Transformer Impedance

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3.2.5 Transformer The turns ratio of a-two-winding transformer determines the
ratio between primary and secondary terminal voltages, when the transformer load current is
zero. When load is applied to the however, the load current encounters an apparent
impedance within the transformer which causes the ratio of terminal voltages to depart
from the actual turns ratio. This internal impedance consists of two components:
(a) A reactance derived from the effect of leakage flux in the windings.
(b) An equivalent resistance which represents all losses traceable to the flow of load
current, such as conductor I 2 R loss and stray eddy-current loss.
3.2.5.1 Percent Impedance. Transformer impedance is conveniently expressed in percent,
and is determined by the ratio of impedance voltage to rated primary voltage. In three-phase
transformer banks, it is usually appropriate to refer both impedance voltage and rated voltage to a
line-to-neutral basis. Percent impedance is also equal to measured ohmic impedance, expressed
as a percentage of base ohms. Base ohms for a transformer circuit is defined as the rated current
(per phase) divided into rated voltage (line-to-neutral), with the rated current derived from the
self-cooled rating of the transformer.
3.2.5.2 Impedance Values. The percent impedance values depend on various factors
including the number of windings, particular phase, high and low voltage ratings, transformer
kVA rating, and the insulation medium and cooling class. In general, impedance values increase
with higher voltage and kVA ratings. For most purposes, the impedance of power transformers
may be considered equal to their reactances because the resistance component is negligible.
3.2.5.3 Fault Current Stresses. The standard transformer is designed with a limited ability
to withstand the stresses imposed by external short circuits. The maximum short-circuit current
magnitude and duration that a transformer can endure without incurring thermal and magnetic
damage varies with transformer size and the number of phases. ANSI Std. C57.1200
defines the fault current withstand capability for liquid-immersed transformers.
3.2.6 Regulation. The full load regulation of a power transformer is the change in secondary
voltage, expressed in percent of rated secondary voltage, that occurs when the rated kVA output
at a specified power factor is reduced to zero, with the primary voltage maintained constant. The
percent regulation can be calculated from the measured impedance characteristics at any load and
power factor. The regulation can also be determined by loading the transformer according to the
required conditions at rated voltage and measuring the rise in secondary voltage when the load is
disconnected.
3.2.6.1 Effect of Variables. The percent regulation varies inversely with respect to the
power factor (being lowest at unity power factor) and directly with the impedance. Transformers
having very good regulation (low impedance), however, are also susceptible to higher fault
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