If we need to add a current transformer to a high-power equipment distribution meter, how should we choose the current transformer?

1、 Principles for Selection and Inspection of Current Transformers

1) The rated voltage of the current transformer shall not be less than the rated voltage of the installation point line;

2) Calculate the current IC based on a load and select the current transformer variation;

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 Conversion Ratio

The ratio of the primary rated current I1n to the secondary rated current I2n of a current transformer is called the rated conversion ratio of the current transformer, Ki＝I1n／I2n≈N2／N1。 In the formula, N1 and N2 are the number of turns of the primary and secondary windings of the current transformer. The primary side rated current standard ratio of current transformers (such as 20, 30, 40, 50, 75, 100, 150 (A), 2Xa/C) and other specifications, and the secondary side rated current is usually 1A or 5A. Among them, 2Xa/C represents that there are two current ratios for the same product, which can be achieved by changing the wiring method of the connection piece outside the top oil storage tank of the product. When connected in series, the current ratio is a/c, and when connected in parallel, the current ratio is 2Xa/C. In general, the selection of the current conversion ratio for measuring current transformers should ensure that their rated current I1n is not less than the load current in the line (i.e., the calculated IC). If the load calculation current in the line is 350A, the conversion ratio of the current transformer should be selected as 400/5. To ensure the accuracy requirements of the current transformer used for protection, the transformation ratio can be selected to be larger.

You can also choose a current transformer in this way!

Function of current transformerIn the power supply system, it is still extremely important. As a current detection device, the current transformer can be matched with different functional instruments or equipment to monitor the current, power, electrical energy, power factor, and relay protection of secondary lines and motor overload protection in the circuit.

According to voltage levels, it can be divided into high-voltage current transformers and low-voltage current transformers.

Divided by structure: closed current transformer and open current transformer.

According to the installation method, it can be divided into busbar type and enclosed type.

According to accuracy, it can be divided into: 0.5, 0.5S, 0.2, 0.2S, and 0.1 and 0.01.

Divided by function: line current transformer and zero sequence transformer.

According to the mutual inductance ratio, it can be divided into secondary side currents of 1A and 5A.

0.5 and 0.5S level transformers are generally used for motor or line detection, monitoring and sampling of current, and can also be used for energy measurement.

0.2 and 0.2S level transformers are usually used as electrical appliances for measuring and detecting electrical energy and current.

0.1 and 0.01 level transformers are used as standard current detection devices in laboratories.

Below, Wei Kewei will provide the following selection explanation for the LMZJ1-0.5 series of busbar current transformers commonly used by electricians.

LMZJ1-0.5 current transformer

The significance of its model is shown in the figure

Meaning of Transformer Model

LMZJ1-0.5 has a wide measurement range from 30A to 4000A, with accuracy levels of 0.5, 0.5S, and 0.2

The rated current of the secondary side of the current transformer is 5A; For example, LMZJ1-0.5 Φ 30 150/5 refers to a busbar casting type current transformer with a working voltage of 0.5KV, a threading hole diameter of 30mm, a primary current of 150A, a secondary mutual inductance current of 5A, and a through turn current transformer. The mutual inductance ratio of a current transformer is related to the number of wires passing through the holes of the transformer. For example, in the transformer mentioned earlier, if the mutual inductance ratio of a through turn is 150/5, it is 30 times, and if the mutual inductance ratio of a through turn is 75/5, it is 15 times. In other words, the mutual inductance ratio is inversely proportional to the number of through turns.

Capacity of transformer:

From the above figure, it can be seen that the capacity of transformers is very small. Previously, the capacity of transformers was labeled in ohms. Through Ohm's law, we can know their load capacity. Taking the 2.5VA transformer in the figure as an example;

X=S/I=2.5/25=0.1 ohms

X is impedance, S is apparent power, and I is secondary mutual inductance current.

The maximum impedance of the load carried by a 2.5VA transformer is calculated to be 0.1 ohms, otherwise it will affect the measurement accuracy of the transformer. Another issue that affects the accuracy of transformers is how to coordinate with measuring instruments. The highest measurement accuracy of analog instruments and meters is at 2/3 of the range, so the optimal measurement point for a 150/5 transformer is:

I=150*2/3=100A

The 150/5 transformer has the best current measurement and monitoring effect for 55KW motors or circuits!

The above is the introduction of Weikewei Technology on how to choose current transformers. If you are interested inCurrent transformerIf you are unsure about how to choose, please contact Weikewei customer service directly.