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Update:12-03-2021

1. The role of ordinary shunt resistors

Ordinary current transformers can convert a primary current with a larger value into a secondary current with a smaller value through a certain transformation ratio, which is used 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 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, and 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.

3. The role of zero sequence current transformer

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

4. Operating conditions of zero sequence current transformer

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 the three-phase The vector sum of currents.

The specific application of zero-sequence current protection can be to install a current transformer (CT) on each of the three-phase lines, or let the three-phase wires pass through a zero-sequence CT together, or install a zero-sequence CT on the neutral line N. These CTs 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 fully 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, the zero sequence current at this time is the unbalanced current IN; when a ground fault occurs in a certain phase, a single-phase grounding is bound to occur The fault current Id, the zero sequence current detected at this time IO=IN+Id, is the vector sum of the three-phase unbalanced current and the single-phase ground current.