Instrument operating characteristics

- Jul 24, 2019-

(1) Working characteristics of current transformer

1. In normal operation, the secondary winding approximates the short-circuit operation.

2. The magnitude of the primary current is determined by the primary load current, independent of the magnitude of the secondary current.

3. The secondary circuit of the current transformer in operation is not allowed to open. Otherwise, high voltage will be generated at both ends of the open circuit to endanger the personal equipment or cause the current transformer to heat up.

4. In normal operation, due to the small impedance of the secondary winding, the magnetomotive force generated by the primary current is mostly compensated by the magnetomotive force generated by the secondary current, the total magnetic flux density is not large, and the electromotive force induced by the secondary winding Not too big, generally no more than a few tens of volts. When the secondary circuit is open, the impedance increases indefinitely, the secondary current becomes zero, the secondary winding magnetomotive force also becomes zero, and the primary winding current does not become smaller with the secondary open circuit, and the secondary winding is lost. The compensation effect of the potential, the primary magnetomotive force is very large, all used for excitation, the synthetic magnetic flux suddenly increases many times and many times, so that the magnetic circuit of the iron core is highly saturated, at this time, the primary current all becomes the excitation current, in the second time. A high electromotive force is generated in the winding, and its peak value can reach several thousand volts or even tens of thousands of volts, which may threaten personal safety or damage the secondary insulation of instruments, protection devices and transformers. In addition, due to the high saturation of the magnetic circuit, the magnetic induction intensity suddenly increases, and the hysteresis and eddy current loss in the iron core rise sharply, which may cause the iron core to overheat or even burn the current transformer. Therefore, when it is necessary to repair and calibrate the secondary instrument during operation, the secondary winding or circuit of the current transformer must be short-circuited before disassembly.

5. The primary current of the current transformer varies widely.

6. The structure of the current transformer should meet the requirements of thermal stability and electric stability.

(B), the working characteristics of the voltage transformer

1. During normal operation, the secondary winding of the voltage transformer works in an open state.

2. The voltage on the primary side of the voltage transformer is determined by the voltage of the primary power grid and is not affected by the secondary load.

3. The secondary winding of the voltage transformer in operation is not allowed to be short-circuited.

The current passed by the secondary side of the voltage transformer is determined by the magnitude of the impedance of the secondary circuit. When the secondary side is short-circuited, a large short-circuit current will be generated to damage the voltage transformer. In order to protect the voltage transformer, some fuses or fast automatic air switches are generally installed at the secondary side outlet for overload and short circuit protection. Where possible, the primary side should also be equipped with a fuse to protect the high voltage grid from the safety of the primary system due to transformer high voltage windings or lead failure.

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