The function of the current transformer is to convert a large primary current into a smaller secondary current through a certain transformation ratio, which is used for protection, measurement and other purposes. For example, a current transformer with a transformation ratio of 400/5 can convert an actual current of 400A into a current of 5A.
The working principle and equivalent circuit of the current transformer are the same as those of the general transformer, except that its primary winding is connected in series in the measured circuit with few turns, while its secondary winding is connected to a low impedance load such as a current meter or relay coil, which is similar to a short circuit. The primary current (i.e. the measured current) and the secondary current depend on the load of the measured line, and are independent of the load of the secondary side of the current transformer. During the operation of the current transformer, the secondary side must not be opened. Because in this case, the primary current becomes excitation current, which will cause the magnetic flux and secondary voltage to greatly exceed the normal value, endangering personal and equipment safety. Therefore, a fuse is not allowed to be inserted in the secondary circuit of the current transformer, and equipment such as current meters and relays cannot be disassembled without shunting during operation.
The basic structure of the current transformer mainly consists of a primary winding, a secondary winding, and an iron core, all of which are insulated from each other. The simplest current transformer has one primary winding, one secondary winding, and one iron core. Such a current transformer also has only one current ratio.
During the operation of the current transformer, there will be a saturation point, which directly affects the correctness of the protection action of the current transformer, so it needs to be determined. In practical applications, what kind of test methods can be used to detect the saturation point of a current transformer?
One of the more direct ways is to load the secondary side, that is, input current from the primary side to the current transformer and observe the secondary current to find the saturation point of the current transformer. However, when the transformation ratio of the current transformer is large, such a test method is not suitable because the saturation point of the protection-class current transformer may exceed 15-20 times the rated current.
Therefore, the saturation point of the current transformer can also be measured through the volt-ampere characteristic test. Since the saturation of the current transformer is caused by the excessive magnetic flux density of its iron core. Therefore, as long as the primary side is open and current is input from the secondary side, the voltage drop on the secondary winding can be measured to detect the saturation point of the current transformer.
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