The Application of Current Transformer

February 20 , 2020
The current transformer is very similar to the transformer. The transformer is mainly used to change the voltage of the line on the line, while the current transformer is mainly used to change the current of the line.
There are two types of current transformers for power systems and instruments.
Current transformers for power systems are an indispensable electrical appliance in output and power supply systems such as power plants and substations. They are connected to the line for a long time and generally have a current ratio. It is used to measure the current and power on high-voltage lines, and plays the role of insulation and isolation to ensure operation Personnel and instrument safety.

The purpose of measuring current transformers is mainly in the following two aspects: to measure high current and power on high-voltage or low-voltage lines, using a unified 5A secondary circuit and measuring instruments.

Therefore, there are three main requirements for measuring current transformers:
First, the insulation must be reliable to ensure safety;
Second, there must be a certain measurement accuracy;
Third, the instrument security factor Fs is small.
Protection current transformer is to change the current on the line into a certain amount of current. Supplying Power for relays, and some protective devices. When a short circuit or other fault occurs on the line, causing the current on the line to increase sharply, the current through the current supplied by transformers to protection devices such as relays also increases sharply, causing relay protection devices to operate and cut off fault lines
For current transformers for protection, there are three main requirements:
(1) Insulation must be reliable to ensure safety;
(2) There must be a sufficiently large accurate limit factor;
(3) Must have sufficient thermal and dynamic stability

Why do we need variable current on the line? This is because according to the different conditions of power generation and power consumption, the currents on the lines are different, and there are huge differences, some are only a few Amps, while others are as large as tens of thousands of amps. To directly measure these large and small currents will bring great difficulties to instrument manufacturing. In addition, some lines are high-voltage, such as high-voltage transmission and power supply lines. It is extremely dangerous to measure the current on high-voltage lines with electrical instruments, which is absolutely not allowed. If the current transformer is connected to the line to change the current, then the large and small currents on the line can be uniformly changed into similar currents according to different proportions, as long as it is measured with an electric instrument with a current specification.

It can be seen that the current transformer can not only change the current of different sizes on the line into a certain amount of current for the convenience of measurement, but also play a role of insulation with the line to ensure the safety of operators and instruments.

Since the current transformer is used to change the current, its most important parameter is the current ratio. The ratio of the primary current I1 to the secondary current I2 is called the actual current ratio and is usually expressed in K. For the convenience of production and use, the primary current and secondary current of the current transformer are both specified standards, which are called rated primary current and rated secondary current. The rated current means that under this current, the winding can be energized for a long time without being burned out.

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