Principles and methods of current transformer selection

1. Rated voltage

The rated voltage of the current transformer should be greater than the rated voltage of the line at the installation point.

2. Ratio

The current IC should be calculated according to the primary load to select the transformation ratio of the current transformer. Current transformer primary side rated current standard ratio (such as 20, 30, 40, 50, 75, 100, 150, 2×a/C) and other specifications, the secondary side rated current is usually 1A or 5A. Among them, 2×a/C means that the same product has two current ratios, which are realized by changing the connecting piece wiring method of the product. When connected in series, the current ratio is a/c, and when connected in parallel, the current ratio is 2×a/C. Under normal circumstances, the selection of the transformation ratio of the current transformer for measurement should make its primary rated current I1n not less than the load current in the line (that is, calculate IC). If the calculated current of the load in the line is 350A, the transformation ratio of the current transformer should be 400/5. In order to ensure the accuracy requirements of the current transformer used for protection, the transformation ratio can be selected to be larger.

3. Accuracy level

The accuracy level of the current transformer should be selected and calibrated according to the measurement accuracy requirements.

The principle of selecting the accuracy level: the accuracy level of the current transformer used for billing and metering is not lower than 0.5; the ammeter used to monitor the load current in each incoming and outgoing line circuit should use a 1.0-3.0-level current transformer. In order to ensure that the accuracy error does not exceed the specified value, the secondary load (volt-ampere) of the current transformer is generally checked. The secondary load S2 of the transformer is not greater than the rated load S2n, and the selected accuracy can be guaranteed. Accuracy verification formula: S2≤S2n.

4. Dynamic and thermal stability

It is necessary to check the dynamic stability and thermal stability of the current transformer [9]. The manufacturer’s product technical parameters have given the dynamic stability multiple Kes and the thermal stability multiple Kt, so the dynamic stability and thermal stability are respectively checked according to the following formulas degree can be.

1) Dynamic stability verification Kes×I1N≥iSh

2) Thermal stability verification (KtI1n) 2t≥I⑶∞tima

In the formula, t is the thermal stable current time.

5. Rated capacity

The secondary rated capacity of the current transformer should be greater than the actual secondary load, and the actual secondary load should be 25-100% of the secondary rated capacity. The capacity determines the load impedance of the secondary side, and the load impedance affects the measurement or control accuracy. Load impedance is mainly affected by measuring instrument and relay coil resistance, reactance and wiring contact resistance, and secondary connection wire resistance.