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Research of sweep frequency impedance to determine transformer winding deformation after short-circuit impact

The reliable running of the transformer is extremely important for the electric system. However, transformers will inevitably suffer various impacts, such as short-circuit current and mechanical vibration, which lead to permanent winding deformation, such as inner short circuit, distortion and bulge, lead and winding integral displacement. Failure to promptly identify the defects may result in their accumulation over time and ultimately result in transformer failure. This motivated the development of multiple methods for monitoring transformer winding condition, such as short circuit impedance (SCI) and frequency response analysis (FRA). However, these tests have certain limitations. For instance, SCI is insensitive to some little deformation while FRA has no quantitative criterion and is easily influenced by onsite condition. Sweep frequency impedance method is a new method to detect the winding deformation that combines the advantages of FRA and SCI. It can effectively detect winding deformation and reduce the rate of error detection while also having high signal-noise ratio, good reproducibility and strong anti-interference ability. This paper used sweep frequency impedance to detect the fault of an 110kV power transformer before and after impacts. Identify the deformation by short circuit impedance and correlation coefficient. The test results show that short circuit faults lead to changes in both the 50Hz impedance and the correlation coefficient of the sweep frequency impedance curve. The magnitude of these changes increases with the severity of the short circuit.