Magnetizing Inrush (Initial, Recovery, Sympathetic) Initial: When a transformer is energized after being de-energized, a transient magnetizing or exciting current that may reach instantaneous peaks of up to 30 times full load current may flow. ... This can cause operation of overcurrent or differential relays protecting the transformer. The magnetizing current flows in only one winding, thus it will appear to a differentially connected relay as an internal fault. Techniques used to prevent differential relays from operating on inrush include detection of current harmonics and zero current periods, both being characteristics of the magnetizing inrush current. The former takes advantage of the presence of harmonics, especially the second harmonic, in the magnetizing inrush current to restrain the relay from operation. The latter differentiates between the fault and inrush currents by measuring the zero current periods, which will be much longer for the inrush than for the fault current. Recovery Inrush: A magnetizing inrush current can also flow if a voltage dip is followed by recovery to normal voltage.Typically, this occurs upon removal of an external fault. The magnetizing inrush is usually less severe in this case than in initial energization as the transformer was not totally de-energized prior to voltage recovery. Sympathetic Inrush: A magnetizing inrush current can flow in an energized transformer when a nearby transformer is energized. The offset inrush current of the bank being energized will find a parallel path in the energized bank. Again, the magnitude is usually less than the case of initial inrush. Both the recovery and sympathetic inrush phenomena suggest that restraining the transformer protection on magnetizing inrush current is required at all times, not only when switching the transformer in service after a period of de-energization.
Posted on: Sat, 06 Jul 2013 08:21:16 +0000
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