MAGNETIZING CURRENT INRUSH - PROTECTION CONSIDERATION

This is a phenomenon that causes the violation of the basic principle of differential relaying since the magnetizing branch of the transformer can have a very low impedance without a transformer fault. Current produced by magnetizing inrush can reach many times the transformer rating, and these currents appear in the differential relay.

The inrush current duration can range from a few cycles up to many seconds. Although usually considered only in conjunction with the energizing of a transformer, magnetizing current inrush can be caused by any abrupt change of magnetizing voltage.

Such transients include the occurrence of a fault, the removal of a fault, the change of character of a fault (for example, the change from a single phase- to-ground fault to a two-phase-to-ground fault), and out-of-phase synchronizing.

Thus, a desensitizing scheme that is operative only when energizing a transformer may not be adequate. There are several conditions that cause particularly severe magnetizing inrush phenomena.

One involves the energizing of a transformer at a station at which at least one other transformer is already energized. The inrush phenomenon will involve transformers that are already energized as well as the transformer being energized.

This inrush transient may be particularly long in duration. It is important to realize that the inrush into the transformer being energized occurs during the opposite half-cycle to that of the already energized transformer.

Thus, the net inrush into all transformers may approximate a sine wave of fundamental frequency, and therefore not operate the harmonic restraint unit of a differential relay if it is protecting both parallel transformers. Another inrush phenomenon involves the energizing of a transformer by means of an air switch.

Arcing of the switch can result in successive half-cycles of arc of the same polarity. Thus if the first half-cycle results in substantial residual magnetism in a transformer core, succeeding half-cycles can cause a cumulative increase in residual magnetism, each time resulting in a more severe inrush.

Two important characteristics of magnetizing inrush current are
a)It contains substantial harmonics, particularly the second harmonics. These harmonics are not always present in high quantities in all three phases.

b)That there is always a time during each cycle where the current magnitude is almost zero. This time is always greater than a quarter cycle.

The harmonic content of the inrush current depends on various factors such as remnant flux in the core, switching angle, and load on the transformer. Harmonic analysis of the inrush current during the events in

Figure below shows that the second harmonic content of the inrush current is sensitive to all these conditions. The second harmonic content reduces appreciably with increasing load at lagging power factor conditions.

Typical magnetizing inrush current wave