Research on vibration characteristics of power transformer under no-load condition

The deformation of the transformer winding core directly or indirectly damages the transformer. This kind of fault hidden danger is generally not diagnosed by conventional electrical tests. The vibration online monitoring method of transformers and similar structural power equipment was first used in foreign countries in the past few years to reflect the condition of the winding and core through online monitoring of the vibration of the transformer body. Compared with methods such as FRALVI and online or offline measurement of short-circuit reactance, the vibration method can not only detect faulty windings, but also detect core conditions, and this method has nothing to do with the power system. There is an electrical connection, which is safe and reliable. Therefore, the vibration characteristics of the power transformer under no-load load conditions and short circuits should be studied and understood. Among them, the no-load vibration characteristics are the basic principle. When the power transformer is operating stably, the silicon steel core and windings are in the electromagnetic state. Vibration is generated under the action of the field and causes body vibration through the transmission of transformer oil. The vibration on the surface of the transformer body is closely related to the compression status, displacement and deformation state of the transformer winding and core. Therefore, the condition of the winding and core can be monitored by online measurement of body vibration.

The magnitude of the main magnetic flux generated in the core by the excitation current at the same tap position of the transformer remains basically unchanged at no-load, load and load changes. Therefore, the core vibration caused by magnetostriction also remains basically unchanged and can be obtained at different taps. For the vibration characteristics of the transformer core, you only need to measure the vibration of the transformer body under no-load conditions. Since the vibration of the transformer body under load conditions also includes the winding vibration under the load current, the winding vibration signal can be measured by measuring the vibration of the transformer under load conditions. The vibration signal is obtained by comparing it with the vibration signal at no-load. Compared with the normal vibration signal, the vibration signal measured when the transformer core or winding is displaced, loose or deformed will have a higher frequency component, the amplitude at the original frequency will also change, and the displacement will change. The larger the shape, the greater the change in high-frequency components and amplitude. Because the vibration characteristics at each position of the transformer body are most closely related to the nearest vibration source, it is easy to determine which part of the winding or iron is based on the degree of change in vibration signals measured at various places on the transformer body. The core has failed, that is, the vibration method can be used to monitor power transformers online. Therefore, when the vibration method is used to monitor power transformers online, the vibration signal of the core must be measured under no-load conditions to obtain the vibration status of the core, so as to determine whether the core has failed. The winding vibration signal must be removed from the vibration signal of the core during load, so as to determine whether the winding has failed. 2 Tests and results 2.1 Test objects and test wiring Simulation experiments show that the transformer body vibrates The dynamic signal test system can correctly measure the acceleration signal of the vibration of the transformer body (converted into a voltage signal proportional to it through the charge amplifier). Therefore, the test system was used to conduct a vibration test on the low and high voltage side of a power transformer during a long-term no-load test. When the transformer cooling system was shut down during the test, the parameters of the transformer were as follows: Model: OSFPSZI2.2 Test results and analysis The vibration sensors were attached under the high and low voltage arm outlet necks with double-sided tape respectively. Because the C-phase low-voltage side is connected to the test power supply, the vibration test test wiring phase of the transformer body was not measured. The vibration signal spectrum of each phase on the high and low voltage side is shown in 3. After the test, the analysis shows that the fundamental frequency of the transformer body vibration during the no-load test is 100Hz, and there are other high-order harmonic components. The harmonics after 1000Hz basically attenuate to 0, which is consistent with the results of theoretical analysis.

The vibration characteristics of the high and low voltage sides of the body are different. The amplitudes in the frequency domain of the vibration signal at the same position on the three-phase high-voltage side appear at the same frequency position, that is, the high-voltage side is at 400Hz; while the amplitudes in the frequency domain of the low-voltage side vibration signal are both at 100Hz. The fundamental frequency and amplitude of each harmonic of the vibration signal at the same position on the high- and low-voltage sides are on the same order of magnitude, that is, the frequency domain characteristics are basically the same. However, it can be seen from the slight difference in the amplitude-frequency characteristics of each phase on the high-voltage side that if the amplitude of the main frequency is relatively large, the harmonic test of the no-load vibration characteristics of another 300MVA power transformer found that except for the main frequency of the high-voltage side appearing at 300Hz, other vibration characteristics are the same as the above-mentioned transformers. 3 Conclusion The vibration signal of the transformer body has 100Hz as the fundamental frequency and has other harmonic components. The harmonic amplitude after 1000Hz basically attenuates to 0. For the same position on the high-voltage side or low-voltage side, the vibration signals of the body have a common pattern, that is, the main frequency of vibration is the same, and the amplitude-frequency characteristics of each phase vibration signal are basically similar. However, the main frequencies of different types of transformers may Due to the influence of factors such as the transformer structure, the compression conditions of each phase core and winding, and the structure of the transformer box/the vibration amplitudes at the same frequency of each phase on the same side of the transformer are somewhat different, but when the main frequency amplitude is relatively large, the amplitude of its harmonic components is also slightly higher, and vice versa.