When the oil-immersed transformer manufacturer sells the transformer, they will remind the staff to refuel in the oil-immersed power transformer, because the oil-immersed power transformer has a magnetic core inside the transformer, which can be covered with oil, because the transformer works on AC.
When working, it flows through the coil to generate a magnetic field, which will generate noise in the case of mutual inductance, and the oil can reduce the noise and also play the role of insulation and cooling.
The magnetizing current generated when the transformer is air-dropped is called magnetizing inrush current. The magnitude of the magnetizing inrush current is related to the structure of the transformer, the closing angle, the capacity, and the remanence before closing. Measurements show that when the transformer is air-dropped, the inrush current due to the saturation of the iron core is very large, usually 2 to 6 times the rated current, and the maximum can reach more than 8 times. Since the magnetizing inrush current only flows into the china oil type transformer on the charging side, a large differential current will be generated in the differential circuit, resulting in a malfunction of the differential protection.
The exciting inrush current has the following characteristics: a. The inrush current has a large value and contains obvious aperiodic components; b. The waveform is star-shaped and intermittent; c. It contains obvious high-order harmonic components, especially the second harmonic component. Obviously; d, the magnetizing inrush current is attenuated.
According to the above characteristics of the magnetizing inrush current, in order to prevent the misoperation of the differential protection of the oil-immersed transformer caused by the magnetizing inrush current, the three principles of high second harmonic content, asymmetric waveform and large waveform discontinuity angle are used in the project to realize the differential protection. atresia.
When a serious fault occurs in the transformer and the CT is saturated due to excessive fault current, the secondary current of the CT also contains a large amount of harmonic components. Differential protection may latch or delay due to second harmonic braking. This will seriously damage the oil immersed transformer. In order to solve this problem, differential quick-break protection is usually set.
The differential quick-break element is actually a high-set differential element of longitudinal differential protection. Different from the general differential element, it reflects the effective value of the differential current. Regardless of the differential current waveform and the magnitude of the harmonic components contained, as long as the differential current rms exceeds the differential quick-break setting value (usually higher than the differential protection setting value), it will take immediate action to cut off the transformer without excitation Blocking of criteria such as inrush current.
The essence of the second harmonic braking is to use the second harmonic component in the differential current to judge whether the differential current is a fault current or an excitation inrush current.
When the percentage of the second harmonic component and the fundamental wave component is greater than a certain value (usually 20%), it is judged that the differential current of the oil-immersed power transformer is caused by the excitation inrush current, and the differential protection is blocked.
Therefore, the larger the second harmonic braking ratio, the more second harmonic current contained in the allowable fundamental wave, and the worse the braking effect.
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