Harmonic current phase failure protection circuit one a

Harmonic current phase failure protection uses the detection of the third harmonic current in the secondary winding of the current transformer to achieve phase failure protection
. Because the third harmonic is in the same phase and is equivalent to a zero-sequence system, harmonic current protection is also called zero-sequence current protection.
    The third harmonic current is used to reflect the operating status of the motor. The current transformer used is not a linear current transformer, but
a fast saturation current transformer.
    The circuit is shown in Figure 4-25. When normal, relay KA is closed; when the power supply phase is missing, KA is released and the motor stops.
In Figure 4-25 (a), KA makes and breaks the main contact (3A), so it is only suitable for no-load starting of motors below 1.lkW.