Increasing of Power Oil-Filled Transformers’ Operation Reliability and Safety

Abstract

This paper is devoted to the currently urgent problem — increasing reliability and safety for power oil transformers’ operation with considerable long and short overloads through cooling systems’ efficiency upgrading, allowing increase the power oil transformers’ operation safety, engine life, reliability and economical efficiency. Statistical data on fault causes for transformers of different voltage classes and lifecycles have been given. Based on the study of statistical data it has been established that one of the main causes for transformers faults was the inefficiency of oil cooling systems during the summer period of operation and with considerable long and short overloads. For improvement of efficiency of power oil transformers’ cooling system the agitation of cooling oil by circulating sulfur hexafluoride (SF6) and subsequent sulfur hexafluoride cooling with a thermoelectric cooler is proposed. The system of sulfur hexafluoride circulation and cooling switches on automatically at considerable long and short overloads. The design and operation of the proposed cooling system for oil transformers have been considered. A series of laboratory experiments for transformer oil cooling in a power transformer’s tank without and with a compressor has been carried out. Graphs of temperature-time relationship at natural cooling and when using agitation have been presented. The coefficients of transformer oil’s thermal diffusivity and heat transfer at various distances from heating element have been experimentally defined. The mathematical problem of a heat flux distribution in a rectangular parallelepiped has been considered, a solution for thermal conductivity equation in the power transformer’s tank, which is a rectangular parallelepiped, has been presented. A laboratory setup design has been described in detail. Brands of used thermocouples, compressor and analog-digital converter have been presented. Use of the upgraded cooling system will allow increase the transformers operation safety and reliability.