An Evaluation Method for Short-Circuit Cumulative Mechanical Damage of Power Transformer
Ou Qiang1,2, Luo Longfu2, Li Yong2, Yang Xian3, Zhou Lawu4
1. College of Information Engineering Southwest University of Science and Technology Mianyang 621010 China; 2. College of Electrical and Information Engineering Hunan University Changsha 410082 China; 3. Electric Power Research Institute Guangdong Power Grid Co. Ltd Guangzhou 510080 China; 4. College of Electrical and Information Engineering Changsha University of Science and Technology Changsha 410114 China
Abstract Multiple short circuits can gradually accumulate mechanical damage to the windings, leading to the failure of power transformers. However, a quantitative assessment method is lacking to evaluate such damage, and grasping the mechanical state of the transformer winding after a short-circuit impact is challenging. The cumulative damage of windings caused by multiple short circuits is still an urgent problem in the industry, which poses a threat to the secure operation of power grids. Therefore, this paper proposes an evaluation method for assessing the accumulated mechanical damage caused by short circuits in power transformer windings, considering the nonlinear characteristics of windings and the cumulative process. Firstly, the method utilizes the damping ratio and reliability loss coefficient as features. Exponential curves are used to simulate the mechanical weakening process of windings. Additionally, the initial short-circuit withstanding capability, short-circuit current magnitude, and the number of faults are considered. The influence of a short circuit on the mechanical performance of the winding is theoretically analyzed, the number of times that it can withstand a current impact is calculated, and the residual mechanical life coefficient is predicted. The correctness of this evaluation method is validated through destructive short-circuit tests on a 50 MVA/110 kV power transformer. This three-phase, three-winding, differential-designed transformer provides six sets of short-circuit test data. The end of the test is determined by gradually increasing the loading current and the relative change rate of the impedance. 122 short circuit tests are implemented, ranging from 8 times to a maximum of 33 times. All the windings are disassembled after short-circuit tests. Several significant deformations are observed, consistent with predicted failure forms. Moreover, no secondary insulation breakdown or internal short circuit faults caused by mechanical damage are reported. Taking multiple short circuit tests as an example, the dynamic curve of winding mechanical life is obtained, and the dynamic ability of transformer winding to withstand short circuits in operation is characterized. By analyzing critical parameters of the evaluation method, it is found that the influence deviation of different checking systems on cumulative effect evaluation results is less than 1%, and the influence of the damping ratio on the cumulative effect is more than 10%. Therefore, this method is suitable for different checking algorithms. Process control and auxiliary strengthening measures can enhance the withstanding ability for short circuits and reduce cumulative damage or winding destruction. In a word, the method provides a quantitative calculation method for evaluating short-circuit cumulative mechanical damage. Moreover, this method can better describe the winding mechanical state after short circuits, which can provide a reference to transformer design, operation, and maintenance.
Ou Qiang,Luo Longfu,Li Yong等. An Evaluation Method for Short-Circuit Cumulative Mechanical Damage of Power Transformer[J]. Transactions of China Electrotechnical Society, 2024, 39(8): 2578-2590.
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