Study on Vibration Characteristics of Power Transformer under No-load Condition

The deformation of transformer winding core directly or indirectly damages the transformer. This kind of fault hidden danger cannot be diagnosed by conventional electrical tests. The vibration online monitoring method of transformer and similar structure power equipment was first applied to shunt reactors abroad in recent years. It reflects the winding and core conditions by online monitoring of transformer body vibration. Compared with FRALVI and online or offline measurement of short-circuit reactance, the vibration method can not only detect faulty windings, but also detect the core condition. Moreover, this method has no electrical connection with the power system and is safe and reliable. Therefore, it is necessary to study and understand the body vibration characteristics of power transformers under no-load and load conditions and when they are short-circuited. Among them, the no-load vibration characteristics are the basic principle. When the power transformer is running stably, the silicon steel core and winding vibrate under the action of the electromagnetic field and cause body vibration through the transmission of transformer oil. The vibration of the transformer body surface is closely related to the compression, displacement and deformation of the transformer winding and core. Therefore, the winding and core conditions can be monitored by online measurement of body vibration.

The main magnetic flux generated by the exciting current in the iron core of the transformer at the same tap position remains basically unchanged when it is unloaded, loaded, and when the load changes. Therefore, the iron core vibration caused by magnetostriction also remains basically unchanged. In order to obtain the vibration characteristics of the transformer core at different tap positions, it is only necessary to measure the vibration of the transformer body under no-load conditions. Because the vibration of the transformer body under load conditions also includes the vibration of the winding under the action of the load current, the winding vibration signal can be obtained by measuring the vibration signal of the transformer under load conditions and comparing it with the vibration signal under no-load conditions. Compared with the normal vibration signal, the vibration signal measured when the transformer core or winding is displaced, loosened, or deformed will have higher-frequency components, and the amplitude at the original frequency will also change. The greater the displacement and deformation, the greater the change in high-frequency components and amplitude. Because the vibration characteristics at various positions of the transformer body are most closely related to the nearest vibration source, the degree of change in the vibration signal measured at various locations of the transformer body can easily determine which part of the winding or core has occurred. Fault, that is, the use of vibration method to monitor power transformers online can achieve fault location. Therefore, when the vibration method is used for online monitoring of power transformers, the vibration signal of the body must be measured under no-load conditions to obtain the vibration condition of the core, so as to determine whether the core has a fault; the winding vibration signal must be removed from the body vibration signal under load to determine whether the winding has a fault. 2 Test and results 2.1 Test object and test wiring The simulation experiment shows that the transformer body vibration signal test system can correctly measure the acceleration signal of the transformer body vibration (converted into a voltage signal proportional to it through a charge amplifier). Therefore, the test system was used to perform a vibration test on the low and high voltage sides of a power transformer in a long-term no-load test. During the test, the transformer cooling system was closed. The parameters of the transformer are as follows: Model: OSFPSZI 2.2 Test results and analysis The vibration sensor was attached to the lower part of the high and low voltage arm outlet necks with double-sided tape. Because the low voltage side of phase C is connected to the test power supply, the transformer body vibration test test wiring phase was not tested. The spectrum of the vibration signal of each phase on the high and low voltage sides is shown in 3. After the test, it is analyzed that the fundamental frequency of the vibration of the transformer body during the no-load test is 100Hz, and there are other high-order harmonic components. The harmonics after 1000Hz basically decay to 0, which is consistent with the results of theoretical analysis.

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