Considerations for Operating Current Transformers

Analysis of the operation of current transformers

The structure of an electromagnetic CT (current transformer) is similar to that of a general power transformer. It consists of a core and a coil. The difference is that the primary coil of the CT (current transformer) is connected to high current and high voltage, while the secondary coil is connected to low voltage measurement instruments or relay current coils. The CT (current transformer) approaches a short-circuit state.

During operation of the current transformers, the secondary circuit should not be opened. When a CT (current transformer) is operating normally, its primary magnetic flux and secondary magnetic flux are in balance, and the excitation magnetic flux is very small. The magnetic density in the core and the induced electromotive force in the secondary coil are not high. If the secondary coil is opened, the primary magnetic flux will be used for excitation, and the core will be deeply saturated. The magnetic density will increase to the plateau state. The open secondary coil will produce a high-amplitude induction electromotive force with a waveform of a peak waveform, which means that the secondary coil will produce a high transient overvoltage that poses a great danger to electrical equipment and personal safety.

At the same time, when the secondary circuit is opened, the core will be seriously saturated, causing overheating and burning of the current transformers. Therefore, it is never allowed to open the secondary circuit of the current transformers during operation.

Precautions during operation of current transformers

In power supply and consumption lines, the current and voltage vary greatly, from a few amperes to tens of thousands of amperes. In order to facilitate the measurement by secondary instruments, it is necessary to convert them into more uniform currents. In addition, since the voltage on the line is quite high, it is very dangerous to measure it directly. The CT (current transformer) plays the role of current conversion and electrical isolation. Here are some precautions for the operation of current transformers:

The secondary side of the CT (current transformer) should not be opened during operation. If the secondary circuit is opened, the core loss will be too high and the temperature will be too high, which will burn out the secondary winding of the transformer or cause the insulation of the winding to break down, posing a risk of high-voltage electric shock. Therefore, when replacing measuring instruments, such as current meters, active meters, and reactive meters, the current loop should be short-circuited first before replacing the measuring instruments.

After the meter is calibrated, it should be connected to the secondary circuit first, then the short-circuit is removed, and the meter is checked for normal operation. If sparks are found when removing the short-circuit wire, the CT (current transformer) has already been opened, and it should be immediately short-circuited again. Only when it is confirmed that there is no open circuit in the metering circuit can the short-circuit wire be removed again.

When removing the short-circuit of the CT (current transformer), it is necessary to stand on an insulated mat. In addition, the protective device of the current transformer circuit should be considered for shutdown, and can only be put into operation after the work is completed.

When the insulation resistance of the secondary coil of the CT (current transformer) is lower than 10-20 megohms, it must be dried before being used to restore the insulation.

Both ends of the secondary coil and the shell of the current transformers must be reliably grounded.

If the current transformers emit a hum, check whether the internal core is loose and tighten the core bolts.

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