Difference between Current Transformer and Transmitter

1. Types of current transformers:

1) Measuring Current Transformer: In the normal working current range, the current information of the power grid is provided to the measuring and measuring devices.

2) Current transformer for protection: Under the condition of power network failure, it provides information of power network fault current to relay protection and other devices.

2. Principles

Current transformers, like transformers, work according to the principle of electromagnetic induction. Current varies widely in power generation, substation, transmission, distribution and power supply lines, ranging from Gui'an to tens of thousands of amperes. In order to facilitate measurement, protection and control, it is necessary to convert to a more uniform current. In addition, the voltage on the line is generally higher, such as direct measurement is very dangerous. Current transformer plays the role of current conversion and electrical isolation. If the ratio of current transformer is 400/5, the actual current of 400 A can be converted to that of 5A.

Current transformer connected to the winding of the measured current (turn number N1) is called primary winding; winding connected to the measuring instrument (turn number N2) is called secondary winding.

The current ratio of primary winding current I1 to secondary winding current I2 of current transformer is called actual current ratio K. The current ratio of current transformer working under rated current is called rated current ratio of current transformer, expressed in Kn.

3. Warm tips:

When the current transformer is running, the secondary side is not allowed to open. Because once the circuit is opened, the primary current becomes the excitation current, which makes the flux and the secondary voltage exceed the normal value greatly and endangers the safety of people and equipment.

4. Current Transmitter

The current transducer is a measuring instrument that converts the measured current into a linear proportional DC current or voltage output. It has the characteristics of high precision, small volume, low power consumption, wide frequency response, anti-interference and so on. It is not only suitable for intelligent monitoring system of load current such as generator, motor, intelligent low-voltage distribution cabinet, air conditioning, fan and street lamp, but also widely used in electric measurement, automatic control and dispatching system of electric power, petroleum, coal, metallurgy, railway, municipal and other departments.

Current transducer is mainly divided into DC current transducer and AC current transducer.

How to convert the 4-20mA output of the current transducer?

The output of the two-wire system is 4-20 mA. The analog voltage signal is converted into 1-5 V or 2-10 V analog voltage signal through 250_precision resistance. There are many ways to convert digital signal. The input module of the system uses voltage-frequency converter LM231 to convert analog voltage signal into frequency signal, and uses photoelectric coupler TL117 to separate analog signal from digital signal. Leave.

At the same time, the analog signal processing circuit and the digital signal processing circuit use two independent power sources, which are separated from each other analogically and digitally, so as to improve the security of the system. The voltage-frequency converter LM231 also has the function of resisting high frequency interference.

5. The difference between current transformer and transmitter:

1) Working Current

Voltage transformer can only measure AC current, and does not need external power supply.

The current transducer can measure both AC and DC currents and needs external power supply.

Current transformers are only proportional to the magnitude of the induced current on the primary side. Current transmitters convert the magnitude of the induced current into other signals, such as 0-20 mA signals.

2) output signal

Current transformers usually output AC5A or AC1A.

The current transmitter usually outputs small signals of 50mA or 100mA or 5V, or 4-20mA, which is convenient for automatic control.

3) Measurement accuracy

In measurement accuracy, the current transducer is superior to the current transformer. The measured current distortion, multiple harmonics, non-sinusoidal wave and so on have a greater impact on the accuracy of current transformer measurement results, while the impact on the current transmitter is relatively small.