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After the transformer manufacturer sells the transformer, it will remind the staff to refuel in the transformer, because there is a magnetic core in the transformer, which can be covered with oil, because the transformer uses AC work when it is working. Flow through the coil to produce a magnetic field, in the case of mutual inductance, will produce noise, oil can reduce the noise, but also can play the role of insulation and cooling.
(1) transformer excitation inrush current
The excitation current generated when the transformer is air-dropped is called the excitation inrush current. The magnitude of the inrush current is related to the structure, closing Angle, capacity and remanence before closing. The measurement shows that the inrush current of the air-dropped transformer due to the saturation of the iron core is very large, usually 2 to 6 times of the rated current, and the maximum can reach more than 8 times. Because the inrush current only flows into the transformer on the charging side, it will produce a large differential current in the differential circuit, resulting in differential protection misoperation.
(2) Second harmonic braking principle
The essence of second harmonic braking is to use the second harmonic component in the differential current to judge whether the differential current is fault current or 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 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 allowed fundamental wave, the worse the braking effect.
(3) differential speed break protection
When there is a serious fault inside the transformer, and the fault current is large, resulting in CT saturation, the CT secondary current also contains a large number of harmonic components, according to the above description, which is likely to cause the differential protection lock or delay action due to the second harmonic braking. This will seriously damage the transformer. To solve this problem, differential speed break protection is usually set.
The differential quick-break element is actually a high fixed value differential element with longitudinal differential protection. Different from the general differential components, it reflects the effective value of the differential current. Regardless of the waveform of the differential current and the size of the harmonic component, as long as the effective value of the differential current exceeds the setting value of the differential speed break (usually higher than the differential protection setting), it will immediately act to cut the transformer without blocking the criterion such as the excitation inrush current.
