Current transformer is a special type of transformer. Its role is to change high current into standard small current. It works with measuring instruments, meters and relays. This can play a role in expanding the measuring range of the instrument and improving the reliability and safety of the circuit.
The parameters of the current transformer are mainly based on the following six points:
1. Rated primary voltage-Determined by the nominal voltage of the system. You can choose a high voltage level current transformer for use in a low voltage level system, such as a 10kv current transformer for use in a 6kv system.
2. Rated primary current-Select according to the rated current level specified in GB1208. If the primary current cannot be selected according to these specified levels, it can be solved by the following methods:
① When the transformation circuit requirements of the protection circuit and the measurement circuit are different, a secondary winding with tap current transformer can be used. It is also possible to change the current transformer with a single tap. Generally, it is divided into series and parallel connection methods to obtain multiple or half current transformers. ② The current transformer for measurement circuit has s-class for special purpose, and it maintains accuracy requirements within the range of 10 ~ 110% of rated current.
3. Rated secondary current-1A and 5A.
Selection principles: ① For all conditions of new power plants and substations, 1A should be selected. ② If the original TA of the transformer installation or expansion project is 5A, and in some cases to reduce the secondary open circuit voltage of TA, the rated secondary current can be 5A. ③ In a factory, the rated secondary current in the station can be selected at the same time as 1A and 5A.
4. The accuracy level and transient characteristics are described in the following topics.
5. Number of iron cores-There are two types of current transformer cores: one type is a single core type with only one primary winding and secondary winding of a current transformer, and most low-voltage current transformers are of this type; The type is a multi-core type in which one primary winding has two or more secondary windings, and each secondary winding is configured differently according to the purpose. The energy meter and meter can share a secondary winding on the premise of meeting the accuracy level.
6. According to the structure, it can be divided into oil immersion type, resin casting type and SF6 type current transformer.
