The characteristics of modern power transformers are that the capacity of single-machines is increasing and the voltage level is getting higher and higher, which makes the internal electric field and magnetic field strength high, which causes electrical and resistance to the coils, leads, tap-changers and other components of the power transformer. Performance issues such as heat and machinery. In addition, power transformers may also experience mechanical collisions during transportation and installation. Under the influence of the above factors, the electric fund project: Xi'an Jiaotong University doctoral thesis funded project power transformer core and winding may be loose. Due to the reduction of the core and winding clamping force, the short-circuit resistance of the power transformer is significantly reduced, which may induce serious accidents. In general, the important purpose of basic maintenance of the transformer is to compress the coil and the core. Therefore, when deciding whether to repair large power transformers, it is very necessary to predict the compression of the windings and the core. Currently, there are several offline methods to predict the compression of the windings and core.
Under no-load conditions, the load current of the power transformer is zero, so the vibration of the winding is negligible, and the vibration of the body is mainly caused by the vibration of the core under the action of the exciting current. Therefore, the vibration characteristics of the transformer core can be obtained by measuring the vibration signal of the transformer body operating under no-load conditions. Under load conditions, a load current flows through the coil, so in addition to the vibration of the core, the vibration of the transformer body also includes the vibration of the winding coil under the load current.
It is generally believed that the main magnetic flux generated by the excitation current of the transformer in the core remains basically unchanged at no load and load change, so the vibration of the core is basically unchanged when the no-load, load and load change. The vibration sensor is used to measure the vibration signal of the transformer body under no-load and load conditions. The vibration signal of the transformer core is measured at no load, and the vibration signal of the core and the winding is superimposed when the load is applied. It is obvious that the vibration signal of the winding can be obtained by separating the vibration signal at the time of no-load from the vibration signal at the time of load.
Of course, the variation of the winding vibration caused by the change of the load current should also be determined. From a foreign perspective, the influence of load current on the noise of the transformer can be evaluated by the following equation: the amount of change (dB); the load current of the transformer (A); /N is the rated current of the transformer (A).
Because transformer noise is closely related to vibration, it can be seen from the above equation that there should be a similar relationship between the level of vibration of the transformer body and the magnitude of the load current. Therefore, it is not difficult to judge the vibration level of the transformer body under the load current according to different load currents, thereby effectively avoiding false positives.
When the rated working magnetic density of the transformer is relatively high (greater than 1.4T), the vibration of the core is much larger than the vibration of the winding, and the vibration of the winding can be ignored. When the transformer is working stably, it can be considered that the vibration of the body is caused by the vibration of the core. The change of the vibration condition is also caused by the failure of the core, so that the large inrush current of the core of the power transformer can be diagnosed, and the vibration signal of the winding is no longer Weak, it can be considered that it is much larger than the core vibration. Therefore, the vibration of the transformer body is mainly caused by the vibration of the winding under the short-circuit condition, so that the vibration signal of the transformer body in the event of a short-circuit accident can be used to monitor whether the winding coil is deformed or loosened.
After one of the high and low voltage windings is deformed, displaced or collapsed, the windings are not tight enough, and the height difference between the high and low voltage windings is gradually enlarged, resulting in an increase in the unbalance of the winding ampere, and the axial force caused by the magnetic flux leakage is large. Therefore, the vibration of the winding is intensified; when the pressing force of the iron core is not large enough, the self-weight of the silicon steel sheet causes the core to be bent and deformed, resulting in large magnetostriction, that is, the vibration of the iron core is intensified. These phenomena are characterized by the fact that the vibration signal is characterized by a higher harmonic component added to the vibration frequency, and the amplitude of the vibration becomes larger.
2.2 Method for judging transformer fault by vibration signal The characteristic vector of transformer vibration in good state can be used as fingerprint quantity (can be used as the feature vector of fingerprint quantity including spectrum, power spectrum, energy spectrum of winding and core vibration signal, etc.) It can be seen that the change of the transformer winding and the core compression condition and the displacement and deformation of the winding will cause the change of the characteristic vector as the fingerprint quantity, so that the indication of the transformer core and the winding condition can be given online and in time, once the transformer fails It can be quickly reflected by comparing the current feature vector with the fingerprint amount.
Compared with the existing transformer winding deformation testing technology (LVI, FRA and short-circuit reactance method, etc.), the biggest advantage of the vibration method for on-line monitoring of the power transformer is to monitor the winding and core condition on-line through the vibration sensor attached to the transformer. There is no electrical connection with the entire power system, which has no effect on the normal operation of the entire power system, and can achieve the purpose of online monitoring safely and reliably.
3 Basic characteristics of the vibration signal of the power transformer body In order to use the vibration method for online monitoring of the power transformer, it is necessary to study the basic characteristics of the vibration signal of the power transformer body. The spectrum of the vibration signal of the body when the measured transformer is working stably (operating frequency 50Hz) is given, as shown. Among them, in the range of less than 100 Hz, the basic vibration caused by the cooling system is concentrated, and above 100 Hz (mainly frequencies of 200, 300, 400 Hz, etc.), the vibration of the body due to the core and the winding is concentrated.
In addition, this paper uses a complete vibration test system to measure the vibration signal of a 110kV (100MVA) power transformer body in a power plant. The spectrum is shown in the figure.
situation. When there is a short circuit accident in the compressor, there is a very high frequency in the winding coil. The core vibration caused by magnetostriction and the amplitude of the fundamental frequency of the transformer body vibration frequency (100Hz) below 500Hz are shown. High, the amplitude above 1000Hz is attenuated to a small value. The vibration of the core is caused by the excitation current. The vibration of the winding is caused by the load current. Therefore, the vibration frequency of the core and the winding does not change under the condition of constant frequency. If the winding or the core fails, at these frequencies. The vibration condition (magnitude, energy, etc.) will change.
After determining the basic vibration characteristics of the winding and the core, the interference of the vibration signals in other frequency ranges can be eliminated, and a more effective criterion for the vibration method for online monitoring of the power transformer is proposed.
4 Vibration signal and noise signal monitoring Compared with the vibration of the core and the winding, noise will be generated. In theory, the noise of the transformer can also reflect the condition of the winding and the core. In foreign countries, the noise method is first used to detect the winding and core condition on the reactor. As shown in the figure, the noise response diagram of a reactor fault and after fault elimination is measured.
It can be seen from the noise that it is indeed possible to detect whether a reactor and a transformer similar to the reactor structure have failed.
It can be seen that the noise spectrum also has the characteristics of twice the power frequency as the fundamental frequency (100 Hz), which is consistent with the vibration spectrum of the body; as an online monitoring method, it is obvious that the method of tens of meters and several hundred meters away from the object is not adopted ( Such as B, C). Therefore, the feasibility of the noise method and the vibration method is equivalent. In addition, the noise method has the convenience and superiority of not requiring a buried body vibration sensor.
However, the method of measuring transformer noise for on-line monitoring of power transformers has not progressed so far. The reason is that the standard method for measuring transformer vibration and noise has strict requirements on the test environment, and accurate measurement results can only be obtained in a special test room. The measurement error of the standard method is very large in the production workshop and the operation site.
This method cannot be applied to on-site measurements, not to mention the funding of a series of experimental studies. In 1991, a new method for noise measurement of 'sound intensity measurement method' was proposed, which can be used in the background. Measurements are made in the production workshop where noise and acoustic reflection are large, but when the background noise is much higher than the noise of the transformer under test, the method also cannot measure the noise level of the transformer under the background noise. It is conceivable that the transformer works. The environment is generally outdoors, the level of background noise cannot be estimated, and it is inevitable that when the background noise is much higher than the noise of the transformer itself, if the noise method is used to monitor the operation status of the power transformer online, it will inevitably lead to misjudgment.
If the vibration signal on the transformer body is measured, the above problem does not exist, and the method of directly measuring the vibration of the body does not require the embedded body vibration sensor. The vibration sensor that measures the vibration signal measures only the vibration signal on the body, and has no connection with the surrounding environmental noise. And the vibration signal of the core and the winding is transmitted to the body through the core pad, the insulating oil and the clamping member, and there is no any attenuation. The vibration sensor attached to the transformer body can obtain the complete vibration information of the core and the winding, thereby being online. Monitoring provides reliable information.
5 Conclusion The vibration signal of the power transformer body is closely related to the internal winding and core condition, and contains a wealth of information. It has a good application prospect for the online monitoring of transformer vibration.
Compared with the on-line monitoring by monitoring transformer noise, directly monitoring the vibration of the transformer body can not only take advantage of the noise method, but also overcome the shortcomings of the noise method, and has higher feasibility.
The research of vibration method for on-line monitoring of power transformers is still in its infancy, and there is still much work to be done. For example: how to put forward effective criteria, the characteristics of the transformer body vibration in the event of a short circuit, and so on.
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