Study On Verification Techniques Of Anti-short-circuit Ability For Operating Power Transformers

Power transformers are known as the most important large equipments forpower transforming. However, due to deficiency of winding mechanical strength,transformer short-circuit damage occurs frequently in operation. As a result,shortage of anti-short-circuit ability has become one of the main causes oftransformer damages. How to discriminate windings which are weak inwithstanding short-circuit, and how to take corresponding improvementmeasures, are critical tasks for both transformer manufacturer and operationdepartment nowadays.For verification of transformer winding strength in the transition process ofshort-circuit, electromagnetic force should be calculated exactly first of all.Therefore, two key affecting factors of electromagnetic force, short-circuitcurrent and magnetic flux density, are analyzed and calculated in detail, and2Dstatic electromagnetic field is simulated by finite element software ANSYS inthis paper. Results show that radial magnetic components which are generatedby magnetic lines when they bend at winding ends distribute unevenly along thewinding height, and components at winding ends are the maximum. It is alsoshown that axial magnetic components distribute evenly along the windingheight, and they are much larger than radial magnetic components.Considering that the main reason for short-circuit damages of transformersis radial buckling of inner windings, three checking methods with properimprovements for winding short-circuit strength are proposed. Meanwhile, anew viewpoint is presented in this paper, which is inner windings should bechecked using corresponding method, according to different radial supportstructure and processing techniques.Radial support stiffness is estimated basedon support structure, including insulation cylinder, support stays and supportsticks among iron core steps. Whether stress equilibrium condition satisfies therequest is judged by processing techniques. And then corresponding checkmethod is selected to be used according to complacency status of both radial support stiffness condition and stress equilibrium condition. As to axialshort-circuit strength, this paper evaluates processing techniques and materialsof transformers, which have most significant influence on winding axialshort-circuit strength.Then, Transformers of110kV and the above voltage classes in Fujianpower grid are checked in aspects of both radial short-circuit strength and axialshort-circuit strength, and classified by short-circuit strength grade as well.Grade classification are based on the checking result of radial short-circuitstrength and winding deformation test result, and the checking result of axialshort-circuit strength is just provided to be referential rule. To validate accuracyof the algorithm presented above, some of transformers which have sufferedshort-circuit damages are check using the algorithm, and computed strength ofall these transformers can’t meet the requirement of short-circuit strength.Results show that the algorithm is simple and reasonable, with relatively strongengineering practicabilityTo reduce the probability of operating transformer suffering short-circuitdamage, preventive measures for transformer manufacturer and operationdepartment are presented, and also preventive measures regarding to differentshort-circuit strength grade are refined to in the end.