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

The transformer winding core deformation directly or indirectly damages the transformer, this kind of hidden trouble general electrical test can not diagnose the transformer and similar structure of power equipment vibration monitoring method online, foreign first applied to shunt reactor through online monitoring transformer body vibration to reflect the winding and core condition is a matter of recent years. Compared with FRALVI and online or offline short-circuit reactance measurement methods, vibration method can not only detect the faulty winding, but also detect the condition of the core, and this method has no electrical connection with the power system, so it is safe and reliable. Therefore, it is necessary to study and understand the vibration characteristics of the body of the power transformer under no-load condition and short-circuit. When the power transformer runs stably, the silicon steel core and winding vibrate under the action of electromagnetic field and the vibration of the transformer body is caused by the transfer of transformer oil. The vibration of the transformer body surface is closely related to the displacement and deformation of the transformer winding and the iron core. Therefore, the condition of the winding and the iron core can be monitored by online measurement of the vibration of the transformer body.

The size of the main magnetic flux generated by the excitation current at the same tapping position of the transformer in the core basically remains unchanged when no load, load and load change, so the vibration of the core caused by magnetostriction also basically remains unchanged to obtain the vibration characteristics of the transformer core at different tapping positions. It is only necessary to measure the vibration of the transformer body under no-load condition. Since the vibration of the transformer body under load condition also includes the winding vibration under load current, the winding vibration signal can be obtained by measuring the vibration signal under load condition and comparing it with the vibration signal under no-load condition to obtain the comparison with the normal vibration signal. When the transformer core or winding is displacedloose or deformed, the vibration signal measured will have a relatively high frequency component, and the amplitude at the original frequency will also change. The greater the displacement and deformation, the greater the change in the high frequency component and amplitude, because the vibration characteristics at each position of the transformer body are most closely related to the nearest vibration source. According to the change degree of vibration signal measured from various parts of the transformer body, it is convenient to determine which part of the winding or core fault occurs, that is, the online monitoring of the power transformer by vibration method can realize fault location. Therefore, when the vibration method is used for online monitoring of the power transformer, the vibration signal of the body must be measured under no-load condition to obtain the vibration condition of the core. To determine whether the core failure; The winding vibration signal must be removed from the vibration signal of the body under load, and the vibration signal of the core must be removed. 2.1 Test objects and test connection simulation experiments show that the vibration signal test system of transformer body can correctly measure the acceleration signal of transformer body vibration (converted into a voltage signal proportional to the vibration signal by charge amplifier). Therefore, the test system is used to test the vibration of the low-voltage side of a power transformer in a long-term no-load test. When the cooling system of the transformer is closed, the parameters of the transformer are as follows: Model: OSFPSZI2.2 Test results and analysis The vibration sensor is pasted with double-sided tape respectively under the outlet abductor of the high and low voltage arms. Because the C-phase low-voltage side is connected to the test power supply, the transformer body vibration test test connection phase is not measured. The frequency spectrum of each phase vibration signal at the high and low voltage sides is shown in 3 respectively. After the analysis, it is concluded that the fundamental frequency of the vibration of the transformer body during no-load test is 100Hz, and there are other high-order harmonics components. After 1000Hz, the harmonics basically attenuates to 0, which is consistent with the results of theoretical analysis.

The vibration characteristics of high and low pressure body are different. The amplitude of vibration signal in frequency domain of the same position on the three-phase high voltage side appears at the same frequency position, that is, the high voltage side is 400Hz; The amplitude of the low-voltage vibration signal in the frequency domain is all in the same position of the 100Hz high-low-voltage side. The fundamental frequency and the amplitude of each harmonic of the vibration signal are on an order of magnitude, that is, the frequency domain characteristics are basically the same. However, it can be seen from the slightly different phase amplitude-frequency characteristics of the high-voltage side that if the amplitude of the main frequency is relatively large, the harmonic test shows that the no-load vibration characteristics of another 300MVA power transformer are the same as those of the above transformer except that the main frequency of the high-voltage side is 300Hz. 3 Conclusion The vibration signal of the transformer body takes 100Hz as the fundamental frequency. And there are other harmonic components. After 1000Hz, the harmonic amplitude basically attenuates to 0. For the same position on the high voltage side or the low voltage side, the vibration signal of the body has a common rule, that is, the main frequency of vibration is the same. The amplitude-frequency characteristics of each phase vibration signal are basically similar, but the master frequency of different types of transformers may be different. Due to the influence of transformer structure, the compacting state of each phase core and winding, and the structure of transformer housing and other factors/the vibration amplitude of each phase at the same frequency on the same side of the transformer has certain differences. However, when the master frequency amplitude is relatively large, the amplitude of the harmonic component is also slightly higher, and vice versa.