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

2021-12-24

General current transformers can convert the primary current with large scale value into the secondary current with small scale value according to a certain transformation ratio, which is used for maintenance, accurate measurement and other main purposes. The function of zero sequence current transformer is to maintain the posture and disconnect the switching power supply when an electric shock accident or common fault occurs in the power supply circuit. Their names are very similar, but their functions are very different. What are their differences and functions? As a zero sequence transformer, the through core current transformer has no problem in principle, but its sensitivity will be very different.

1. Effect of general current transformer

General current transformers can convert the primary current with large scale value into the secondary current with small scale value according to a certain transformation ratio, which is used for maintenance, accurate measurement and other main purposes. For example, a current transformer with a transformation ratio of 400 / 5 can change the specific current of 400A to 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 complex current injected into any connection point in the power supply circuit is equal to zero. When the route and electrical equipment are all normal, the vector sum of each phase current is equal to zero. Therefore, the winding resistance signal on the secondary side of the zero sequence current transformer is not easy to output, and the components are not easy to operate. When a common fault occurs in the grounding device, the vector sum of each phase current is not zero. The common fault current causes the magnetic flux in the ring transformer core of the zero sequence current transformer. The induced voltage at the secondary side of the zero sequence current transformer makes the components pose, promotes the disconnection equipment, and converts the Internet of the power supply system, so as to achieve the purpose of common fault maintenance of the grounding device.

3. Efficiency of zero sequence transformer

In case of electric shock accident or common electric failure in the power supply circuit, maintain the posture and disconnect the switching power supply.

4. Application standard of zero sequence transformer

A current transformer can be installed on the three-phase line, or the three-phase transmission line can cross a zero sequence current transformer together, or a zero sequence current transformer can be installed on the neutral point n to check the current vector sum of the three phases.

For the practical application of zero sequence current maintenance, a current transformer (C.T) can be installed on the three-phase line, or the three-phase transmission line can cross a zero sequence C.T together, or a zero sequence C.T can be installed on the neutral point n. this C.T can be used to check the current vector sum of the three phases, that is, the zero sequence Current IO, Ia + IB + ic = io. When the three-phase loads connected on the line are completely balanced (there is no common fault of grounding device, and the leakage current of route and electrical equipment is not considered), IO = 0; when the three-phase load connected on the route is unbalanced, IO = in, and the zero sequence current at this time is unbalanced current in; when a common fault of grounding device occurs in a phase, a single-phase grounding common fault current ID must be generated, and the zero sequence Current IO = in + ID detected at this time is three-phase Vector sum of unbalanced current and single-phase grounding current.

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