Scientists from Tomsk Polytechnic University and Université Grenoble Alpes (France) have proposed a more accurate method for loading capability assessment of power transformers. As an example, the scientists defined the loading capability of the power transformer in Tomsk and Grenoble. The research findings are published in the International Journal of Electrical Power and Energy Systems (IF: 3,588, Q1).
The cost of a power transformer can reach a few hundred million rubles that makes it the most expensive element of an electrical grid. Therefore, power engineers tend to use the loading capability of transformers in full. The method proposed by the TPU scientists can help to operate power transformers closer to their thermal limit. It will allow operators to control power systems of higher transfer capability, grid electricity suppliers to postpone investment to change power transformers, end-users to use cheaper electrical energy.
“Nominal rating of most kinds of power transformers is designed for an ambient air temperature of 20°C. Nevertheless, the ambient air temperature is inconstant and changes during a day, month, year. Due to the temperature changes, the actual admissible loading of a transformer can be either higher or lower than its nominal rating.
In both cases, the admissible loading has to meet four limitations: current of a transformer, the hot-spot temperature of a winding, a top-oil temperature and thermal wear of insulation. The existing methods of the admissible loading assessment do not take into consideration all of these limitations simultaneously, as well as admissible short-term overloading of transformers in normal operating conditions,” Ildar Daminov, a postgraduate of TPU and Université Grenoble Alpes, a co-author of the article, explains.
The TPU researchers proposed a feasible region method. Due to this method, using the full-scale current, the winding and oil temperatures, it is possible to calculate the entire region of admissible loadings. Moreover, besides the characteristics, it is enough to know only the air temperature to calculate the region of admissible loadings. It allows defining admissible long-term loadings and admissible short-term overloadings of transformers taking into account practically all possible load profiles.
As a calculation example, the scientists defined the loading capability of an ONAF transformer based on the analysis results of the ambient air temperature in Tomsk and Grenoble during the last 35 years. The results showed that depending on the air temperature, the loading limit of the researched transformer can surpass its nominal rating on average from 15% to 45% in Tomsk and from 5% to 41% in Grenoble. Furthermore, the loading capability of the transformer in Tomsk can surpass the nominal rating for 88,5% of the time (even without taking into account admissible short-term overloading), as cold Siberian climate allows effectively withdraw heat from power transformers. This indicator is lower and equal to 79% in Grenoble.
“The power transformer researched by us is designed based on International Standard IEC 60076-7. It operates using rather universal thermal characteristics, which in general meet a variety of transformers. Hence, the research findings can be applied to transformers of alternative design,” Anton Prokhorov, Associate Professor of the TPU Division for Power and Electrical Engineering, a co-author of the article, says.
As a result, the scientists listed recommendations on transformer overloading. Thus, it is admissible to increase loading to 120°C during 26 days per year or to 140°C during two weeks for a transformer with a winding temperature of 98°C in normal operating conditions. Transformers with a standard temperature of 110°C can be overloaded to 120°C during 98 days per year or to 140°C during 20 days per year.
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