What Are The Inrush Current Consideration For Power Transformers?

Two distinctly different definitions for inrush current have been offered because one definition cannot serve all the purposes where inrush current is of interest.

Inrush Current:
is the maximum root-mean-square or average current value, determined for a specified interval, resulting from the excitation of the transformer with no connected load, and with essentially zero source impedance, and using the minimum primary turns tap available and its rated voltage.

Peak Inrush Current:
is the peak instantaneous current value resulting from the excitation of the transformer with no connected load, and with essentially zero source impedance, and using the minimum turns primary tap and rated voltage.

Magnetic and thermal cut-out devices usually are not responsive to one-half cycle of energy regardless of magnitude, hence two or more half cycles are needed to define the trip-out characteristics. Furthermore, these devices are not responsive to peak values, but rather to energy content. (I2 t) becomes the parameter of interest, using root-meansquare current values for fusing characteristics.

Relays and magnetic cut-outs are responsive to the average current value. Therefore, when inrush current is cited it should be made clear which of the two values (root mean square) (average) is indicated.

It should be noted that the inrush current of a transformer is seldom the same value as the steady-state exciting current, but is typically larger and decays to steady state after several cycles, depending on the condition of the core, the instantaneous value of applied voltage, etc.

It is important to consider this asymmetry of inrush current in the design and use of transformers and particularly in the specification of protective devices for the transformer. Maximum inrush current values occur when a transformer core that has an existing maximum residual flux is switched on at zero instantaneous voltage so the residual flux and the instantaneous magnetizing flux are additive.

Circuits are available using silicon controlled rectifier switching to cause this to happen deliberately. Alternately, random switch on twenty or more times will usually produce a near maximum value for a single-phase transformer.

It may take more times for a three-phase transformer unless all three lines are monitored. For the measurement of root-mean-square or average current it is necessary to use an adequate X-axis spread or chart speed so that curve area per cycle can be measured.

Peak inrush current values are of interest in connection with contact welding problems and with devices sensitive to instantaneous current magnitude. The measurement of true inrush current with any degree of accuracy can be very difficult because of the usual nonavailability of zero source impedance power lines for larger systems.

This problem can best be circumvented when the installed source capacity is known and specified in terms of impedance and phase angle, and rated capacity.

These values can then be used in test or computation to determine the installed inrush characteristics of a system which, of course, is the final value of interest. When inrush current values are presented for conditions other than essentially zero source impedance, the actual source impedance values applicable to the data should also be given.

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