are very vital and costliest apparatus in electrical power network. A long lead time for delivery and huge cost warrants for optimize usages of Power transformer. Transformer loading, a function of current through its winding, leads to ohmic losses in transformer winding and enclosure. ohmic losses responsible for producing heat inside the transformer. Heat, temperature has an adverse effect on life of transformer insulation. Oil impregnated paper (OIP) insulation is the sole choice as insulating medium in power transformer. Life of the transformer which is life of OIP is a time -temperature function. Substation engineers are often interested in keeping track of consumed life and remnant life of transformer. Overloading of the transformer beyond nameplate rating involves risk and leads to accelerated rating. Before taking on any call on overloading, substation engineer needs to evaluate the impact on transformer reliability and impact on life. At any moment allowable overload (with additional loss of life) will depend on prevailing load and ambient conditions. Understanding thermodynamic towards temperature rise, mapping of temperature at various zone like top oil, winding and winding hot-spot becomes essential. Based on these; Loading (current) - Time - temperature characteristics can be forecasted. In this review paper, calculations of average winding, hot-spot temperature, acceleration factor for ageing, cumulative ageing will be covered and will produce time-load characteristics for long-time and short-time overloading. The calculation models shall cover all type of cooling modes in power transformers. As an outcome of the presented algorithm, a computer program shall be developed to compute top-oil and hottest-spot temperatures as a function of time and load, the allowable limit for overload that can be impressed on a transformer to meet specified limitations with loss of life. Based on calculations models, computer program can forecast loading (current) - time - temperature characteristics (winding hot-spot temperature, ageing acceleration factor and percentage loss of life).