POWER TRANSFORMER INRUSH CURRENT
CONSIDERATION
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.
