What Is a Current Transformer?
A current transformer is an instrument that converts a large current on the primary side into a small current on the secondary side based on the principle of electromagnetic induction. The current transformer is composed of a closed core and windings. Its primary winding has a few turns and is stringed in the line of the current that needs to be measured. According to different uses, current transformers can be roughly divided into two categories: current transformers for measurement and current transformers for protection.
- Current transformer for measurement
When measuring the large current of alternating current, in order to facilitate the measurement of the secondary instrument, it needs to be converted into a relatively uniform current (China stipulates that the secondary rating of current transformer is 5A or 1A), and the voltage on the line is relatively high, such as direct measurement It is very dangerous. The current transformer plays the role of current conversion and electrical isolation. It is a sensor used by secondary equipment such as measuring instruments and relay protection in the power system to obtain electrical primary circuit current information. The current transformer converts high current into low current proportionally. The primary side of the current transformer is connected to the primary system and the secondary side Connect to measuring instruments, relay protection, etc.
The secondary side of the transformer is in an approximate short-circuit state during normal operation, and the output voltage is very low. During operation, if the secondary winding is open or the primary winding flows through abnormal currents (such as lightning current, resonance overcurrent, capacitor charging current, inductor starting current, etc.), overvoltages of thousands of volts or even tens of thousands of volts will be generated on the secondary side . This not only causes harm to the insulation of the secondary system, but also causes the transformer to be over-excited and burned, and even endangers the life and safety of operators.
The primary side has only 1 to a few turns, and the wire has a large cross-sectional area, which is serially connected to the circuit under test. The secondary side has a large number of turns and a thin wire, which forms a closed circuit with a meter with a small impedance (current coil of the ammeter/power meter).
- Current transformer for protection
Current transformers for protection are divided into:
- Overload protection current transformer
- Differential protection current transformer
- Grounding protection current transformer (zero sequence current transformer)
Main requirements for protection transformers:
- Reliable insulation
- A sufficiently large accurate limit coefficient
- Sufficient thermal stability and dynamic stability.
The current transformer for protection is mainly coordinated with the relay device. In the event of a short circuit overload and other faults on the line, it provides a signal to the relay device to cut off the fault circuit to protect the safety of the power supply system. The working conditions of the current transformer for protection are completely different from the current transformer for measurement. The protection transformer only starts to work effectively when the current is several tens of times larger than the normal current.
The maximum primary current of the protection transformer that can meet the requirements of the accuracy level under the rated load is called the rated accuracy limit primary current. The accurate limit coefficient is the ratio of the rated accurate limit primary current to the rated primary current. When the primary current is large enough, the iron core will saturate and fail to reflect the primary current, and the accurate limit coefficient represents this characteristic. The accuracy level of the protection transformer is 5P and 10P, which means that the allowable current error is 1% and 3% when the primary current is rated accurately and the composite error is 5% and 10% respectively.
When the line fails, the inrush current generates heat and electromagnetic force, and the current transformer for protection must withstand it. When the secondary winding is short-circuited, the effective value of the primary current that the current transformer can withstand within one second without damage is called the rated short-term thermal current. When the secondary winding is short-circuited, the peak value of the primary current that the current transformer can withstand without damage is called the rated dynamic stable current. The accuracy grade of the protection current transformer is 5P/10P, and the 10P indicates that the composite error does not exceed 10%.