Magnetizing inrush:

The magnetizing inrush phenomenon is associated with a transformer winding which is being energized where no balancing current is present in the other winding(s). This current appears as a large operating signal for the differential protection. Special measures are taken with the relay design to ensure that no mal-operation occurs during inrush.

The phenomenon of magnetizing inrush is a transient condition that occurs when a transformer is energized. To minimize the cost, weight and size of the power transformer, core of the transformer is designed to operate near to ‘knee point’ of the magnetizing characteristic. Consequently, only a small increase in core flux above normal operating level will result in high magnetizing current. Under normal steady–state condition, the magnetizing current associated with the operating flux level is relatively small. However, if a transformer winding is charged at voltage zero, with no remnant flux, flux level during first voltage cycle (2 x normal flux) will result in core saturation and high non-sinusoidal magnetizing current waveform – as shown in Figure 3.0 .This current is referred as magnetizing inrush current and will be seen by the only winding which is charged and may persist for several cycles. Following factors affect the magnitude and duration of magnetizing inrush current.

• Residual flux.
• Point on wave switching
• Number of banked transformers
• Transformer design and rating
• System fault level

The magnetizing inrush phenomenon described above produces current input to the energized winding which has no equivalent on other windings. The whole of the inrush current therefore appears as unbalance and the differential protection is unable to distinguish it from current due to internal fault. This will lead to mal operation of the differential protection during the charging of the transformer. Since, this phenomenon is transient; stability can be maintained by providing small time delay in operation. However, this time delay also delays operation of the relay in the event of internal faultoccurring at switch-on-to fault, this method isnotused.

The inrush current, although generally resembling normal in-zone fault current, differs greatly when waveforms are compared. This difference in the waveforms of normal fault and inrush current can be used to distinguish between conditions. The inrush current contains all harmonics. Predominant is second harmonic. Proportion of second harmonic varies somewhat with the degree of saturation of core and exists always in inrush current. Normal fault current does not contain second harmonic component. Hence, the differential protection can be blocked if percentage of certain level of second harmonic current is detected during the charging of transformer, thereby mal operation of the protection can be avoided.