doble M4110 - Comparing Subsequent Test Results

Interpretation of Test Results
4-2 72A-2243-01 Rev. B 9/04
September 17, 2004
Typically, the results of the per-phase measurements are used as a benchmark
for subsequent measurements. On some units, however, the per-phase results
are influenced by the reluctance of the leakage flux path outside the leakage
channel (due to the peculiarities of the single-phase excitation), thus masking
the changes in the leakage channel. Under these conditions, all three per-phase
values may exceed the factory data by as much as 10-30%, while the three-
phase equivalent test may be comparable with the factory data. Therefore, in
these cases the results of the three-phase equivalent measurement may be used
as a benchmark for subsequent tests.
In a three-winding unit, the sign of a change in leakage reactance associated
with three pairs of windings can be used to identify the distorted winding.
These limits simply state whether the distortion is present. The real challenge
is in defining the limits that, if not exceeded, will allow the unit to remain in
service even with distorted windings. At the same time, the same percent
change in the measured leakage reactance can be caused by different levels of
distortion in different transformers. Therefore, these limits may be different
for different transformer designs. In the meantime, experience suggests that a
winding distortion could be suspected if both the three-phase equivalent
results deviate from the factory data, and the per-phase results deviate from
each other by more than 3% of the measured value.
In conclusion, the Delta Benchmark is generally used for the initial test data
analysis, using the three phase equivalent test, and for subsequent tests, using
the per-phase tests. The Delta Average is used mainly for the initial per-phase
test, before a real benchmark for these values has been established.
Subsequent Test
During the subsequent test, results of the per-phase test or (if applicable) the
three-phase equivalent test are compared with results of initial (benchmark)
measurements. We recommend treating changes exceeding 2% of the leakage
reactance measured during the initial tests as an indication of winding
distortion.