Research On Power Transformer Temperature Field Under Multiple Operating Conditions

Power transformer is indispensable electrical equipment in the power system and the reliable operation of power transformer is the basis for the stable power supply.Key factors that affect the operational status of power transformers include thermal issues and insulation issues.Paying attention to the temperature characteristics of the transformer in multiple operating conditions is of great significance to ensure its stable operation.When the transformer is in fault status and abnormal operation,the internal electromagnetic field will also change.The most common conditions of oil temperature rise include overcurrent caused by overload and out-of-area ground faults and the drop of oil level caused by oil leakage in transformer oil tank.This article compares the overload and single-phase ground fault conditions with the rated operating conditions and analyses the loss and temperature rise under the most common abnormal operating condition and fault status.Using the established model explores the factors that influence the temperature rise of the transformer.A 50MVA/110 kV oil immersed power transformer is set for an example in this thesis.Based on the theory of electromagnetic field,an electromagnetic field calculation model was established.Due to uneven three-phase core loss distribution,the three phase iron cores of transformer were modeled individually and the loss is calculated.The paper calculated the loss of iron cores and winding when power transformer is under rated conditions,occurs overload and Single-phase ground fault.It found that three-phase iron core loss distribution trends is similar to rated conditions and overload by comparison.When single-phase ground fault occurs,core loss decreases from rated operating conditions and the distribution trend is also different from the rated operating conditions.And winding loss is far greater than core loss and more vulnerable to operating conditions.Based on the theory of fluid and thermal field,a temperature field calculation model was established and the loss calculated by the model is as a heat source to input into the temperature field calculation model.Then this paper analyses the temperature field distribution characteristics of transformer core and winding under multiple conditions.It turns out that when single-phase ground fault occurs the temperature distribution is different from the rated operating conditions and the core hot-spot appears at the corresponding maximum load current.Hot-spot in the transformer exceeds the limit under single-phase ground fault condition.The temperature distribution trends of cores and windings under rated conditions and overload conditions are similar.The average temperature rise of the winding exceeds the limit value under overload conditions.The effectiveness of the proposed method is verified by comparing the calculation results of this paper with the traditional methods.This paper used the established temperature field model to explore the possibility of FR3 plant insulation oil instead of mineral oil.Based on calculating the influence on the transformers temperature rise when the temperature of external heat source changes,this paper calculated the relative life loss rate of the transformer under different ambient temperatures.According to the calculation results,FR3 vegetable oil can replace mineral oil in terms of temperature rise characteristics;The transformer core and the winding hot spot temperature increase with the ambient temperature rise.When the ambient temperature exceeds 293 K,the loss rate of transformer’s relative life increases very quickly.Therefore,the transformer should not be exposed to temperatures exceeding 293 K for a long period of time.