1 Introduction Transformers are one of the main electrical devices in power systems, playing a crucial role in the safe operation of the power system. During the operation of a transformer, its windings inevitably bear various short-circuit electrodynamic forces, resulting in varying degrees of winding deformation. After winding deformation, the transformer's short-circuit withstand capability decreases sharply, potentially leading to complete damage under further short-circuit impacts or even normal operating current. To prevent the expansion of transformer defects, in accordance with the relevant anti-accident technical measures for transformer equipment issued by the East China Power Company and the Provincial Power Bureau, transformer winding deformation tests must be conducted on transformers that have already withstood short-circuit impacts. There are three main methods for transformer winding deformation testing: short-circuit impedance method, low-voltage pulse method, and frequency response analysis method. This paper further analyzes and studies the technical issues of transformer winding deformation testing using the short-circuit impedance method. [b]2 Basic Principles of Transformer Winding Deformation Testing Using the Short-Circuit Impedance Method[/b] The short-circuit impedance of a transformer refers to the equivalent impedance at the transformer input terminal when the load impedance is zero. Short-circuit impedance can be divided into resistive and reactive components. For large transformers of 110kV and above, the resistive component accounts for a very small proportion of the short-circuit impedance; the short-circuit impedance value is mainly determined by the reactive component. The short-circuit reactive component of a transformer is the leakage reactance of the transformer windings. The leakage reactance of a transformer can be divided into longitudinal leakage reactance and transverse leakage reactance. Generally, the transverse leakage reactance accounts for a smaller proportion. The leakage reactance value of a transformer is determined by the geometric dimensions of the windings. Changes in the transformer winding structure will inevitably cause changes in the transformer leakage reactance, thereby causing changes in the transformer's short-circuit impedance value. Taking a cylindrical two-winding transformer as an example, the winding arrangement diagram is shown in Figure 1. [img=229,180]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/zjdl/0001/image1/t2401.gif[/img] Figure 1. Schematic diagram of a two-winding transformer. Assumptions: The winding height is equal to the height of its axial configuration; the ampere-turns are uniformly distributed; the proximity effect of the core and the DC resistance of the winding are ignored. Then the short-circuit impedance can be expressed by the following formula: [img=181,59]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/zjdl/0001/image1/25-1.gif[/img][font=SimSun]　(1) Where Zk is the short-circuit impedance; Xk is the leakage reactance; μ0 = 4π × 0 - 7; ω is the number of turns in the x winding; Q1 is the Rogoff coefficient; h is the winding height; DCP is the average diameter of the main flood discharge channel; δ is the effective width of the main flood discharge channel; Since Dcp >> b1, b2, δ≈C + (b1 + b2)/3. From equation (1), it can be seen that the change of ZK actually depends only on the deformation of the winding, that is, the change of the winding geometry. If the diameter of the transformer coil is reduced by 2ΔX under the action of the squeezing force (see Figure 1), in equation (1), D′CP = DCP - ΔX is used to replace DCP, and δ′ = δ + ΔX is used to replace δ to obtain Z′K. Therefore, the change in short-circuit impedance ZK caused by winding deformation is: ΔZK=Z′K－ZK≈（m－n）ΔX (2) Where [img=181,59]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/zjdl/0001/image1/25-1.gif[/img] As can be seen from equation (2), the change in short-circuit impedance ΔZK is directly related to the deformation ΔX. To determine whether the winding is deformed based on the change in short-circuit impedance, simply compare the measured short-circuit impedance with the measured value when the transformer is in normal operation (such as the factory data). ... The interference at the site mainly comes from the following aspects: (1) the influence of harmonics in the test power supply; (2) the instability of the test power supply voltage; (3) the 50Hz co-frequency interference at the test site. The influence of the above three factors on the short-circuit impedance test value and the elimination measures are briefly described below. 3.1 Eliminating the influence of harmonics in the test power supply on the test results The power supply used for the test inevitably has various harmonics, and the amplitude of the harmonic components is unstable. Higher harmonics have a significant impact on the test value of the transformer short-circuit impedance. Let the short-circuit impedance of the transformer under test be Z under harmonic-free conditions. When a test voltage with harmonic components u = α1sin(ωt + ψ) + α2sin(3ωt + ψ1) is applied, the current flowing through the transformer is: [img=259,37]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/zjdl/0001/image1/25-2.gif[/img] The effective value of the transformer's short-circuit impedance after considering harmonics is: [img=121,52]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/zjdl/0001/image1/25-3.gif[/img] As can be seen from the above formula, due to the presence of harmonics in the test power supply, there is a certain difference between the measured short-circuit impedance value and the short-circuit impedance value under harmonic-free conditions. To eliminate the influence of test power supply harmonics on short-circuit impedance test results, the short-circuit impedance tester must have excellent filtering performance. A combination of hardware and software methods can generally eliminate the influence of test power supply harmonics on short-circuit impedance test results, meeting the needs of transformer winding deformation testing, analysis, and judgment. 3.2 Influence of Test Power Supply Voltage Instability on Test Results The instability of the fundamental component of the test power supply voltage during the measurement period has a direct impact on the test results. Since short-circuit impedance is an inductive impedance, there is a certain phase difference between the current and voltage. When the fundamental voltage component changes during the test period, the current cannot change synchronously, resulting in measurement errors. To reduce the short-circuit impedance test error caused by the instability of the test power supply voltage, the common method is to calculate the average value of multiple measurements, but the effect is not ideal and it also prolongs the test time. To effectively solve the above problems, the short-circuit impedance tester must analyze and process the signals collected during the measurement period to minimize test errors without prolonging the test time. 3.3 50Hz Co-channel Interference at the Test Site The 50Hz co-channel interference at the test site mainly comes from corona interference from the substation operating equipment and interference generated by the coupling of the 220V AC power supply used by the test instrument to the measurement circuit. To reduce the impact of the 50Hz co-channel interference at the test site on the short-circuit impedance test results, the test instrument must suppress the co-channel interference caused by the coupling of the 220V AC power supply to the greatest extent possible from a hardware perspective. When the corona interference at the test site is large, the polarity of the test instrument can be changed, and the test voltage and current of the transformer under test can be appropriately increased. [b]4 Short-circuit Impedance Test Connection Method[/b] The transformer short-circuit impedance is measured using the volt-ampere method. This method is applicable to single-phase and three-phase transformers. Before testing, one side of the transformer's output lines is short-circuited. The wire used for short-circuiting must have sufficient cross-sectional area, and the terminals of each output line must be kept in good contact to reduce the loop resistance of the leads. A test voltage is applied to the other side of the transformer, thereby generating a current flowing through the impedance. At the same time, the current and voltage applied to the impedance are measured. The ratio of the fundamental components of this voltage and current is the short-circuit impedance of the transformer under test. During transformer short-circuit impedance testing, voltage is typically applied to the high-voltage winding side of the transformer, while a short circuit is applied to the low-voltage winding side. To ensure test accuracy, the voltage measurement circuit should be directly connected to the output terminals of the transformer under test to avoid introducing voltage drop on the current leads. The rated current of the voltage regulator used for testing should not be less than 10A. The test current flowing through the windings of the transformer under test should ideally be on the order of 0.5% to 0.1% of its rated current, or 2 to 10A. The test current should not be too large; otherwise, overload of the power supply will cause severe distortion of the test voltage waveform, affecting test accuracy. Analysis of the short-circuit impedance measurements of 46 transformers of various types measured using a CS-8 short-circuit impedance meter revealed that the differences in three-phase indirect short-circuit impedance values ​​were all less than 2%. The field test values ​​showed significant dispersion compared to the factory values, but were generally less than 4%. [b]5 Principles for Analysis and Judgment of Transformer Winding Deformation Test[/b] After the transformer windings are deformed, the changes in the detection parameters during operation include both electrical and mechanical aspects. Therefore, the analysis and judgment of transformer winding deformation is not a one-sided issue but a comprehensive one. The transformer winding deformation test using the short-circuit impedance method must comprehensively consider the following aspects: (1) Whether the short-circuit impedance test results between the three phases of the transformer are balanced; (2) The change in short-circuit impedance compared with the factory value; (3) The electrical test and insulating oil chromatographic analysis during operation; (4) Whether there are abnormal sounds in the transformer during operation and the operating temperature of the insulating oil, etc. [b]6 Conclusion[/b] (1) The change in short-circuit impedance value is directly related to the structure of the transformer windings. The short-circuit impedance method can be used for transformer winding deformation testing. (2) Conventional ammeters and voltmeters cannot meet the requirements for transformer short-circuit impedance measurement for the purpose of transformer winding deformation testing. (3) A transformer winding short-circuit impedance tester for deformation testing should not only have good testing accuracy but also good anti-interference capability. (4) The difference in short-circuit impedance values ​​between the three phases of a transformer winding is generally less than 2%. A difference of 3% in the short-circuit impedance values ​​between the three phases should be considered a significant change in the transformer's short-circuit impedance and must be given sufficient attention. (5) When analyzing and judging transformer winding deformation tests, attention should be paid if the difference in test results using the same testing instrument and the same testing method is greater than 2%.