Transformer differential protection is the main protection of the electrical quantities of the transformer, and its protection range is the part surrounded by the current transformers on each side. When faults such as winding phase-to-phase short circuit and inter-turn short circuit occur within this range, the differential protection must operate.
1. Excitation inrush current of transformer
The excitation current generated when the transformer is air-dropped is called excitation inrush current. The size of the excitation inrush current is related to factors such as the structure of the transformer, closing angle, capacity, residual magnetism before closing, etc. Measurements show that when the transformer is airdropped, the core saturation excitation inrush current is very large, usually 2 to 6 times the rated current, and the maximum can reach more than 8 times. Since the excitation inrush current only flows into the transformer on the charging side, a large differential current will be generated in the differential circuit, causing the differential protection to malfunction.
The excitation inrush current has the following characteristics: a. The inrush current value is very large and contains obvious non-periodic components; b. The waveform is peak-shaped and intermittent; c. It contains obvious high-order harmonic components, especially the second harmonic component. Obviously; d. The excitation inrush current is attenuated.
According to the above characteristics of the excitation inrush current, in order to prevent the misoperation of the transformer differential protection caused by the excitation inrush current, three principles are used in the project: high second harmonic content, asymmetric waveform, and large waveform discontinuity angle to realize the blocking of the differential protection.
2. Second harmonic braking principle
The essence of second harmonic braking is to use the second harmonic component in the differential current to determine whether the differential current is a fault current or an excitation inrush current. When the percentage of the second harmonic component and the fundamental 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 is, the more second harmonic current is allowed to be included in the fundamental wave, and the worse the braking effect is.
3. Differential quick-break protection
When a serious fault occurs inside the transformer and the fault current is large causing CT saturation, the CT secondary current also contains a large amount of harmonic components. According to the above description, this is likely to cause differential protection due to second harmonic braking. Block or delay action. This will seriously damage the transformer. In order to solve this problem, differential quick-break protection is usually set up.
The differential quick-break element is actually a high-set value differential element for longitudinal difference protection. Different from general differential components, it reflects the effective value of the differential current. Regardless of the waveform of the differential current or the size of the harmonic components, as long as the effective value of the differential current exceeds the differential quick-break setting value (usually higher than the differential protection setting value), it will immediately cut off the transformer without excitation. Blocking of inrush and other criteria.
