Reference Suggestion For Design And Selection Of Current Micro Transformer

The so-called current micro-transformer belongs to a kind of CT set on the cable, its primary winding is through the CT inner hole of the three-phase primary conductor cable, its primary current is a three-phase current vector and.

When the three-phase equilibrium relationship is broken when the system is single-phase, the current micro-transformer has a current output of two times, a supply protection device, to achieve protection and monitoring. In fact, the primary insulation of the current miniature transformer refers to the insulation of the cable itself. As a result, it can be used on cables at any voltage level. At present, it is mostly used in systems where the neutral point is not grounded or grounded by the arc extinguishing coil.

In practical application, the small current line selection device itself has no set value, 0 sequence current is only one of the criteria of the device, requiring the zero sequence current transformer in the primary grounding current is small, and non-metallic grounding, 0 sequence current transformer should also have a certain output, to meet the threshold value of the device startup.

It is important to note that the current micro-transformer itself is not loaded with a small impedance, but the need to connect each 0 sequence current transformer to the device through the cable, so the impedance of the cable is the main load impedance of the 0 sequence current transformer, so it needs to be selected according to its load impedance. In the selection, it is also necessary to pay attention to the reasonable determination of the existing protection of the fixed value time-varying ratio selection and resistance grounding system ratio selection. The connection location of the resistance grounding system is composed of two components, one is the capacitance current and the other is the neutral point resistance current, and the difference between the two is 90 º.

The 0 sequence current of the fault loop of the current miniature transformer is equal to the difference between the current vector of the connection location and the grounding capacitance current of the line, that is, the negative value of the grounding capacitance current and the resistive current vector of all non-fault lines. In addition, attention should be paid to reasonably determine the selection of the ratio of large current grounding system. Usually if the neutral point grounding system is single-phase short-circuit, then the ratio can be selected larger. At the same time, about the current micro-transformer two rated current selection, mainly considering that the zero-sequence current transformer is generally small variable ratio, so try to choose 1 A, to improve the load capacity.