1. The function of a common current transformer

A standard current transformer can convert a primary current with a relatively large 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. Operating principle of zero-sequence current transformer

The fundamental 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 an electric circuit is equal to zero. Under normal conditions of the line and electrical equipment, the vector sum of the currents in each phase is equal to zero. Therefore, there is no signal output from the secondary winding of the zero-sequence current transformer, and the actuator does not operate. When a ground fault occurs, the vector sum of the currents in each phase is not equal to zero. The fault current generates magnetic flux in the toroidal core of the zero-sequence current transformer, and the induced voltage on the secondary side of the transformer causes the actuator to operate, driving the tripping device to switch the power supply network, thus achieving the purpose of ground fault protection.

3. Function of zero-sequence current transformer

When an electric shock or leakage fault occurs in the circuit, the protective action is triggered to cut off the power supply.

4. Operating conditions of zero-sequence current transformer

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

For the specific application of zero-sequence current protection, a current transformer (C.T) can be installed on each of the three-phase lines, or the three-phase conductors can pass through a zero-sequence C.T together. Alternatively, a zero-sequence C.T can be installed on the neutral line N. These C.Ts are used to detect the vector sum of the three-phase currents, which is the zero-sequence current Io, as expressed by IA + IB + IC = IO. When the three-phase loads connected to the line are completely balanced (without grounding faults and without considering the leakage currents of the lines and electrical equipment), IO = 0; when the three-phase loads connected to the line are unbalanced, IO = IN, and the zero-sequence current at this time is the unbalanced current IN; when a grounding fault occurs in one phase, a single-phase grounding fault current Id is inevitably generated. At this time, the detected zero-sequence current IO = IN + Id, which is the vector sum of the three-phase unbalanced current and the single-phase grounding current.