Can Ordinary Current Transformers Be Used As Zero-sequence Current Transformers?

The function of the zero-sequence current transformer is to protect the action and cut off the power supply when an electric shock or leakage fault occurs in the circuit. The names of the two are very similar, but their functions are very different. What are their differences and functions? There is no problem in principle when the through-core current transformer is used as a zero-sequence transformer, but its sensitivity is very different.

1. The role of ordinary current transformers

Ordinary current transformers can convert a larger value of primary current through a certain transformation ratio into a smaller value of secondary current 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.

2. Working principle of zero sequence current transformer

The basic principle of zero-sequence current protection is based on Kirchhoff's current law: the algebraic sum of the complex currents flowing into any node in the circuit is equal to zero. When the line and electrical equipment are normal, the vector sum of the currents of each phase is equal to zero. Therefore, the secondary winding of the zero-sequence CT current transformer has no signal output, and the actuator does not act.

When the ground fault occurs, the vector sum of the currents of each phase is not zero, the fault current causes magnetic flux to be generated in the annular iron core of the zero-sequence current transformer, and the induced voltage on the secondary side of the zero-sequence current transformer causes the actuator to act, driving the tripping device, switching the power supply network, and achieving the purpose of ground fault protection.

3. The role of zero-sequence current transformer

When an electric shock or leakage fault occurs in the circuit, the protection action will cut off the power supply.

4. Conditions of use of zero-sequence current transformers

A current transformer can be installed on each of the three-phase lines, or the three-phase wires can be passed through a zero-sequence current transformer together. A zero-sequence current transformer can also be installed on the neutral line N to detect the current vector sum of the three phases.

The specific application of zero-sequence current protection can install a current transformer (C.T) on each of the three-phase lines, or let the three-phase wires pass through a zero-sequence C.T together, or install a zero-sequence C.T on the neutral line N, using These C.Ts are used to detect the three-phase current vector sum, that is, the zero-sequence current Io, IA+IB+IC=IO, when the three-phase load connected to the line is completely balanced (no ground fault, and the leakage current of the line and electrical equipment is not considered) ), IO=0; when the three-phase load connected to the line is unbalanced, then IO=IN, and the zero-sequence current at this time is the unbalanced current IN; When a ground fault occurs in a phase, a single-phase grounding fault current Id is bound to occur. The zero-sequence current IO=IN+Id detected at this time is the vector sum of the three-phase unbalanced current and the single-phase grounding current.

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