Zero-sequence current transformer principle, function and how to use - Database & Sql Blog Articles

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Principle: 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. In the case of normal line and electrical equipment, the vector sum of the currents of the phases is equal to zero. Therefore, the secondary winding of the zero-sequence current transformer has no signal output, and the actuator does not operate. When the ground fault occurs, the vector sum of each phase current is not zero, the fault current causes magnetic flux to be generated in the toroidal core of the zero-sequence current transformer, and the secondary side induced voltage of the zero-sequence current transformer causes the actuator to act, driving The tripping device switches the power supply network to achieve the purpose of ground fault protection.

Function: When an electric shock or leakage fault occurs in the circuit, the protection action is taken and the power is cut off.

Use: can install a current transformer on each of the three-phase lines, or let the three-phase wires pass through a zero-sequence current transformer together, or install a zero-sequence current transformer on the neutral line N to detect Three-phase current vector sum.

In a three-phase four-wire circuit, the phasor sum of the three-phase current is equal to zero, that is, 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, a leakage current flows through the loop. At this time, the three-phase current phasor and unequal zero passing through the transformer are lumped sum: Ia+Ib+Ic=I (leakage) Current)
In this way, the transformer secondary coil has an induced voltage, and the voltage is applied to the electronic amplifying circuit of the detecting portion, and is compared with the predetermined operating current value of the protection zone device, such as greater than the operating current, even if the sensitive relay operates, acting on the actuator brake. The transformer connected here is called zero-sequence current transformer, the phasor sum of the three-phase current is not equal to zero, and the generated current is the zero-sequence current.
Two conditions for generating zero sequence current:
1. Whether it is a vertical fault, a lateral fault, or an asymmetry between normal and abnormal, as long as there is zero sequence voltage generation;
2. The zero sequence current has a path.
The above two conditions are indispensable. Because of the lack of the first one, it is a passive spring; the lack of the second one is what we usually discuss "whether there is a voltage or not."
Zero-sequence formula: 3U0=UA+UB+UC, 3I0=IA+IB+IC

There is no need for wiring at one time, and the core is used. Secondary relay.
The protected three-phase cable passes through the core of the zero-sequence current transformer. Normally, the three-phase current vector sum is zero. When single-phase grounding occurs, the secondary coil will have an induced current to drive the relay to activate the protection device.