Zero sequence transformer is generally used to detect the main circuit through the transformer circuit "unbalance" current (leakage current), because the main circuit through the transformer circuit, the current should be from the power supply, through the transformer in a line to the power equipment (load), and then by the load back through the transformer through the other lines and then back to the power supply. Other lines back to the power supply through the transformer, the size of the current, when passing through the transformer should be "equal", that is, the size of the same, just a different direction. 

Because the same size and direction of the current in the opposite direction of the primary side of the transformer (the main circuit line can be regarded as the primary side of the transformer), the zero-row ground row will not be in the secondary side of the induction of current or electrical signal to (electromagnetic induction due to its size and direction of the same and the opposite direction of the cancel each other out), so there is no zero-sequence transformer on the "signal output So there is no "signal output" on the zero sequence transformer. If one of the routes in the main circuit of the current from the power supply after the transformer "primary side", no longer through the transformer primary side of the other lines or not through the transformer primary side of the other lines and then back to the power supply, but through other "pathway "back to the power supply, then the secondary side of the transformer (transformer coil) will make the transformer induction of this "through and not back to" the transformer current signal (such as the human body electric shock or equipment leakage), and this signal is amplified, you can control and zero sequence transformer has an electrical connection between the protection equipment Action, disconnect the main circuit of the protected power supply, to ensure the safety of people and equipment.