As mentioned in the previous articleCurrent transformerWhat is it and how does it work? Today, let's talk about how to correctly choose a 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.

Different models of current transformers often have significant differences in structure (including copper bar overlap form, shape, iron core, and dynamic and thermal stability tolerance), and are mainly installed in different switchgear. For example, mid mounted handcart cabinets are mostly equipped with models such as LZZBJ9 or AS12, while there are many models with fixed cabinets.

There are also many different specifications for the same model, mainly based on the combination of different transformation ratios, secondary coil capacity, protection coil, and measurement coil accuracy.

When selecting the transformation ratio of a current transformer, the actual load rated current should be obtained first, preferably at 65-85% of the transformer's measurement range. For example, if the rated current is 70A, a transformer with a 100/5 ratio should be selected.

The current transformer ratio of 100/5 indicates that the measurement accuracy within the current range of 100 * 120% can meet the measurement accuracy indicated on the transformer nameplate, such as 0.2 level (measurement accuracy error of 0.2%) and 0.5 level (measurement accuracy error of 0.5%). Measurement results exceeding this current may have significant errors compared to the actual current, and excessively high currents can enter the saturation zone of the iron core, rendering the measurement data meaningless.

100/5 means that when the primary current is 100A, the secondary output current is 5A, describing the transformation ratio, rated measurement, and rated output value.

The transformation ratio of the transformer and ammeter must be consistent, and a 100/5 transformer must be equipped with a 100/5 meter. The main impact is on the readings. Of course, if you can recalculate the display scale of the ammeter according to the transformer ratio, or you can make a new scale and stick it on the ammeter yourself.

give an example:

LZZBJ9-12/150b/2 100/5 0.2/15VA 10P15

The model of the transformer is LZZBJ9-12/150b/2, which is the type installed in the central cabinet.

Rated current of 100A, transformation ratio of 100/5

The measurement grade coil has an accuracy of 0.2 level and a capacity of 15VA

The protection level coil has an accuracy of 10P and a capacity of 15VA

You can also choose a suitable current transformer based on the following points:

1、 Calculate the current based on the load power and select the appropriate current ratio.

2、 Select the transformer accurately based on its purpose (such as protection, metering, measurement, multi winding, etc.)

Degree.

3、 Select the size of the transformer based on the size of the through conductor (such as cable type, busbar type, etc.), and

Fixed method.

4、 Select the voltage level based on the voltage level.

The above is Weikewei's experience on how to choose current transformers. If you have any other thoughts or suggestions about current transformers, you can directly contact Weikewei Technology customer service.