Investigation Of Cooling Efficiency Of Separated Cooling System For Transformers In Underground Substations

Underground substations are mostly used in large cities to solve the contradiction between the sharp increase of electricity demand and the limited land resources.Due to different noise limitation and heat dissipation requirements,underground substations usually adopt separated structure,which means the transformer fuel tank and cooling system are arranged separately.The cooling efficiency of separated transformer cooling system is one of the important factors that influence the temperature field distribution of transformer and determine the aging rate of insulation.Accurate and fast calculation of cooling efficiency for separated transformer cooling system based on the transformer’s temperature field and flow field distribution is of great significance for evaluating the cooling performance of transformer cooling system and discovering the malfunction or maintenance deficiencies of cooling system.And the calculation of cooling efficiency for transformer cooling system is also an important project focused by domestic and foreign transformer manufacturers and scientific research units.Oil-immersed self-cooling separated transformer has many advantages,such as complete voltage capacity,good insulation performance and good reliability.It is most common in underground substations.The SZ11-80000 k VA/110 k V and SZ11-31500 k VA/35 k V oil-immersed self-cooling separated transformers are taken as the main research objects in this paper.Depth research on the calculation of temperature field and flow field for separated transformer is conducted,and the influence on distribution of temperature field and flow field for separated transformer caused by the arrangement of cooling system and the separation distance is analyzed in this paper.The accuracy of the calculation results is verified through the experiment of temperature rise of separated transformer.A calculation method of cooling efficiency for separated transformer cooling system is proposed in this paper.The calculation results of temperature field and flow field for SZ11-80000 k VA/110 k V and SZ11-31500 k VA/35 k V oil-immersed self-cooling separated transformer are used to calculate cooling efficiency of their cooling systems.The feasibility and validity of the calculation method for cooling efficiency are verified.The main research contents and achievements are as follows:(1)A typical oil-immersed separated transformer model is established.The finite element method is used to calculate the temperature filed and flow field for separated transformer.The effects of material parameters varying with temperature,mesh quality and simplification of transformer model on the calculation results are discussed.(2)SZ11-80000 k VA/110 k V and SZ11-31500 k VA/35 k V oil-immersed self-cooling separated transformers are taken as research objects.A three-dimensional calculation model is established for two types of separated transformer.The temperature field and flow field distribution of the transformer under different arrangement and separation distance are calculated.The influence of horizontal separation distance and vertical separation distance on temperature field and flow field distribution for separated transformer is analyzed.The temperature rise experiment of separated transformer is carried out through short circuit method.The temperature distribution of transformer fuel tank and radiator is measured by thermal resistance temperature measurement system during the experiment.The experiment results are in good agreement with the calculation results of model,which verifies the accuracy of the calculation model.(3)A method for calculating cooling efficiency for separated transformer cooling system based on energy conservation law is proposed.The cooling efficiency of separated transformer cooling system is calculated by using the calculation results of temperature field and flow field for SZ11-80000 k VA/110 k V and SZ11-31500 k VA/35 k V oil-immersed self-cooling separated transformer.The feasibility and effectiveness of the calculation method for cooling efficiency are verified.It can be used for the evaluation of transformer cooling system.