Working principle of residual current standard transformer
Dec 01, 2023| The standard transformer allows the three-phase four-wire conductors to pass through a residual (zero sequence) current transformer CT. You can also install a residual (zero sequence) current transformer CT on the neutral line N and use these CTs to detect the three phases. The vector sum of currents is the residual current.
According to the principle of the circuit, when there is no equipment leakage or grounding fault in the circuit and the three-phase load is completely balanced, the vector sum of the instantaneous currents in the primary side is zero, that is, Ia+Ib+Ic+IN=0. When the current mutual inductance The vector sum of the magnetic flux generated in the device is equal to zero. At this time, the induced current IL=0 in the secondary coil.
When an insulation fault occurs in the protected circuit, there is a leakage current to the ground somewhere on the load side, and the vector sum of the remaining (zero sequence) current transformer is not zero, that is, Ia+Ib+Ic+IN≠0. When the current mutual inductance The vector sum of the magnetic flux generated in the transformer is not equal to zero. At this time, an induced current is generated in the secondary winding of the zero-sequence current standard transformer, that is, the residual current IL≠0.