1. The principle of current transformer selection and inspection
1) The rated voltage of the current transformer is not less than the rated voltage of the line at the installation point;
2) Calculate the current IC according to the primary load to select the current transformer change;
3) Select the accuracy of the current transformer according to the requirements of the secondary circuit and verify the accuracy; 4) Verify the dynamic stability and thermal stability.
2. Selection of current transformer ratio
The ratio of the primary rated current I1n of the current transformer to the secondary rated current I2n is called the rated conversion ratio of the current transformer, Ki=I1n/I2n≈N2/N1. In the formula, N1 and N2 are the turns of the primary winding and the secondary winding of the current transformer. Current transformer primary side rated current standard ratio (such as 20, 30, 40, 50, 75, 100, 150 (A), 2Xa/C) and other specifications, the secondary side rated current is usually 1A or 5A. Among them, 2Xa/C means that the same product has two current ratios, which can be realized by changing the connection method of the connecting piece outside the oil conservator on the top of the product. When connected in series, the current ratio is a/c, and when connected in parallel, the current ratio is 2Xa/C. Under normal circumstances, the selection of the conversion ratio of the current transformer for measurement should make the primary rated current I1n not less than the load current in the line (ie, calculate the IC). If the calculated current of the load in the line is 350A, the current transformer ratio should be 400/5. In order to ensure the accuracy requirements of the current transformer for protection, the transformation ratio can be selected larger.
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