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Can general current transformers be used as zero-sequence current transformers?

by：Transmart     2022-05-02

Generally, the current transformer can convert the primary current with a large scale value into a secondary current with a small scale value according to a certain transformation ratio, which is used for main purposes such as maintenance and accurate measurement. The function of the zero-sequence current transformer is to maintain the posture and disconnect the switching power supply when an electric shock accident or a common fault occurs in the power circuit. These names are similar to each other, but the functions are very different. What is the difference and effect in practice? There is no problem in the basic principle of the through-core current transformer as a zero-sequence transformer, but its sensitivity will be quite different.

1. The function of general current transformers General current transformers can convert a primary current with a large scale value into a secondary current with a small scale value according to a certain transformation ratio, which is used for main purposes such as maintenance and accurate measurement . For example, a current transformer with a transformation ratio of 400/5 can change a current of 400A to a current of 5A.

2. Principle of zero-sequence transformer The basic concept of zero-sequence current maintenance is based on Kirchhoff's basic law of current: the algebraic sum of the complex current injected into any connection point in the power circuit is equal to zero. Under the condition that the circuit and electrical equipment are all normal, the vector sum of the currents of each phase is equal to zero. Therefore, the secondary winding signal of the zero-sequence current transformer is not easy to output, and the components are not postured. When the common fault of the grounding device occurs, the vector sum of the currents of each phase is not zero. The common fault current causes magnetic flux in the ring transformer core of the zero-sequence current transformer, and the induced voltage on the secondary side of the zero-sequence current transformer makes the The posture of components, promotes the stand-off equipment, converts the power supply system to the Internet, and achieves the purpose of maintenance of common faults of the grounding device.

3. Zero-sequence transformer function When an electric shock accident or a common fault occurs in the power supply circuit, maintain the posture and disconnect the switching power supply.

4. The zero-sequence transformer application standard can install a current transformer on each of the three-phase lines, or let the three-phase transmission lines pass a zero-sequence current transformer together, or install a zero-sequence current transformer on the neutral point N The sequence current transformer is used to check the current vector sum of the three phases. In the actual application of zero-sequence current maintenance, a current transformer (C.T) can be installed on each of the three-phase lines, or the three-phase transmission lines can pass a zero-sequence C.T together, or a zero-sequence C.T can be installed on the neutral point N. This C.T is used to test the current vector sum of the three phases, that is, the zero sequence current Io, IA+IB+ICu003dIO, when the three-phase loads connected on the route are completely balanced (there is no common fault of the grounding device, and the route, electrical leakage current of equipment), IOu003d0; when the three-phase load connected to the route is unbalanced, then IOu003dIN, and the zero-sequence current at this time is the unbalanced current IN; when a common fault occurs in the grounding device in one phase, It must cause a common fault current Id of single-phase grounding. At this time, the detected zero-sequence current IOu003dIN+Id is the vector sum of three-phase unbalanced current and single-phase grounding current.
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