Selection of Transformation Ratio of Zero Sequence Transformer
Mar 31, 2023| 1. Transformation ratio: the ratio of the rated primary current to the rated secondary current
The application of zero-sequence current transformers generally chooses smaller transformation ratios, such as: 50/5, 75/5, 100/5, 150/5, 200/5, 20/1, 50/1, 100/1 , 150/1, 200/1, etc., because the zero-sequence current transformer has an output only when a ground fault occurs. People don't make the protection action when the grounding current is very large (don't consider avoiding the load current).
However, since the primary winding is a power cable with only one turn, the secondary rated turns of the 50/5 zero-sequence current transformer are only 10 turns, so the load characteristics of the 50/5 zero-sequence current transformer are poor. When the load impedance is inconsistent with the capacity of the zero-sequence current transformer, there will be a large error, and the error will also increase when the current is lower than the rated current, so try to choose a larger transformation ratio if possible.
2. The time-varying ratio selection of the existing protection setting value
With the protection setting, the transformation ratio is easy to choose. If the fixed value is a primary current of 80A, the protection will act, and the reliable national standard should be 100/5 or 100/1.
3. Selection of transformation ratio of resistance grounding system
The grounding point current of the resistance grounding system is composed of two components, one is the capacitive current, and the other is the neutral point resistance current, and the difference between the two is 90o. The zero-sequence current of the fault circuit is equal to the vector difference between the ground point current and the ground capacitance current of this line, which is equal to the negative value of the vector sum of the ground capacitance current and resistive current of all non-fault lines.
4. Selection of transformation ratio of large current grounding system
The single-phase grounding of the neutral point grounding system is a single-phase short circuit. The transformation ratio can be selected larger, such as: 150/5, 150/1 or more transformation ratio, not too small, otherwise the unbalanced current cannot be avoided. Note that the neutral wire (N) does not cross the CT.