MAGNETISING INRUSH CURRENT:-

Magnetising inrush current refers to the current which flows in the primary winding of the transformer while the secondary winding is open-circuited just after the switch is closed on the primary side. The magnitude of the magnetising inrush current depends upon several factors such as the quality of the steel used for core construction, the magnitude and polarity of residual magnetism, the instant of switching, and so on. The initial asymmetrical flux requirement penetrates core deep into saturation, which leads to an abnormally high magnetising current. Most power transformer operate fairly close to the saturation level. The magnetising characteristic determines the exact shape of the magnetising inrush current, which sometimes goes up to 10 - 15 times the rated value. In many cases, this current takes nearly 1-2 seconds to decay owing to a large magnetising inductance. This effect is more severe in a 3-phase transformer as the current flowing in a winding is dependent upon currents of the remaining winding is dependent upon currents of the remaining windings as well. The magnetising inrush current, which is rich in second harmonics, may cause false tripping of the relay. It is observed that the second harmonics content is always more that 16 % and is found to be very useful in detecting the difference between fault current and magnetising inrush current for relaying purposes.
https://electricalenergyworld.blogspot.com/2019/05/what-is-inrush-current-of-transformer.html

OVER-VOLTAGE INRUSH CURRENT:-

During abnormal system conditions, a short duration voltage increase may take place, which in turn may cause saturation of transformer resulting in high differential currents.
Increased current rises the magnitude of internal fault current causing an erroneous operation of the protective relay. It is observed that a 20-50 % over-voltage in grain steel may cause an increase as high as 10-100 % in exicitation currents.
The over-voltage magnitude is determined  by different factors :-

  • Generator excitation
  • Remaining line length connected to a generating station
  • System shunt reactance
  • Total generating capacity
**An increase in the magnetising current due to an over-voltage causes an increase in third and fifth harmonics components, which in turn leads to malfunctioning of the relay.