Research On Load Capacity And Life Loss Of Oil Immersed Distribution Transformer Under Three-phase Unbalanced Disturbance

In the distribution network,three phase unbalanced operation of distribution transformer are widespread.This will cause an increase in transformer losses,raise the temperature of hot-spot,accelerated thermal aging rate of oil paper insulation,and it will reduce the transformer operation life and load capacity.In this paper,equivalent circuit model is established for Dyn11 and YynO two kinds of connection ways of distribution transformer.The loss calculation method of distribution transformer under unbalanced three-phase power disturbance is analyzed,and the loss is studied.Theoretical analysis and experimental results show that,the transformer which work under the situation that three-phase unbalanced makes loss increased dramatically,and with the increase of the unbalanced degree,the loss curve is a quadratic function.When the unbalanced degree reaches 50%,the wastage of the transformer will be doubled.In order to study the influence of three-phase unbalanced on the temperature rise of distribution transformer.To solve the above problems,this paper derives the calculation model of the hot-spot temperature of transformer windings under three-phase unbalance.A test platform for the temperature rise test of a distribution transformer under three-phase unbalanced is designed and built.Through the analysis of the theoretical and experimental results,According to the hot spot temperature limit and the allowable overload multiple,the evaluation model of the overload capacity of the distribution transformer is given,and the constraint conditions of the distribution transformer are determined.These can provide reference for the safety and economic operation of the oil immersed distribution transformer.The increase of loss and hot spot temperature will not only affect the economic performance and load capacity of distribution transformers,but also affect the safety and operation life of it.So in this paper,according to the hot spot temperature calculation model of distribution transformer in three phase unbalance,and the oil immersed transformer insulation life loss calculation model of "oil immersed power transformer load guide",the life loss assessment model of distribution transformer under three phase unbalance based on hot spot temperature is given.This provides some technical support to get the appropriate time window for examine and repair of the distribution transformer and get the right time through the life assessment to replacement of the equipment.