Recently, the effect of harmonic current content and ambient temperature has been linked to many of the failures seen in distribution transformers. The reason is the increase in transformer losses due to increased harmonics, specifically current harmonics. An increase in the harmonic content of the load current will create extra losses in windings, leading to an increase in hot spot temperatures and in the stress on the insulation of the transformer. These above-mentioned factors will decrease the useful life of the transformer due to the decrease of the useful life of the insulation and the transformer's loading capacity.
Distribution transformers are typically created to provide power to devices that require a consistent and steady electrical current. However, there are instances where the voltage and current can become distorted due to non-linear loads, resulting in an increase in harmonics and other irregularities. In the past, non-linear loads made up only about 15% of the total power consumption, but in the year 2000, this number increased to 50%. As a result, the presence of non-linear loads leads to a higher harmonic content in the network. This increased non-linear load has many disadvantages such as an increase in the loss and reduction of the efficiency of power system equipment. Therefore, the impact of harmonics on the loss of useful life and reduction of efficiency in power transformers needs to be investigated. The reduction of life in transformers can be due to hot spot temperatures in the transformer winding, and this increased temperature can be a direct effect due to harmonic presence. The hot spot temperature is in direct correlation to the winding temperature and top oil temperature which is an important parameter to ensure efficient transformer monitoring. One of the most limiting factors in transformer loading is the hot spot temperatures. To evaluate loading ability awareness, the hot spot temperature is required. Thermal stresses are one of the most important factors influencing insulation deterioration, therefore hot spot temperature data are crucial to ensure better evaluation of the loading ability of a transformer, the used lifetime as well as the determination of the possible remaining lifetime of the transformer.
At an IEEE Transformers Committee meeting in March 1980, it was recommended that a standard is provided for guidance in estimating the loading capacity of the transformer in a network with distorted currents. After this, IEEE C 57.110 entitled "Recommended procedure for determination of the transformer capacity under non-sinusoidal load currents" was published. This standard determines the procedure to decrease the level of the rated current for risen harmonics.