Current Transformer

It is made of the characteristics that the original and secondary currents of the transformer are proportional. The working principle and equivalent circuit are also the same as the general transformer, except that the primary winding is connected in series in the circuit under test, and the number of turns is small; the secondary winding is connected to a low-impedance load such as an ammeter or a relay current coil, which is approximately short-circuited. The primary current (ie, the current being measured) and the secondary current depend on the load of the line under test, regardless of the secondary load of the current transformer.

Since the secondary side is close to the short circuit, the primary and secondary voltages U1 are both small, and the exciting current I0 is also small. When the current transformer is running, the secondary side is not allowed to open. Because once the circuit is opened, the primary current becomes the excitation current, which makes the magnetic flux and the secondary voltage greatly exceed the normal value and endangers the safety of people and equipment. Therefore, fuses are not allowed in the secondary circuit of the current transformer, and it is not allowed to remove the ammeter, relay and other equipment without bypassing during operation. The wiring method of the current transformer is determined according to the operation requirements of the load to which it is connected. The most common wiring methods are single phase, three phase star and incomplete star.