CP TD1 Reference Manual V 1.44
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to fingerprints is recommended. If no such test results are available, comparison
should be made to transformers of similar design. On three-phase transformers,
results are also compared between phases. On a three-phase, y-delta or delta-y
transformer, the excitation current will be higher on the two outer phases than
on the middle phase. Only the two higher currents can be compared. If the
excitation current is less than 50 mA, the difference between the two higher
currents should be less than 10%. If the excitation current is more than 50 mA,
the difference should be less than 5%. In general, if there is an internal problem,
these differences will be greater. In this case other tests will also show
abnormalities, and an internal inspection should be considered.
12.9.12 Leakage Reactance
The measurement of the short circuit impedance is done as part of the initial
acceptance test in the factory. The leakage reactance can be calculated from
the short circuit impedance. The difference between the average of Lk of the
three phases should be within 3% of the calculated value from the short circuit
impedance factory test. However, the percentage impedance should not vary
more than 1% from any fingerprint results taken in a good condition.
After events such as through faults, nearby lightning strikes, other surges and
transport of the transformer, this test is used in the field to detect winding
deformation or displacement. This can lead to immediate transformer failure
after a severe through fault, or a small deformation can lead to a failure several
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