The zero sequence current transformer

The zero sequence current transformer 

Zero sequence current transformer is a line fault current monitor. Generally, there is only one iron core and secondary winding. When used, the primary three-core cable passes through the iron core window hole of the transformer, and is connected to the special relay through the lead wire twice. Then, the output terminal of the relay is connected to the signal device or the alarm system. Under normal conditions, the three-phase current in the primary circuit is basically balanced, and the synthesized flux produced by the three-phase current is close to zero. There is no induced current in the secondary winding of transformer. When single-phase grounding fault occurs in the primary circuit, unbalanced current (zero-sequence current) is generated in the primary circuit, and tiny current is induced in the secondary winding to make the relay operate and signal. That is to say, when the zero sequence current is generated in the power system, the protection or monitoring of the power system is realized through the cooperation of the zero sequence current transformer and the relay protection device or signal device.

Principle:

The basic principle of zero sequence current protection is based on Kirchhoff's current law: the algebraic sum of complex currents flowing into any node of the circuit equals zero. Under the normal condition of circuit and electrical equipment, the vector sum of each phase current equals zero. Therefore, the secondary winding of zero sequence current transformer has no signal output and the actuator does not act. When grounding fault occurs, the vector of each phase current is not zero. The fault current causes flux to be generated in the annular core of zero sequence current transformer. The secondary induced voltage of zero sequence current transformer causes the actuator to act, drives the tripping device, and switches the power supply network to achieve the purpose of grounding fault protection.

Application:

A current transformer can be installed on each three-phase line, or three-phase conductors can pass through a zero-sequence current transformer together, or a zero-sequence current transformer can be installed on the neutral line N to detect the three-phase current vector sum.

In three-phase four-wire circuit, the phasor of three-phase current is equal to zero, i.e. Ia+Ib+IC=0.

If a current transformer is connected to the three-phase four-wire, the induced current is zero. When an electric shock or leakage fault occurs in the circuit, the leakage current flows through the circuit. At this time, the three-phase current phases passing through the transformer and the unequal zero phases are: Ia+Ib+Ic=I (leakage current).

In this way, there is an induction voltage in the secondary coil of the transformer, which is applied to the electronic amplifier circuit of the detection part. Compared with the predetermined action current value of the protective device, if it is larger than the action current, even if the sensitive relay acts, it acts on the actuator to switch off. The transformer connected here is called zero-sequence current transformer. The phasor of three-phase current is not equal to zero, and the generated current is zero-sequence current.

Two conditions for generating zero-sequence current:

1. Whether it is vertical fault, transverse fault, or normal and abnormal asymmetry, as long as there is zero-sequence voltage;

2. Zero sequence current has a path.

The above two conditions are indispensable. For the lack of the first, there is no source; for the lack of the second, we usually discuss the question of whether there must be a voltage or a current.

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