Research On Short-circuit Resistance Of Oil-immersed Distribution Transformer

The distribution transformer is one of the important equipment in the distribution network,and its safe operation plays a very important role in the normal operation of the entire power grid.However,China’s distribution failures are frequent,and the short circuit failure rate accounts for more than half of the total distribution failures,which has caused great problems to people’s production and life.At the same time,due to the variety of distribution core structure and winding structure,distribution Accurate verification of variable short-circuit resistance has become an urgent problem to be solved in the transformer design industry.Therefore,it is of great practical significance for the engineering design to improve the research method for the analysis and calculation of the distribution short-circuit fault.The main content of the research of this article includes three parts:First,the electromagnetic characteristics of the distribution transformer short-circuit reclosing and the calculation method of fault inrush current are studied.Based on the JA hysteresis theory to consider the magnetic hysteresis characteristics of the core,a permanent fault simulation model of the power system with the distribution as the core is established,the influence of the residual magnetic direction on the short-circuit current after reclosing is verified,and the different short-circuit faults are subjected to closing The law of the short-circuit current affected by the angle,and the focus is on the analysis of the dual influencing factors of the residual magnetism and closing angle of the three-phase short-circuit fault.The law of the change of the short-circuit current and the influence of the two on the current are obtained.At the same time,the different connections of the transformer are analyzed.The current variation law of the mode under the closing angle factor verifies that the influence of the connection mode of the distribution transformer cannot be ignored.According to the requirements of GB1094.5,the short-circuit thermal stability of the windings before and after the distribution transformer short-circuit reclosing is quantified,and the thermal stability of the distribution transformer is evaluated.Secondly,the short-term stability calculation method of rectangular winding and circular winding is studied.Taking the single-phase transformer as an example,the calculation method of short-circuit impulse current is introduced,and then the magnetic circuit method and energy method are used to calculate the short-circuit impedance,and the two methods are compared for the calculation of the short-circuit impedance of rectangular and circular distribution transformers.The effectiveness of the method is also studied.The distribution law of the current and magnetic field of the foil winding of the distribution transformer is summarized.On this basis,the calculation method of the electromagnetic force of the foil winding is summarized.The stress distribution characteristics of the rectangular winding of the distribution transformer are analyzed.According to the structural characteristics and stress distribution of rectangular windings and circular windings,the verification criterion of the short-circuit resistance of distribution transformers is summarized,and the examples of rectangular distribution transformers and circular distribution transformers are calculated according to the national standards of distribution transformers.Check and quantify the weak position of the transformer winding,evaluate the transformer’s ability to withstand short circuit,and provide designers with a basis for improvement.Finally,the short-circuit stability of three-dimensional wound core and planar wound core transformers is studied.First,the magnetic circuit analysis of the three-dimensional wound core transformer is carried out,and its field-circuit coupling analysis model is established.The transient short-circuit current and dynamic electromagnetic force change rules of high and low voltage windings are simply analyzed.The radial distribution law was studied,and then the leakage magnetic field and electromagnetic force in the window of the three-dimensional core structure and the plane structure were compared and analyzed,and the simulation results of the coupled field of the electromagnetic structure were analyzed to verify that the short-circuit stability of the three-dimensional core structure was stronger than that of the planar structure transformer.