Investigation On Evaluation Of Mechanical Damage And Cumulative Effect For Power Transformer Windings Under Short Circuit Impact

The electromagnetic force of the winding generated by the short-circuit fault current will be more than ten times the rated operation,which may cause winding damage or even collapse.It is quite important to investigate the winding mechanical damage and evaluate the cumulative effect of a power transformer short circuit.To reduce cumulative damage and avoid winding collapse,accurately characterizing the ability to withstand short circuits,designing windings with high mechanical strength,and ensuring process control is pivotal research.However,due to the inf luence of differentiated manufacturing,complex operating conditions,and lack of mechanical state characterization of windings,it is still difficult to evaluate the winding state accurately after mechanical damage.On the one hand,the current method of checking the mechanical strength does not consider the operation short-circuit characteristics,ignores the influence of the thermal property,and rarely involves the spatial difference and time characteristics of the dynamic force.Therefore,there is a l arge deviation in the check results at present,and the mechanical damage assessment is more challenging.On the other hand,the winding collapse may be the cumulative result of multiple damages.However,few characteristic quantities and judgment methods can present the cumulative effect,and the process physical quantity of cumulative damage is difficult to be acquired by experimental method.It has been a lack of systematic evaluation methods for winding mechanical damage and cumulative effect.This paper investigates the winding mechanical damage and the cumulative effect of transformer short circuit in axial and radial directions respectively,supplements the characteristic quantity and judgment method,and completes the experimental verification through the actual transformer.The main innovation work is as follows:(1)This paper sets up an analysis platform for the research of all the chapters by implementing the research on short-circuit impact of power transformer,describing the impact current and electromagnetic force,analyzing the factors affecting the mechanical strength of winding,developing the software for checking the winding mechanical strength,manufacturing the experimental research transformer.(2)A reliability evaluation method based on dynamic relative displacement(DRDE)is proposed.This method takes into account the effects of the core and yoke on the magnetic field and the inevitable spatial differences in the compression systems.The experimental results show that DRDE method can judge axial force and damage more accurately.Finally,the cumulative effect of the axial force is investigated based on the changes in the support stiffness caused by cumulative micro-deformation.(3)The improved radial buckling analysis(IRBA)method is proposed based on defining equivalent flexibility and manufacturing deviation.Flexibility establishes the relationship among buckling stress,conductor thickness,and winding radius.And the manufacturing deviation represents the influence of the asse mbly gap and support weakening on radial buckling.Then,the cumulative effect of the radial force is investigated by analyzing changes in the buckling stress,considering the influence of supports on the radial stability of the inner winding.(4)An evaluation method of winding mechanical damage and cumulative effect based on reliability loss coefficient is proposed for the first time.The short-circuit damage and the dynamic mechanical life of the winding are quantified by this method considering the initial ability to withstand short-circuit impact,the amplitude of short-circuit currents,and the number of failures.The research results of this paper can guide transformer design and operation reliability evaluation with a high engineering application value.