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8. Factory Testing
CAUTION: The information below is not intended to be as a guideline for testing transformers, please refer to
IEE/ANSI C57.12.90-2006 and
8.1 Routine Tests
Ratio
The turn ratio of a transformer is the ratio of the number of turns in the high-voltage winding to that in the
Lowvoltage winding.Whenthetransformerhastaps,theturnratioshallbedeterminedforalltapsandfor
the full winding.
Note: The ratio test can also be used to test polarity, phase relation, and phase sequence.
Winding Resistance
Resistance measurements are of fundamental importance for the calculation of the
conductor losses and calculation of winding temperatures at the end of a temperature rise test.
No Load and Excitation Current
No-load (excitation) losses are losses that are incident to the excitation of the transformer. No-load losses
include core loss, dielectric loss, conductor loss in the winding due to excitation current, and conductor loss
due to circulating current in parallel windings. These losses change with the excitation voltage.
Excitation current (no-load current) is the current that flows in any winding used to excite the transformer
when all other windings are open-circuited. It is generally expressed in percent of the rated current of the
winding in which it is measured. The no-load losses consist primarily of the core loss in the transformer core,
which is a function of the magnitude, frequency, and waveform of the impressed voltage. No-load losses also
vary with temperature and are particularly sensitive to differences in waveform; therefore, no-load loss
measurements will vary markedly with the waveform of the test voltage.
Load Losses and Impedance Voltage
The load losses of a transformer are losses incident to a specified load carried by the transformer. Load losses
include
R loss in the windings due to load current and stray losses due to eddy currents induced by leakage
2
flux in the windings, core clamps, magnetic shields, tank walls, and other conducting parts
The impedancevoltage ofa transformer is the voltage required to circulate rated currentthrough oneof two
specified windings when the other winding is short-circuited, with the windings connected as for rated voltage
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