# Principle and function of zero sequence current transformer Principle and function of zero sequence current transformer

Principle:
The basic principle of zero sequence current protection is based on Kirchhoff's current law: the algebraic sum of the complex current flowing into any node in the circuit is equal to zero. When the circuit and electrical equipment are normal, the vector sum of each phase current 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 a 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. The induced voltage on the secondary side of the zero sequence current transformer causes the actuator to act and drive 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 will act and cut off the power supply.

use:
A current transformer can be installed on each of the three-phase lines, or the three-phase wires can pass through a zero-sequence current transformer together, or a zero-sequence current transformer can be installed on the neutral line N to use it to detect three-phase The vector sum of currents.

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 at this time. When an electric shock or leakage fault occurs in the circuit, a leakage current flows in the circuit. At this time, the phasor sum of the three-phase current passing through the transformer is not equal to zero, and the phasor sum is: Ia Ib Ic=I (leakage current)

In this way, there is an induced voltage in the secondary coil of the transformer. This voltage is applied to the electronic amplifying circuit of the detection part and compared with the predetermined operating current value of the protection area device. If it is greater than the operating current, even if the sensitive relay operates, the actuator will be lost. brake. The transformer connected here is called a zero-sequence current transformer. The phasor sum of the three-phase current is not equal to zero, and the current generated is the zero-sequence current.

Two conditions for zero sequence current:

1. Whether it is a longitudinal fault, a lateral fault, or asymmetry between normal and abnormal conditions, as long as there is a zero sequence voltage;

2, the zero sequence current has a path.

The above two conditions are indispensable. Because of the lack of the first one, there is no source; the lack of the second one is the question of "whether there must be a current if there is voltage."

Zero sequence formula: 3U0=UA UB UC, 3I0=IA IB IC

Date:2020-08-12 15:06