The selection of the ratio of 0 sequential current miniature transformer to the grounding system of Neutral point grounding and arc suppression coil is mentioned.
When the grounding current on this system is relatively large, or when the minimum start current on its protection is relatively small, it is necessary to pay attention to the selection of a larger variable ratio of 0 sequence current micro-transformer, such as 50/1, 100/1, 100/5, 150/5 and above. However, in the neutral point grounding system generally does not allow grounding current on more than 10A, in view of this, the general 10A below the protection is required to move. ARC suppression coil grounding system because of the inductance current and capacitance current neutralization, generally speaking, that is, to pay attention to can not exceed 10A grounding current (generally is compensated, the actual grounding current is already inductive current) because of the comprehensive protection used (sometimes with high sensitivity 0 sequence current transformer without comprehensive protection,and its accompanying relays). However, in the above use, will be required to have a fixed value, the general fixed value of ≤10a, such as the set value of a current is 5 A, this time should consider 100/5a or 20/1A, when the current is 5 A, two times current 0.25A, generally for the comprehensive protection of the start current.
When its comprehensive protection of the minimum start current >0.25a can only choose 75/5, 50/5, 15/1, 10/1 of the variable ratio, these variable ratio of 0 sequence current micro-transformer is best to choose the overall type, otherwise the accuracy is less. For the selection of the ratio of the high current grounding system, it refers to the single phase short circuit of the neutral point grounding system. Variable ratio can be selected larger: 150/5, more than 150/1 of the ratio, can not be too small, otherwise do not hide the unbalanced current.
Note that line 0 (N) cannot pass through the CT. 0 sequence current micro-transformer in two rated current selection, in terms of national standard provisions will also have 1 A, 2 A, 5 a. Considering that the zero-sequence current micro-transformer is generally a small variable ratio, so in the actual use, should pay attention to the choice of 1 A, which is to improve the load capacity.