Harmonic restraint feature is imparted to transformer differentia

SOLUTION

• When an unloaded transformer is switched on, it draws a large initial magnetizing current. It is called as magnetizing inrush current.
• As the inrush current flows only in the primary winding, the differential protection will see this inrush current as an internal fault.
• The harmonic contents in the inrush currents are different than those in the usual fault current.
• The dc component varies from 40 to 60%, the second harmonic 30 to 70% and the third harmonic 10 to 30%.
• The third harmonic and its multiples do not appear in CT leads connected as these harmonics circulate in the delta winding of the transformer.
• As the second harmonic is more in the inrush current than in the fault current, this feature can be utilized to distinguish between a fault and magnetizing inrush current.

The figure shows a high-speed biased differential scheme incorporating a harmonic restraint feature.

Operating principle:

• The operating principle is o filter out the harmonics from the differential current, rectify them and add them to percentage restraint.
• The tuned circuit XCXL allows only the current of the fundamental frequency to flow through the operating coil.
• The dc and harmonics, mostly second harmonics in case of magnetic inrush current, are diverted into the restraining coil.
• The relay is adjusted so as not to operate when the second harmonic (restraint) exceeds 15% of fundamental current (operating).