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Measuring current transformer
Apr 07, 2018

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 (the secondary rated current transformer in China is specified as 5A or 1A), and the voltage on the other line is higher, such as direct measurement. It is very dangerous. Current transformers function as current transformers and electrical isolation. It is a sensor in the power system to obtain electrical primary circuit current information from secondary devices such as measuring instruments and relay protection devices. The current transformer converts the high current in proportion to low current, and 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. In operation, if the secondary winding is open or the primary winding flows abnormal current (such as lightning current, resonant over-current, capacitor charging current, inductor starting current, etc.), it will generate thousands of volts or even tens of thousands of volts over the secondary side. . This will not only cause damage to the secondary system insulation, but will also cause the transformer to over-excite and burn, and even endanger the safety of the operating personnel.

There is only 1 to several turns on the primary side, and the cross-sectional area of the lead is large, and the circuit is connected in series. The number of turns on the secondary side is large, the wire is fine, and the instrument with a small impedance (current coil of the ammeter/power meter) constitutes a closed circuit.

The operation of the current transformer is equivalent to the secondary side short-circuited transformer, ignoring the excitation current, 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.

Excitation current is the main source of error.

The accuracy class of current transformers for measurement is 0.2/0.5/1/3, 1 means that the ratio error does not exceed ±1%, and there are also 0.2S and 0.5S stages.