Transformer low-voltage short-circuit impedance test application

1 Application Power transformers are inevitably subjected to various fault short-circuit current impacts or physical shocks during operation or transportation. Under the strong electromotive force generated by the short-circuit current, the transformer windings may lose stability, causing local distortion, bulging or bulging. Permanent deformation such as displacement will seriously affect the safe operation of the transformer. In order to detect the change of the transformer 1 application, the power transformer will inevitably suffer the impact of various fault short-circuit currents or physical shocks during operation or transportation. Under the strong electromotive force generated by the short-circuit current, the transformer windings may lose stability, resulting in Permanent deformation such as local distortion, bulging or displacement will seriously affect the safe operation of the transformer. In order to detect the degree of deformation or whether the transformer is deformed, the national electric power industry standard DL/T911-2004 clearly stipulates the test of transformer deformation. At present, the commonly used methods for detecting transformer winding deformation are: frequency response analysis method, frequency sweep method and low-voltage short-circuit impedance method. Among them, the short-circuit impedance is an important parameter of the transformer, and the low-voltage short-circuit impedance method is the traditional method of judging the deformation of the winding. For this reason, the national standard 1094.5-2003 and IEC60076-5:2000 have stipulated that the change of the short-circuit reactance is the only way to judge whether the transformer winding is deformed. Criterion. 2 Principle The short-circuit impedance of a transformer refers to the equivalent impedance at the input end of the transformer when the load impedance of the transformer is zero. Short-circuit impedance can be divided into resistance component and reactance component. Reactance is the sum of capacitive reactance and inductive reactance. For large transformers of 110kV and above, due to the large inductance, the proportion of the resistance component in the short-circuit impedance is very small, and the short-circuit impedance value is mainly the value of the reactance component. The short-circuit reactance component of the transformer is the leakage reactance of the transformer winding. The leakage reactance of the transformer can be divided into two parts: the longitudinal leakage reactance and the transverse leakage reactance. Generally, the proportion of the transverse leakage reactance is relatively small. The leakage reactance value of the transformer is determined by the geometrical size of the winding. The change of the transformer winding structure state will inevitably cause the change of the leakage reactance of the transformer, which will cause the change of the short-circuit impedance value of the transformer. The impedance formula of the transformer Zu003d R+jX where Z: impedance R: resistance X: reactance j is an imaginary unit. When X> 0, it is called inductive reactance. When X u003d 0, the reactance is 0. When X <0, it is called In general applications of capacitive reactance, you only need to know the strength of the impedance: |Z|u003d√R^2+X^2 For an ideal pure inductive or capacitive element with a resistance of 0, the impedance strength is the size of the reactance. The total reactance of a general circuit is equal to: X u003d XL + XC, where XL is the inductance of the circuit, and XC is the capacitive reactance of the circuit. 3 Measurement method 　 The short-circuit impedance measurement of the transformer adopts the volt-ampere method. This method is suitable for single-phase and three-phase transformers. Before the test, short-circuit one side of the transformer. The wire used for short-circuiting must have sufficient cross-sectional area, and keep the terminals of each outlet in good contact to reduce the loop resistance of the lead. The test voltage is applied to the other side of the transformer to generate a current flowing through the impedance, and the current and voltage added to the impedance are measured at the same time. The ratio of the fundamental wave component of this voltage to the current is the short-circuit impedance of the tested transformer.　　 When testing the short-circuit impedance of a transformer, the high-voltage winding side of the transformer is usually pressurized, and the low-voltage winding side is short-circuited. In order to ensure the accuracy of the test, the voltage measurement circuit should be directly connected to the outlet terminal of the transformer under test to avoid introducing a voltage drop on the current lead. The rated current of the test voltage regulator cannot be less than 10A. The test current flowing through the windings of the tested transformer during the test should be on the order of 0.5%~0.1% of its rated current or 2~10A. The test current should not be too large. Otherwise, the test voltage waveform will be severely distorted due to the overload of the power supply, which will affect the test accuracy