Since the primary winding of the open zero sequence current transformer is the neutral line of the three-phase star connection. Under normal conditions, the sum of the three-phase currents is equal to zero, the neutral line has no current, no magnetic flux is generated in the iron core of the transformer, and no induced current is present in the secondary winding. When a single-phase earth fault occurs on the protected equipment or system, the sum of the three-phase currents is no longer equal to zero, and the primary winding will flow a current equal to three times the zero-sequence current of each phase. At this time, the magnetic flux is generated in the iron core. The secondary winding will induce current.
A current transformer can be installed on each of the three-phase lines, or the three-phase wires can be passed through an open zero sequence current transformer, or an open zero sequence current transformer can be installed 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. 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 passing through the transformer and the unequal zero, so that there is an induced voltage in the secondary coil of the transformer.
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. If the operating current is greater than the operating current, even if the sensitive relay operates, the actuating element is turned off. The transformers connected here are called open zero sequence current transformer