According to different uses, current transformers can be roughly divided into two categories:
Measuring current transformer (or measuring winding of current transformer): Provide current information of power grid to measuring, metering and other devices within the normal working current range.
The current transformer for protection (or the protection winding of the current transformer): in the state of power grid fault, it provides power grid fault current information to relay protection and other devices.
1. Current transformer for measurement
Current Transformer
When measuring the large current of alternating current, in order to facilitate the secondary instrument measurement, it needs to be converted into a relatively uniform current (China stipulates that the secondary rating of the current transformer is 5A or 1A), and the voltage on the line is relatively high, such as direct measurement. is very dangerous. The current transformer plays the role of current conversion and electrical isolation. It is a sensor for 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 measuring instruments, relay protection, etc.
During normal operation, the secondary side of the transformer is in an approximate short-circuit state, and the output voltage is very low. During operation, if the secondary winding is open-circuited or the primary winding flows abnormal current (such as lightning current, resonance overcurrent, capacitor charging current, inductor starting current, etc.), an overvoltage of thousands or even tens of thousands of volts will be generated on the secondary side. . This will not only cause harm to the insulation of the secondary system, but also cause the transformer to be burnt out due to excessive extremes, and even endanger the life safety of the operating personnel.
Current Transformer
There are only 1 to several turns on the primary side, and the cross-sectional area of the wire is large, which is connected in series to the circuit under test. The number of turns on the secondary side is large and the wire is thin, forming a closed circuit with a meter with a small impedance (current coil of an ammeter/power meter).
The operation of the current transformer is equivalent to a transformer with a short circuit on the secondary side, ignoring the excitation current, and the number of ampere turns is equal to I1N1=I2N2
The current ratio of the primary winding current I1 to the secondary winding I2 of the current transformer is called the actual current ratio I1/I2=N2/N1=k.
The excitation current is the main source of error.
The accuracy grade of the current transformer for measurement is 0.2/0.5/1/3, 1 means that the ratio error does not exceed ±1%, and there are 0.2S and 0.5S grades.
2. Current transformer for protection
The protection current transformers are divided into: 1. overload protection current transformers, 2. differential protection current transformers, 3. grounding protection current transformers (zero sequence current transformers)
The protective current transformer mainly cooperates with the relay device, and provides a signal to the relay device to cut off the fault power when the line occurs short-circuit overload and other faults.
road to protect the safety of the power supply system. Working conditions of protective current transformers and measuring currents
Current transformer for protection
The transformer is completely different. The transformer for protection only starts to work effectively when the current is several times and dozens of times larger than the normal current. The main requirements of the protection transformer are: 1. Reliable insulation, 2. Large enough accurate limit coefficient, 3. Sufficient thermal stability and dynamic stability.
The protection transformer can meet the requirements of the accuracy level under the rated load, and the maximum primary current is called the rated accuracy limit primary current. The accurate limit factor 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 cannot 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 at the rated accurate limit primary current is 1% and 3%, 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 rms value of the primary current that the current transformer can withstand within one second without damage is called the rated short-time 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 level of the protection current transformer is 5P/10P, and the compound error of 10P marks does not exceed 10%.
Current clamp meter
A clamp meter is an instrument used to measure the current of a running electrical circuit, and it can measure the current without interruption.
